Building a Water Buffalo Rocket
These modifications of the Suzuki GT-750 water-cooled two-stroke represent 5 years of research and development, and cover numerous engine and chassis modifications. It must be remembered that in order to acquire speed and handling, all facets must be examined. Some may choose not to completely modify their bike and therefore we cannot guarantee total performance. Careful assembly and good judgment will assure that your machine will perform as well as ours.
It will be necessary to obtain a Suzuki GT-750 shop manual in order to insure proper assembly and proper torque settings. These dimensions will remain the same as on the standard machine unless otherwise instructed. All of the modifications have been track and dynomometer tested and should give you the same results.
Again we emphasize careful assembly and accurate modification.
PART 1: Engine
There are three different types of barrels made for the GT-750. The 1972 J with flange mounted carburetors, the late J thru L, 1972 through 1974, spigot mounted, and M through B, 1975 through 1977 spigot barrel with higher exhaust timing. The early ’72 flange mounted is the easiest of the lot to modify. The others require some welding and machining.
The ’76 and ’77 barrels are also favorable since they have more metal around the water jacket and are therefore least likely to be damaged if an error is made.
Alignment, welding and machining should be done first and porting last. (SEE BARREL DIAGRAMS) The porting scheme has slightly more timing than the factory road racers of several years ago, and is the best combination for all around performance. The modifications we will be dealing with will result in 88bhp at the rear wheel and 105 at the crankshaft. Further modifications will only entail more work on the intake port and carburetor size (from 32mm to 38mm).
The 38mm modifications will also require the use of a close ratio gearbox and a more sophisticated ignition system. All other modifications for the engine are the same. The 38mm modifications with Lectron carburetors have given up to 138hp at the crank shaft and 109bhp sustained at the rear wheel.
Porting should follow diagram height in with. We shall begin with the exhaust port. Finished height of 32mm (1 1/4) should be measured with the position at B.D.C. Caution must be exercised when reaching maximum height so that one does not break through the water jacket (see barrel diagram). This problem can best be avoided by raising the port at about a 30° angle.
The width of port is limited somewhat by the battle holes, but primarily by ring flutter. The maximum width that will avoid premature ring wear, is 52mm. We recommend a 50mm oval with a good chamfer around the edges. The volume of the port should be increased until the other edge.
As a bit of caution, be certain to leave enough shoulder on the exit side of the port so the stock exhaust gasket has sufficient support.
Transfer ports are to be raised to a height of 16mm, and are to be well chamfer. Accuracy cannot be stressed enough and this point. The new top of the port must be flat across and at 90° with the bore of the cylinder.
Make certain that whoever does the transfer ports is very, very good board is very good and has a machining setup that will mechanically and rigidly cut the roof of the port. It is essential that the difference between the heights of the transfer ports and the exhaust port be within the range out of 16 ±.5 mm.
There can be no variation from this narrow range because the expansion chambers are tuned to this split and any deviation from port heights will alter the effective tuning speed of the engine, either cutting power slightly with a great increase in peakiness if the split is narrow, or cutting powder considerably but with greater tractability in the split is increased. Do it right the first time.
The intake port should also be sized as in the porting diagram. Widening of the port is not necessary. If it is widened by enough to be significant, the piston skirt will no longer be wide enough to seal off the port.
The result would be increased blowback and a loss of bottom-end the power. The floor of the port should be lowered until the port is 30mm high. The edges of the piston should be chamfer to improve flow.
For AMA superbike, the volume of the intake ports will receive little increase because stock carburetors will have to be used. Be careful not to take meat out of the sides of the port to such an extent that the port exceeds the width of the piston skirt, as mentioned previously. This is very important.
Also, be certain that the correct piston is fitted to the correct cylinder, or the transfer ports passageways will be shrouded, blowback will be greatly increased, and power will drop by at least one-third. The window rib will be retained but its length should be shortened to 3/4 to 7/8 and blended to the cylinder liner. The intake opening should be matched to the carburetor mounting spigots.
For maximal output, the port should be the same as described above at the piston. The volume should be opened up considerably going out to a 38mm opening at the carburetor mount. The back of the cylinder block where the carbs are attached must be milled until it is flush with the back of the block.
Then it should be milled again in order to tilt the big carburetors up about 15° and out of the way of the top of the crankcase. With the early ’72 flanged block this is a simple operation. With a later spigoted blocks, the spigots must be cut off and the face that is left must be filled in with aluminum welding. Then milling can be done and the new, tilted face can be tapped to accept a set of 38mm Mikuni mounted manifolds.
These are available through most Suzuki dealers from Rocky Cycle Supply. Match the Manifolds to the cylinder block. Either 38mm Mikuni or Lectron carburetors will fit the rubber manifolds without modifications. With some modifications, a set of 40mm Dellortos can be mounted. We have never tried them, but we may future.
Back to the AMA superbike. The carb mount spigot is to be fitted as shown in the appropriate diagram. It must be fitted as closely as possible and must clear the clutch hump of the crankcase. The G.P. bike avoids this by having its face tilted up some 15°.
If the bike is to compete in AMA superbike, the standard carbs must be used and modified. If the bike is to be street ridden or is to compete in event to other than G.P. or drags, 32 or 34mm Mikuni VM racing carburetors or 34mm Lectron’s can be fitted. Our machine has 32mm stock carbs (72 J thru 74 L model only). CV carbs 1975 and on are totally useless. Carbs will be discussed later.
After all welding, porting, boring if necessary is done, the barrel is to be faced off to make sure that all surfaces are straight. The bottom first. 005 to .010 and then the top. 020 to .030.
Barrel’s may very from the factory, so be sure that when the top is faced off that not too much metal is removed. When the Pistons are fitted they should have at least .020 clearance to the top of the cylinder block. Finally, all port edges are to be chamfered at .020 -.030 radius to avoid ring damage. All SRIS tubes must be removed.
The intake ports are to remain free from obstruction, so remove the brass tubes. The transfer tubes are to be removed and plugged. We used 1/2-20×1/2 set screws.
The barrel is now finished and ready for its Pistons.
It is advised that ’73 on pistons be used. If yours are within tolerance, they will need to be modified. If new, the same modifications will apply. The ’76 and ‘ 77 piston are thicker and heavier and will withstand more heat, so they are favorable. Modifying ’72 pistons has proven to be somewhat tricky and since the engine will have to be dismantled anyway, the ’72 crankshaft should be modified to accept later pistons.
The ’72 crankshaft in standard form will not withstand high rpm. Stock street Suzuki’s in 1972 had the same problems and were later modified. 1973 and on are acceptable.
The piston skirts are to be cut 3.0mm as shown in the appropriate diagram, and the entire piston should be micro shot-peened or glass beaded. This will eliminate surface fracture and hold oil, reduced friction and increase cooling. Piston clearance should be .0005 over stock when fitted for racing. Honing of the bores is recommended, and standard rings may be used.
It is and a blind recommended that only the top ring the new. The old one may be left in the bottom groove since only the top one does any work. Do not leave the bottom groove empty.
If using new pistons, old rings of the same size can be used on the second ring land. The pistons are now ready.
B. Cylinder head
The cylinder head should be left totally stock. Raising compression ratios to beyond sanity has caused more problems among two-stroke tuners then any other modification. Among these problems are detonation leading to holed pistons, blown gaskets. and warped heads. Remember we are dealing with a stock engine whose actual compression ratio is 11.5-1 uncorrected, and we have already removed .030 to .040 from the block.
You will need to use the late style head gasket part #11121 .31600 which will give about 6.1-1 corrected. Any effort to use a higher compression ratio for sustained running will result in a loss of power due to overheating which results in a less dense fuel mixture. The previously mentioned problems usually follow overheating.
Cylinder head bolts can be modified as shown in a diagram to help in sealing and in weight reduction (1 3/4 lbs. removed). Torque on larger bolts should be 35 ft. lb. and on small bolts, standard as described in the GT-750 manual. The bottom water bypass below the thermostat is to be blocked off since it is unnecessary.
An automotive (Chrysler Corp.) thermostat of 180° is used. It’s outer diameter is to be cut down to fit the Suzuki housing. (Suzuki O.D. is 52mm Chrysler O.D. is 53mm). The head is ready.
The crankcases need little modification, except for 1972. Water bypass hole between cases does not have O-rings as in later models, so care must be taken when fitting the engine. SRIS valve in front of lower cases are to be removed and sealed. Remove the starter, the water bypass should be sealed off, as should be all unnecessary covers and bolts.
It will also be necessary to fabricator a plug for the starter mounting hole. The sure to clean all mating surfaces and used Suzuki No. 4 sealer when closing the cases.
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 cheek. 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. This 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 KM’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 move 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 JR, 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. SW 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 4.00/5.60×18 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.
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