The GT380, GT550, and GT750, have similar virtues. They are totally reliable, inexpensive to own, and are easily repaired if they do break. All of them have more than acceptable performance, not far from today's superbikes.
The GT triples will accept high performance modifications easily. High performance is not only more horsepower, it's also reliability, braking and handling.
High performance also involves correcting deficiencies (i.e. most Japanese motorcycles were equipped with marginal, high-wear shock absorbers) and will benefit from the installation of improved shocks. Touring owners of the GT750 "Water Buffalo" with their heavy loaded, long-distance style of riding, will greatly benefit from suspension modifications.
Since all three Suzukis have excellent engine performance within their respective displacement limitations, modifications to all three should begin by improving the bike's handling and braking.
The first step in suspension modifications is to make sure that all components in the suspension system are well within tolerances. This is especially important on high-mileage bikes, such as touring GT750's
If the stock parts are worn out, then consider using after market replacement parts, many times there is little point in using OEM replacements when heavier-duty, better-quality, and often lower-priced parts are available.
A common cause of poor handling are worn out swing arm bushings. Riders that feel they can improve handling by replacing shocks, installing high performance forks or other modifications will get nowhere if the swing arm has any side play at all.
Checking swing arm play is recommended for any GT Suzuki, and mandatory on a performance-modified machine.
Check by removing the rear wheel, and unbolting the shocks at the bottom. With the bike on the centerstand and braced, attempt to move the swing arm sideways. If there is any noticeable movement, the bushings should be replaced.
How long do they last and when are they worn out? Its been said stock rear shocks on the GT Suzukis usually last no more than 12,000 miles. Unfortunately, there is no way the average rider can check his shocks with the tools ordinarily at hand. If you think you can "feel" if the shocks are worn then you should know this: Shocks wear out slowly over time, and most riders will adjust (subconsciously) to this loss of handling without realizing it.
Here are Three tests, the first Two of whitch are "feel" tests;
1) Have someone who is knowledgeable about shocks, ride the bike through some dips and bumps, and such. Since he may not be familiar with the bike, he may not be able to tell if the shocks are worn out.
2) Have a friend ride behind you, and watch the shocks. If they bounce (repeatedly spring-return~spring~return) without any apparent damping, the shocks need replacement.
As you now know this method of testing is very subjective.
3) Taking the shocks off the Bike, removing the springs and hand compressing reveals nothing of the real world performance.
An old rule says "if you think your shocks are going bad, they are"!
Rather than using OEM parts to replace worn-out shocks, you should think about getting aftermarket shocks. They are usually less expensive than stock shocks, have a longer lifespan, and offer superior performance.
A good set of shocks to use is the inexpensive, high quality, S&W shock absorber,
it has very good damping performance, and a long lifespan in excess of 50,000 miles.
For solo riding on the GT380 and GT550, the S&W's should be equipped with 85-115 lb. dual-rate springs. For canyon riding or touring, 95-125 lb. springs should be used.
For the GT750, the touring riders might consider installing air adjustable shocks that you can set for a soft, plush ride for long highways trips, or stiff for straightening canyon curves. Other shocks for the GT-bikes are from Koni or Number One.
The stock forks are OK for "normal" riding, for performance or touring riders you will want more
As with other components, carefully inspect the entire front end before making modifications.
Look for dry or out-of-round steering head bearings, abrasions on the fork sliders, insufficient or broken-down fork oil, and, worn-out fork springs.
Even if the springs are good the front end will be rather undersprung, particularly if a fairing is mounted.
Adding a one inch diameter shim inside each fork cap will slightly preload the springs. The fork oil should be replaced with 25 to 30-weight fork oil.
These modifications slightly improve preload and damping. More improvement can be made by installing aircaps. It preloads the forks, increasing spring poundage and ground clearance when cornering; it decreases the possibility of seal leakage, since the air pressure forces the seals more tightly into position.
Normally, 8 to 10 pounds air pressure will be sufficient loading for the performance or touring rider.
Here is some information for thoughs who may wish to replace the stock wire wheels with cast alloy wheels.
Cast wheels reduce motorcycle maintenance time, since there are no more spokes to clean, tighten, replace, etc., also the cast alloy wheels have greater rigidity.
Unsprung weight (the parts of the bike affected directly by road shock, the swing arm, brakes, wheels, etc.) will be reduced slightly, Both these changes will improve handling.
The only reason not to switch to cast wheels is cost.
Cast wheels for the GT-triples were available from Morris Industries. If you have the disc brake front end, you will only have to move the rotors from the stock wheels to the Morris wheels and reassemble.
For drum brake GT machines you will have to replace the front drum with a complete disc assembly. Many use the GS-series four-stroke assembly or, if you want dual front discs, the entire assembly from a GT750. A GT750 kit was available that included both lower fork legs intended for the GT750, as the 380 and most 550's lack mounting lugs for dual brakes.
The addition of a second disc brake looks great, but Suzuki's single disc is more than efficient enough for normal and even mildly hard riding, although I would recommend duel brakes for the GT750.
The rear Morris Mag wheel has no rear drum brake like that used on OEM wheels, so, an accessory rear disc system must be installed.
One solution is to purchase the complete rear disc assembly used on 1978 and later GS750C models. The 1978 GS rear brake uses a smaller disc than the 1977, and is therefore less prone to unexpected lockup. You will need to use the GS master cylinder and brake pedal assembly, this requires welding mounting hardware to the frame.
Another solution that has been done is to install a Grimeca rear brake assembly. This consists of master cylinder, brake line, disc, and caliper, from Morris Industries. As with the Suzuki GS assembly, you will have to build or have built mounting hardware for the disc system.
Rear brake performance can be improved by replacing the stock linings with automotive competition metallic linings. Take the stock brake shoes to a high performance automotive brake shop. They will custom fit the metallic linings. However, this will increase brake wear. Also, since less than 40% of braking involves the rear wheel, overall braking is not improved much; the stock Suzuki drum brakes do a satisfactory job of braking. This modification is only recommended for competition riders, who need all of the stopping power they can get.
Metallic linings can be fitted to the front drum brake. However, the four leading-shoe drum used on the GT750's already delivers exceptional braking performance.
Improving Disc brake performance in the rain. When it is wet, water collects on the disc. When the brake is applied, the puck hydroplanes on the water surface for at least one full revolution. At this point, the rider generally squeezes harder on the brake lever. Since the water now has been cleaned from the disc, this excessive force can cause front wheel lock an extremely dangerous condition on wet pavement.
The solution is to remove the disc(s), and have it drilled. In the rain, the water will be squeezed from the disc into the holes, and the puck will not hydroplane. The disk should not be drilled by an amateur machinist, since it is possible to mar the surface of the disc and ruin it, or make too many holes and weaken it (which could possibly collapse under braking, locking up the wheel). These holes should be slightly counter-sunk on either side, to prevent the sharp edges from having a cheese grater effect on the puck.
This service should be done by an experienced machine shop.
Rather than replacing the stock tires with OEM components when worn out, high-quality performance tires should be used.
Tires are a necessary compromise between the high-wear, high-adhesion racing compounds, and the low-wear, but decreased traction touring compounds.
Other excellent performance tires are available from Goodyear, Pirelli, and Dunlop and Yokahoma. The
rider is advised to talk to other riders with the same bike and similar riding style who have used these or other tires.
Touring-oriented owners of the GT750 are advised against changing their rear wheel to the popular 16 inch diameter. Although this permits use of the comfortable large-diameter Harley -type tire, these compounds are more noted for their longevity than for maximum cornering adhesion.
The Suzukis GT-Triples have ample power, but its always nice to have more. The Suzuki GT-Triple engine design is strong and reliable, increased horsepower can be made without loosing engine reliability.
You can easly build either of the three engines to produce much more horsepower. This will of course be at the expense of rideability. A built-up two-stroke can be very peaky and hard to ride. Heavy power increases can be had from port modifications. Be cautious about port modifications which easly make the power band narrower and higher in rpm. If done right you will gain Heavy power increases much like a roadracing bike.
There have been various, rather fascinating bits of exotica available for all three models. For the 380 and 550, replacement cylinders were available from England. These increase displacement, radically alter port timing, and convert the engine to water cooling.
Super performance parts for the GT750, originally for their use in small automobile racing, have been made.
Street riders should remember that these parts were intended for racing. Racers will use parts which last no more than a few races, so long as they significantly increase power within the powerband needed for competition. The street rider must consider this, unless you want to tear your bike down every thousand miles or so. When street riding you spend most of the time riding at low and mid-rpm, a racing modification may not be suitable.
Before any engine modifications are made to the GT750, the crankshaft seals should be closely inspected. If worn (likely on high-mileage Water Buffaloes), they should be replaced with the late-model seals, which are fitted with metal flanges.
Installation of the inner seals requires the crank to be disassemble, reassembled and trued which is a press-fit machine-shop job, its best to have the work done by an experienced machine shop.
Since the Suzuki GT-Triples have low compression ratios, horsepower may be moderately increased by increasing the engine's compression, without sacrificing reliability or longevity. By milling lmm from the base of the head, the compression should be increased to an ideal 150-160 lb. For still further performance, up to 3mm may be milled from the GT750's head without any problems.
In 1976, Two additional transfer ports were added to the GT380's cylinders. Pre-1976 models can be upgraded by replacing their older barrels with the higher-performance late-model part .
Oil Pump Modification
Most racing only two-strokes use premixed oil and gas instead of an oil injection system like that used on the GT's This saves weight and eliminates the chance of an oil injection failure. It should not be done on street machines, having to mix gas and oil at every fuel stop is messy and a real pain in the butt.
A two-stroke is deceptive in its simplicity, even though the two-stroke engine is simple in its component parts, in function it is very complicated. Without valves, camshafts, etc., modification should be simple.
According to theory, port alteration on two-stroke engines is simple. Widen the intake and exhaust ports, you merely increase the mixture amount and therefore performance without altering the timing. If you raise the intake ports 2mm and raise the exhaust ports 2mm, you will get more power at high rpm without making much of a sacrifice at lower rpm.
It seems so simple that some builders buy a Dremel tool, and go after performance. More often thay end up with reduced performance and/or a bike with an unusable or impractical powerband.
Each part of the two-stroke engine interacts with other parts to affect the engine's behavior. What appears to be a minor change can severely alter the engine performance.
Engine port modification is a very critical area. Any changes should be made cautiously.
Its easy to find a local porting service that has a racetrack reputation. The rider should be cautious as the competition porter may automatically think in terms too radical for a street bike, producing an engine far to peaky for normal street riding.
Small improvements in performance may be gained by removing the barrels and smoothing the edges of the ports. This work should be done with emery or sandpaper, by hand, to prevent the possibility of inadvertent engine damage. It is especially important the angle of the port edges are not altered, thay were carefully designed to be the way they are.
The stock exhaust pipes used on the GT-Triples are a compromise between cost, maximum power output, and noise limits. An unmuffled two-stroke engine is one of the loudest and most obnoxious noises known to man. Take care to properly silence exhaust noise. Significant power may be gained by installing a custom-built exhaust system known as expansion chambers.
There are reasons to avoid a loud exhaust systems on the street. A primary cause of fatigue is noise. The louder a motorcycle is, the less time you can ride it comfortably. For touring riders this is highly significant, since the long distance biker must have the option of being able to put down long miles without exhausting himself. Of course a good full faced helmet will stop alot of noise too.
Strader Engineering used to make a three-into-one exhaust system. Installation is bolt-on, and should not require carburetor rejetting. They say power will be noticeably increased in the midrange. The set I had on my 75' GT-750 showed no such improvement but they sounded great when you backed off the throttle, and they were at least 25 lbs lighter. Strader himself moved from Simi Valey California to Indiana somewhere and I do not know his address. If someone knows I would be curious to know where he is but as far as a good exhaust system goes there will be a 3 into 3 stainless steel expansion chamber exhaust system availiable in the near future. E-mail for details
Owners of pre-1974 GT750's could replace their exhaust pipes with the later stock exhaust. The later pipes eliminate the crossover tubes, allowing the engine to put out slightly more power in the midrange.
You could also remove the crossover tubes and plug the holes in the pipes with automotive freeze plugs.
Intake flow may be increased by replacing the stock air filter element with a stock-dimensioned filter from K&N.
Some riders feel that performance will be increased by entirely removing the air cleaner assembly, and installing vacuum stacks, air-horns, bellhorns, etc. Its not recommended for street riders, because of the likelihood of sucking damaging debris into the engine.
After milling the head and possibly mild port modification, you could remove the GT380's 24mm carburetor assembly and replaced them with the 28mrn set of Mikunis used stock on the GT550. This modification is an easy bolt-on.
Rejetting is probably not necessary on a modified 380, since the 550's baseline jetting should be ideal on the modified smaller engine.
Main jet sizing on the GT550 carburetors should be 95 on the center cylinder and 97.5 on the outer carburetors.
Performance will remain stock on the low end, gradually improving through the midrange and the upper rpm range.
A mildly modified GT550 may also have its performance increased by replacing the stock carburetors with three 32mm slide-needle Mikunis.
When the GT550 carburetors are used the stock throttle cable must be replaced with a (pre-CV carburetor model) cable from a GT750.
With a mildly modified GT-750 engine, performance may be further increased by replacing the stock carburetors with a set of 34mm Mikunis. On a pre-1974 GT750, the low end performance will remain stock, gradually improving through the midrange and give a significant boost to the top rpm range.
When the Mikunis are used on 1974 and later GT-750's. you will see an even greater performance boost.
The stock CV (constant vacuum) carburetors are economic and reliabile. But there is one major disadvantage they are sluggish in response to engine demands and they offer less than maximum flow. The performance rider may wish to replaced the CV carburetors with the larger slide-needle carburetors. Performance will increase throughout the engine's rpm range giving the most noticeable boost to the top end. Installing these carburetors on pre-1974 GT750,s may drop 2-4 mpg,. On the 74' and later bikes formerly equipped with CV carburetors, mileage may drop as much as 6-8 mpg
It is possible to modify the stock air cleaner assembly connectors on CV-carburetor GT-750's for use with the slide-needle carburetors. But on earlier models it will be necessary to remove the entire air cleaner assembly and use three separate K&N or Filtron air cleaners mounted directly to the carburetor bellmouths.
Stock coils are rated at 10,000-12,000 volts. They should be replaced with, high-output components. An excellent choice are the Lucas high performance coils. In spite of the bad reputation that Lucas components have (primarily caused by the lowcost, low-reliability parts used as stock on most British bikes), these high performance coils will provide significant power improvements and excellent reliability. They are a wire-in replacement, and do not require modification to the ignition circuitry.
A second coil option is to install three 6-volt coils with ballast resistors wired into circuit between the battery and coil, to prevent overdriving and frying the smaller-voltage parts. With these installed, voltage will be increased. However, the electrical current may still be to great and burn the stock points. You might also change the poor OEM ignition condenser with a higher-quality U.S.-built part. Any automotive ignition condenser rated at around 20 microfarads will be ideal.
Those with 1972 or early 1973 crankshaft will do best to modify the crank or get a new, later one. The old style crank can be identified by the thrust washers between the rod on the crank check. Side movement is only .020" or .030". The late style crank has no washers and about rods move up to 1/8" sideways. The a rod is held in place by the wrist pin thrust washers. Give '72 cranks are to be modified, be sure that rods have lubrication holes on the small and enlarged. Also as in any modification of a high-performance engine, if the crankshaft has 3000 or more miles on it, seals should be replaced or the entire crank assembly should be renewed. The balancing factor can be left stock. Our G.P. engine was lightened and re-balanced, but nothing was gained by it. We have, however, found rods to differ as much as 15 grams since Suzuki uses four different manufacturers to make rods. It therefore will become necessary to carry out only this portion of balancing, which is to even out the rod weight by determining the lightest one, and then taking off enough material from the other two to bring them down to the correct weight. Next, checkout crankshaft alignment. New cranks are often as much as .007" off, while only .001" to .002" is acceptable for racing.
If you have installed new crank that was perfectly aligned and your engine vibrates, you may be one of the unfortunate ones with the 15 gram difference in rod weight. If your low mileage crank was smooth, trueing may be all you need. Poor balance along with excessive crankshaft runout can result in a loss of as much as 10bhp, so spend the money to do the job right. When fitting the crank into the cases, locktiting of the bearing support or spreading a thread lock on the supports will help eliminate pounding on the crankcases at high rpm's. When rebuilding and old-style crank, the #3 inner seal must be replaced with a late style #09263-38008 seal. The old seal will have a plate associated with it that should be discarded.
We have tried three different types of carburetors and have had good results with all three. The question becomes one of money. To begin with, if the standard carbs are to be used, they will require extensive modification and can be made to work well. J, K, or L, '72 through '74 carbs only can be used. The '75 and on CV carbs as mentioned earlier are useless, responsive and flow being inadequate. The other choice are VM type Mikuni racing carbs and Lectron carbs. It must be remembered that AMA rules require stock carbs, but can be modified. All other associations have no limits. Begin with, all carbs will be mounted to the barrel with a Mikuni rubber flange VM 30-34 (38 in the case of a G.P. engine). All three types of carbs will fit. Mikuni intake tracks are not straight. A 32mm carbon is actually 30mm in effectiveness, so it is necessary to bore them so they are straight. The should be done by reaming followed by hand honing. Be sure not to exceed 1.25mm (.050"), as further boring would break through some lines and render the carbs useless. Modify as follows: (1) stock carbs bore an modify intake mouth as shown in diagram, 2.5 slide fitted with 6DP1 needles in top or 2nd notch, 159-R-O ? (bad type, posibly 159-R-C) needle jets, 45 to 50 pilot, increase flow needle to 3 or 3.3, adjust floats on the high extreme, air screws from 1 1/2 to 3 turns, made jets vary from 160 to 200 depending on where or when. (2) VM Mikuni's, bore and modify intake mouth as shown, all other components to be the same as stock carbs including removing air jets, if any. Lectron's will be left as they found except that road race carbs are to be specified, 34mm can be used. Needles vary from 5 series to 7 series depending on what the engine requires. To the Lectron is more difficult, but more rewarding. They have shown better performance throughout the entire range with better acceleration, however, cost is also a factor. They are not cheap.
The components that are to be used inside carburetors may vary from engine to engine depending upon usage. Since out engine is used in competition, no air filters are used. Those who drive on the street will find it necessary to fit filters. Dry K&M's are recommended, as large as can be fitted.
The ignition chosen in our engine is a Lucas Rita LR105. It can be used as a total loss, but it is recommended that the charging system be retained in order to keep spark and maximum intensity. The LR105 bolts on to wall GT-750's; timing should be set at 2.5mm instead of 3.0mm as indicated on the LR105 instructions.
Standard points can be used and modified if limited to street and occasional drags, but since ignition plays and important part in the role of horsepower, an electronic system is essential for serious work. Points are subject to wear and cause is timing changes. Springs can lose tension and float points, causing erratic spark and since they are operated at the crankshaft speed and are mounted on the end of the crank, they g subject to excessive wear and crank vibration. All of these symptoms will cause some form of engine failure, from overheating to detonation and finally, seizure. The LR105 is an electronic trigger not affected by any points problem. Furthermore, all three plugs fire at the same time, which keeps them clean. Timing is changed to 2.5mm for racing and develops the widest power band and insurers safe engine operation.
Any form electronic system can be used, and the results will be the same, accurate firing of the plugs. The LR105 also introduces some 30,000 volts or about twice is much as a stock system. The standard spark plug connectors in should be discarded since these have resisters and should be substituted with a racing type or the ones from RM series Suzuki's. For spark plugs we followed the following to be adequate and best performing; Bosch 290S2S, 310S2S; NGK B8EV, B9EV; Autolite AG701, AG 901. Champions were used, however they must be checked carefully for open circuits. KLG FE220 or FE265 were found to be satisfactory, although not as good as the first three.
As for modifying points, although our tests showed constant weakening, it was advised that an automotive heavy-duty condenser be substituted for the standard ones g. that the stock plug gaps be replaced as previously mentioned. One must also constantly watch for timing changes. Better still, it makes no sense to spend considerable money on developing a superior motor only to have it destroyed by points.
hence transmission is inadequate for street use or racing. Although robust, its ratios are totally inadequate, hence our choice of engine modifications. Although desirable, a close ratio gearbox is not essential. Our best inexpensive movie is to change top gear. First, check all gears and shaft forks as indicated in the Suzuki shop manual. 1974 and earlier transmissions can be modified by installing the later fifth gear. The ratio will be changed by exchanging the 24-26 gears for the newer 25-26, #24351-31201 and #24250-31001. This will make the jump from forth to fifth gear smaller and less painful. Lastly, the proper final ratios as indicated in the gear ratio chart should be referred to for proper gearing depending on what the application will be. As for lubrication, the standard 20w-40 recommended is a adequate for all around use. For drag racers, ATF Dextron is better.
The GT-750 has one of the best clutches and in the business, but can be improved upon. The old-style cork plates will show signs of strain. They can be easily replaced by the Barnett clutch plates or by GS-750 plates parts #21441-45000. Heftier Suzuki plates are heftier, the Barnett's grabbier. Either way, they are better than the all fiber plates. The old-style spacer which is supported by a bushing should be replaced with the '75 and later model roller bearing #09263-51001 and #21251-31600. The stock springs should be replaced by Barnett MT-23 springs. Of course all other clutch related parts should be inspected as per the shop manual.
I. Exhaust System
The expansion chambers outlined in the diagrams have been tested to give the best all-around results. They will work with various size carburetors from 32mm through 39mm, will tolerate minor port timing changes and compression changes. With them our engines have developed as much as 109bhp sustained at I. D. rear wheel. To fabricate them we chose .032" chrome moly sheet metal available at aircraft supply houses. Header pipes for made for stock exhaust pipe U-bends of 2.0" O.D., 1 7/8" I.D. welded to Suzuki T. 500 header pipes that are 1 7/8" O.D., 1 3/4" I.D. If the U-bend tubing cannot be found, one can get away with using the T-500 header alone.
Mounting should be made with springs and the use of the exhaust mount as shown in the diagram. The rear or of the chambers can be supported by using Suzuki MX mounts and welding adjoining tabs to the frame. Silencers can be made oil shown or purchased as weld on's from J&R, Skyway, or Torque Eng. We used Caslers and found them to be the best but also loudest. They cannot be used on the street. For the street rider and drag racer, chambers can be routed in the stock configuration, but for road racing this method does not give adequate ground clearance. All three pipes must be mounted under the framing between the floor rails. The sure to allow clearance of frame, chain, tire, whenever.
There is much room for argument here, but we will state what has worked for us. For street use we used Suzuki CCI, Castrol injection oil, R Full Bore injection oil, pump set stock. Drags , Castrol R-30 in the injector, pump set stock. Road racing, Castrol R-30 in the injector, 32:1 in tank and pump set stock. We found synthetics to be extremely poor, showing the most wear and the least heat control. The injection oils for street use were better and Castrol "R", although messy, was the best even after a solid 8 hours of dyno work. Castrol "R" ONLY is recommended for road racing. We have used it for 15 years and have not yet found anything better. One should be cautious against using mix that is more than 12 hours old. It begins to breakdown and reduces the octane rating of the gasoline as well.
K. Cooling System
The Suzuki cooling system needs little attention, but it will need some modification. Remove the water by-pass from case to head. Replaced the water pump if it has seen more than 3000 miles, and also pre-1975 pumps with pump Kit #17400-31830. Use water only with water pump lubricant, about 4 oz. Water pump lubricant can be purchased in any auto supply house. Suitable oil can also be used in the same proportion. Suzuki Bars Leak can also be used if any minute leaks occur, but care should be taken because if repeated often, it may clog the radiator. To save weight, the radiator filler can be welded directly atop the right side of the radiator and all extra plumbing can be removed.
Part II: Chassis
We have now completed the engine, and now are ready to install 100bhp into a chassis designed for 60bhp. We have enough power to reach 150 mph without streamlining, again for which the chassis was not design. In order to make riding a little less frightening without the cost of a sophisticated and expensive chassis, we have made the following modifications to make the standard chassis as liveable and safe as possible. Keep in mind that those who plan to compete in AMA superbike must use a standard frame which can be modified.
On our machine we chose an earlier frame, but all are adequate. The earlier frame needs only one brace located across the front down tubes just above the lower radiator mount. Newer frames already have this. Remove all unnecessary brackets and mounts that will not be used in an effort to decrease weight. Excessive weight full effect handling, acceleration, stability and breaking. Suzuki's have approximately 9 pounds of wire. One-half can be eliminated along with a lot of decorative junk, such as around radiator hoses, for example. For street used use and drags remove the center stand and keep the side stand. For road racing remove both stands and brackets so that chambers will clear. Since the starter has been removed, when necessary we can use a battery from a T250 for other small bike and save 1/2 the weight of the GT-750 battery. The tail lamp can be replaced by an earlier T-20, K10, K11, S32, etc. to save another pound. The headlight can be changed to an accessory 5" type along with clip on brackets. The front fender can be replaced by a fiberglass or aluminum one. (Try Dick's Cycle West). Finally and most important, the steering head bearing should be replaced with tapered rollers. The standard steering bearings of the same as on the MT50 mini-bike and will cause high-speed wobbles due to lack of thrust pressure. Steering head bearings from the GS1000 can be modified to fit. The rigidity of the tapered roller bearings cannot be over emphasized for high-speed use. Of course, check for cracks and signs of stress, although such is unlikely with the GT-750 frame. This 52 lb. frame can shed 10 pounds. The front portion of the rift fender on the late '75 and later models can be replaced with the earlier plastic type to save an additional two pounds. It may seem like nit-picking, but if you save a pound or two here and there, after fifteen spots are covered and analyzed, you may have saved twenty-five pounds.
Suzuki forks are adequate. We use '73 and later forks since these have disc brakes which we will discuss later. The '72 forks can be used with the light weight drum for street and drag racing with little change, but for road racing 1973 and later discs are better. The drum will fade road racing. '76 and later forks with rubber top covers are more difficult to service, but are adequate and can be used. The best forks are '73 and '74 since the dampers are of Ceriani design. Since much weight has been shed at this point, '72 through '74 springs can be changed with comparable springs from the GT-550 of the same year. 1975 and on forks must remain the same. Do not increase spring tension by using spacers since this will cause early spring bind, but instead choose a heavier spring TZ will allow the entire travel of the fork to be used and will permit proper dampening. S&W will supply fork springs. Damper action and spring weight must be carefully chosen. A lot of people make the mistake of stiffening the spring when they should " stiffened the dampening, or vice-versa. Many also attribute fork bottoming to weak springs when the fault is poor dampers, frame mounted fairings and other types of overloading should be considered.
A. GT-750 fork on a stock weight machine has approximately 4" of travel. When the bike is resting on two wheels, up to 2" is lost. As one sits on the bike, another inch is taken up, and when the bike reaches higher speeds, 1/2 inch of heaviness is gained back. This leaves the rider with only 1 1/2" to work with, so at 150 mph, a small ripple will bottom the forks resulting in what is called taken slapper. Lightness is important.
As was indicated before, the frame is only adequate. It was only 26° fork rake to make up for the heaviness of the bike in an attempt to make its steer easier at low speed, but who's going to drive it at 5 mph? The earlier Yamaha TZ-700's and 750's have the same problem. 28° is recommended and better, but to do this we must make extensive modifications that would require much work, expense, and five pages of instructions. It is not worth it. The next best thing is remove all excess weight from the bike, choose the correct spring, (stock is inadequate in most cases), and experiment with oils so that the dampers work properly. Later on we will see what else affects the front end. As for oils, we have try to ATF, and recommended 10-W40, 20W and 30W racing fork oil, and mixtures of 10W and 25% STP, all at the recommended amount. All work for different applications; trial and error is the only way. A last note. When re-filling, the tubes must be entirely emptied and left to empty for at least one hour.
Shock absorbers play an extremely important part in handling and stability. We shall start first by taking the stock units and making book ends out of them. For street and drag racing, we found Konis to be the best. Konis have adjustable dampers and can be ordered with matching springs. Girling shocks have also been tried with excellent results along with Girling adjustable gas shocks. The Konis, however, allow the dampening rate to be adjusted for street and drag. This will be discussed later.
For road racing the founder the best to be Girling gas MX medium dampening shocks with dual springs, not progressive. Before we go further, do not use progressive wound springs. Girling MX shocks have extra travel, approximately 5" and the rate can be controlled from start to finish. With the bite of road race tires along with 88 horses at the real, we need all of the dampening we can get. The object here is to get constant dampening throughout the travel during compression and rebound. This will, of course, help control the front end.
Confused? Here is an example situation. This particular bike has correct springs but the wrong dampers. It accelerates hard out of a corner, the rear ends compressed making it the equivalent of a rigid frame and leaves the front end to do all of the work. The bike is finally and speed and the rear spring shoots the tail end up but the shocks do not dampen the spring reaction causing the front end to drop, losing some of that critical travel. This nose dive also causes the fork angle to change to 25° or 24°, causing the front wheel to want to turn in under the bike. Conditions are now perfect for a speed wobble (commonly called a tank slapper). Now the rear end is bouncing and hopping wildly with 88 horses twisting the frame, and if the rider is lucky, he will get off with only a few bruises, but he most certainly will get off. It is essential that the spring rate and dampening rate are closely matched. This is the main ingredient to stability. The rules of thumb are: adequate and matched travel, front and rear, spring rate to be as soft as possible, (too stiff a spring will override a good shock) , and dampening that is constant throughout the entire travel.
On our bike we use GT-550 springs with ATF, and in the rear we use Girling gas MX shock #(un-readable) and spring #(un-readable) and (un-readable). Ours is used for road racing and has a modified swing arm. If the swing arm is not to be modified, shock #(un-readable) with spring #(un-readable) can be used. The combination with modified swing arm was stable without streamlining at speeds up to 152 mph.
There seems to be a fallacy that cast wheels are better than spoked wheels. First of all, let us examined the development of cast wheels. The reason was for light weight. For this we must use magnesium such as used in aircraft. Magnesium is soft, expensive and light. Magnesium is also weak. Load factors in drag or road racing are much less violent than in street driving as there are few potholes to worry about. Therefore a racer does not need a strong wheel nearly as much as he needs a light one. Weight, as we mentioned earlier, is very important for acceleration in handling. Since we're dealing with unsprung weight (weight located between shock absorbing device and the road), light weight is essential. An average magnesium wheel with bearings and no tire set up for discs weighs around 7 lbs. and cost about $250. A street type "mag" aluminum wheel weighs about 14 lbs. and costs about $120. A stock Suzuki steel wheel weighs about 131 lbs. and costs nothing as the rider already has it. Last but not least, the spoked wheel with an alloy rim weighs about 8 1/2 pounds and costs about $45. This alloy wheel can also be used on the street. Since we are dealing with dynamic load, (load around the circumference), we find that at any measured point a spoked wheel will withstand some 20,000 lbs. of weight. The cast wheel, magnesium or aluminum, will withstand about the same AT THE POINTS OF ATTACHMENT. These cast wheels have an area of some 5" to 7" of empty space with no support. These areas have less than half the strength of the supported areas. On the racetrack one is not faced with the problem, but if a street rider hits a bad pothole or bump and bends his magesium wheel, he will be out $250. An aluminum wheel would cost him $120, an alloy rim about $45, and a steel rim about $28.
For our money, the best all around buy is the aluminum alloy rim on a spoked wheel. We can buy a lot of shock absorbers to control 1 1/2 lb. of unsprung weight and we can certainly afford to check spoked's. On one application or spoked rear wheel weighed 1/2 lb. more than the magnesium wheel at 1/4 the cost. $90 vs. $360. T-ring the rim and checking spokes is far cheaper. So if money is no object, go magnesium. If you are thrifty and wise, you will use aluminum alloy rim's.
In road racing, stopping is at LEAST as important as going like a bat out of hell. Two-stroke engines offer no breaking force, so all of the stopping force (minus body wind resistance) must come from the breaks. As was mentioned earlier, the 1972 drum breaks should be limited to drag racing and street use. The '73 through '77 double discs are more adequate if the bike has been lightened considerably. If you are into road racing, and are an above average rider, even the double discs are inadequate without considerable modification. We will discuss only maximum or road race use. We are attempting to give you the best stopping power for the money.
Breaking also plays a part in unsprung weight, which help suspension. To begin with, do not mount these calipers on the rear of the forks as they were not designed to perform properly when so mounted. Suzuki brake discs weight 8 lbs. each and are useless. They are stainless steel which has a low coefficient of friction, poor heat dissipation, and great weight. Even so, they can be made to work. We do not like them, but we must think of the expenditure level as well as how well they can be made to work, and this is the best combination. To begin with, we will lightened them by drilling as shown in magazines. This will cut weight to about 5 3/4 lbs., and help throw off heat. The standard calipers were found to be in adequate for road racing (they are adequate for everything else). They flex too much under extreme usage and give a spongy, erratic feel. They should be replaced by GS -1000EC calipers and hoses. Fit to the standard junction.
Caution: calipers on the GS-1000 are mounted in the rear, but on the GT-750, they must be mounted in the front. They have been tried bold ways, and they work best in the stock position. GS-1000 calipers are one pound each heavier, but more than two pounds each have been lost on the discs, and stopping power has been improved.
The let us consider the rear brake. To begin with, the street bike in dragger can get along with the stock unit, but the road racer should change to disk in the rear. Stock breaks would require constant maintenance, for example, some sort of cooling must be provided by fitting an air scoop and providing exhaust holes. Brake shoes would have to be replaced often. Stock units are better and more reliable than metallic shoes. The rubber damper may get destroyed, so carry an extra one and last, the extreme heat generated will cause spokes to loosen, so one must constantly check them.
A disk should be fitted. A GS-750 unit can be modified to fit which would probably be the easiest, but the best and the lightest is the one we have made up in our diagram from stock parts. We also use one of the discarded front calipers in order to save more money. This combination of front and rear brake has been used successfully and is by far the best when all factors are considered: weight, stopping power, and money.
This is an extremely touchy subject since all riders have different preferences. We argue among ourselves on this one. We will only discuss our superbike. First, we will have to back up a bit and discuss rim and tire size. Street and drag racers do not have much of a problem. To save money, one can retain the 19" front wheel and replace only the rim. Use of 18" rim will require an expenditure for spokes. The 19" size will work on road racing, but it will severely limit the selection of tires. You should fit the 2.15 or WM-3 rim. In the rear, one should fit at least a 3.0" or WM-5. This combination is good for street and drags. The WM-5 rim will all use of most tires. We recommend a Michelin M45 3.50 x 18" at the front and a Michelin M45 4.25/85v18 at the rear. Michelin PZ2 compound can be used on the street but life is very short (about 500 miles). On our bike we use a WM-3 x 18" rim at the front and a 3.5" WM -6 x 18" at the rear. S41PZ2 and PZ4 slicks have been used at the front, while a PZ4 4.00/5.60x18 has been used on the rear. We are limited to using only road racing slick tires. With 88bhp it becomes a necessity for all out competition, especially the rider is good. Without the wide back rim, the bike will burn a normal tire.
Tubeless tires should be mentioned. We have seen too many accidents in road racing that were a result of their use. We always use tubes, and recommended that all do likewise.
As one increased tire size, rim width, and the resulting bite, handling will become more accurate since traction becomes greater. The more traction, the more likely the flame will flex, and the more critical the suspension will have to be. When our bike was conceived, flexing and bending from horsepower were taken into account. The combinations we have outlined work the best.
D. Swing arm
In an effort to reduce unsprung weight and to increase rigidity, we have modified ours as shown in the diagrams. The stock arm weighs about 21 lbs., ours about 17 lbs.. The 4 lb. lost aids in controlling unsprung weight. Our modification will also aid rigidity. We also found the stock iron and plastic bushing of the stock Suzuki to be inadequate and replace them with Webco #2429 bronze bushes in order to minimize play. On earlier Suzukis '72 and '73, the thread on the swing arm shaft is too short, so when the nut is tightened, only the shaft is tightened, not the swing arm. This can be cured by adding washers to take up the slack, and torque to 45 ft. lbs. The welding around the reinforcement web will have to be ground away to allow for tire clearance. It must then be re-welded.
Here we will discuss some general problems in building the superbike. In porting the barrel we found that a smooth rather than mirror finish is more effective. Old parts to be reused should be carefully inspected and discarded if in doubt. Stock carb cables can be used with stock choke cables on Mikuni stock and VM racing carburetors. Lectron carburetors will require some modification. Those with '75 and later engines will have to modify the oil pump arm to fit the standard early style cable, also a fitting must be added on the crankcase to accommodate the pump cable adjusting screw. Pumps run in fixed positions will consume more oil and tend to load up at low speeds.
Check crankcase support bushes for wear and replace if necessary. Loosen bushes will cause whipping and twitchy handling with premature chain wear.
Extra strength Denselube chain was used to transmite power to the rear wheel. Chain play should not be less than 1 1/2". Denselube's need no oil. Check all sprockets for wear. Use new countershaft sprockets and alloy rear sprockets. Don't use nylon sprockets, as they are not strong enough.
The neutral indicator switch can be replaced by plate #(un-readable) from the TM-400. This will eliminate the possibility of a loose chain wrecking the neutral indicator switch and will also allow the use of a 17 tooth countershaft sprocket. Rear sprockets can be replaced with alloy types and can be purchased from most dealers. This is in keeping with trimming weighed down to reduce unsprung weight. For those who must use the stock fuel tank (AMA superbike rule), it will be necessary to fashion a petcock with ample volume to feed 3 carburetors. We may ours from old petcocks and made one with three outlets. Keep in mind that the stock petcock is vacuum operated and should not be used in a racing engine because it does not flow enough fuel and the vacuum diaphragms could fail if the engine should backfire.
Since our sheen is used in AMA superbike and WERA superbike classes, we had to conform to both association rules. The only difference between the two is that AMA allows for use of total loss ignition and total removal of all starting mechanism. WERA says it wants a fully operative electrical system and an operating starter system (kick or electric), so he chose WERA rules on these points. The AMA wants handlebars at least 3" high to fit stock mounts. WERA allows any bar but it must fit stock mount; AMA says the footrests must be in front of the swing arm pivot, while the WERA says any type of footrest can be used. We chose AMA on this point. In short we build the machine to comply with both sets of rules. These modifications will also comply with most drag rules.
One note on the footrest. We chose AMA rules. We use stock petals, however, the footrests were modified to accept folding types and relocated some 3" higher to accommodate cornering. We also made them as narrow is possible.
The cooling system of the GT-750 is over design. It is too good. Although we left the radiator alone, size wise, on our superbike, we did cut down on our G.P. machine. Operating temperature of the engine must be around 185° to 190°, but the system will keep it at 180° to 185° so we upped the thermostat. Next season we will make the radiator as small as it is on the G.P. bike, thus losing weight and providing proper engine temperature. We will then go back to the 180° thermostat. Although a thermostat may be labeled to open at, say, 180°, it may vary to such an extent that may open as early as 176° or as late as 189°, so if you have the same error at 185°, your high level may be 194° bringing you closer to disaster should the thermostat fail, and sometimes they do.
At this point you will have completed your rocket and should have your 545 lb. buffalo down to 397 lbs., 60 crankshaft hp up to 105 hp, and streetable. Major engine modifications can be used to boost power to exceed 135 hp, but remember that this will require more exotic ignition, transmission, carburetor and more internal lightening along with extensive chassis work. If you wish to pursue it, contact us for details. As of now you have built an extremely rapid and competitive motorcycle. The machine should be capable of top speeds of as high as 152 mph (honest ones), 1/4 mile speeds of high 10's, and it should be competitive in at least four classes combined in AMA WERA and two or three drag classes not to mention a real rocketship on the street that is comparable to nothing now available. The Water Buffalo lives!