Superbike mag: Supertune Suzuki GSX-R750

[Jelly]

I was working my way back through the archives of OSS and stumbled across this info... Thought I posted/pasted it. 

Some info from Superbike Magazine 1995. 

Supertune Suzuki GSX-R750

I hate you all. You buy millions of GSX-R750s and cause me and three GSX-R tuners (PDQ Developments, V&M Racing and Specials) massive headaches. All the Stage 1 tuning tips for all the GSX-R models. Bastards, bastards, bastards...!

The GSX-R's got a choosy, fussy motor that 'll kick out 10 more bhp with one mod and lose 10bhp with another. Most guilty are the carburettors, from the 29mm flatslides it started life with to the gargantuan 38mm jobs it uses today. Whip off the airbox or stick an exhaust on by itslef and you'll muck everything up... maybe. It's all different on each of the models, so read the section which applies to you and act on it.

Company: PDQ Developments

Contact: Larry Webb
Where: Slough, Berkshire, GB
Phone: (01628) 667644

1985-1987: F,G & H models

"The carbs will be fucked if you buy one of these, guaranteed. In fact, check that the engine is basically sound on these older models before doing any tuning. For the early GSX-R750s, use 750J/K carbs from a breaker. They're 36mm and the best GSX-R carbs ever made."

"Whether you're using the original 29mm carbs or a set of 36mm CVs, run a Stage 3 Dynojet kit with individual twin filters. You have to go to Stage 3 with the bigger carbs anyway because the venturis are too big for the old airbox."

"Early GSX-Rs respond very well to a decent quality pipe. You want a 4-2-1 system, and Micron still does a fitment. Hindle also does a full system which works very well, but it's mild steel so needs to be looked after. You could even try retro-fitting the original exhaust system from a 750L with a slip-on can and that should work well when set up properly. All of the GSX-Rs need dyno time to make them work well. You can just bolt stuff on, but it'll never give the best results."

"With the pipe on and the Stage 3 jet kit with 36mm carbs, you're looking at a 10-12% power increase (94bhp) at peak, and a much livelier mid-range. All GSX-Rs eat coils, so on early ones they definitely need checking or replacing. It's also the only bike ever made that needed an ignition advancer. The standard ignition is pretty woolly and an adjustable Simms & Rohm item will fill in the mid-range and give you better throttle response right across the rev-range."

"Handling? 750J rear wheel goes straight in, as will the front end from any of the GSX-R series. The steering bearings are identical on all models, so there's no reason why you shouldn't stick in upside-downers if you like. The standard 750F/G wheels are so weak that you can flex the spokes with your hand. Best to change them, I think. A 17mm master cylinder (1000 Exup) for the brakes will transform the standard brakes and use SBS-RQ pads for maximum breaking."

1988-1989: J & K models

"Nothing wrong with standard carbs on this model. Keep the airbox: it's the air filter that's the problem. The working filter area is too small, so stick in a K&N replacement. This will flow more air and is fully washable. Use it in conjunction with a stage 1 Dynojet kit for the carbies. If your header pipes are in good order, there's no reason why you can't stick on a pair of carbon cans to the standard exhaust. The look good, cost loads and will take a bit of setting up on the dyno, so you're probably better go for a Skorpion or Sebring system. The Sebrings are good middle-range exhausts which don't mess up the power curve. Yoshimura, especially the Duplex range, work well on GSX-Rs, but they're expensive. I'd stick a Barnett kevlar clutch in there if the standard clutch is getting tired. If a rider wants to bring us his GSX-R and a donor motor, we could do a 750/1100 engine swap in a week, time allowing."

"Oil-cooled GSX-Rs make a lot of heat and they don't like it. Some of the mega-tuned GSX-Rs have bigger head coolers than the main radiator. Good synthetic oil is a must - Motul or Gemini. The 750Js and Ks were top handlers - get Ron Williams at Maxton (0928-74031) to advise on the suspension if it's getting tired."

1990-1991: L & M models

"These need more thought to the carburation. They've got fucking huge carburettors - too big and incredibly sensitive. Give it serious thought if you're thinking of removing the airbox, because getting rid of the 5-6,000 rpm flatspot can be very difficult indeed. Go for a Stage 1 Dynojet kit. Slide speed is all important in these later carbs, but go with the smallest holes in the slides. If you drill them out to their biggest, it makes the flatspot even worse."

"For exhausts, fit Skorpion, Termignoni or Yoshimura systems. If you've taken the carburation to Stage 1 only, you can get away with a slightly cheaper exhaust, like a Micron. There are good gains to be had at the top end with a Stage 3 induction kit and it'll take you over 100bhp easily. The trade-off is with the mid-range and the flatspot. If you rev it everywhere, go for it."

"From a handling point of view, these models were the best. The steering dampers were crap, though. Too heavily damped, too crude. Change'em for a Sprint or Öhlins system. You need to check steering head bearings, because the bottom race absorbs water and rots."

1992-1995: WN, WP & WR models

"Much the same as the L and M models, just with a waterjacket. For the carburation, there's Stage 1 Dynojet kits only, no Stage 3 available. We've had the airboxes off and we've tried but there's no gains to be had with individual filters. Same carbs, so same deal. They're very sensitive to changes. The header pipes on these bikes are pretty good, so use them and stick a Johnson or Quill can on the back."

Company: V&M Racing

Contact: Mark Hindle
Where: Rochdale, GB
Phone: (01706) 840066

1985-1987: F,G & H models

"The 29mm flatslides on the early models were good little carbs but, more than likely, they'll be goosed by now. Either overhaul them or stick on the carbs from a later GSX-R: the 36mm carbs from the 750J will probably fit. Whip off the airbox, install a Stage 3 Dynojet kit and fit individual K&N filters. It's not worth messing about with a Stage 1 jet kit."

"For the exhaust, get a full Cobra system. We've always dealt with these and they've consistently given good results. They're even semi road legal and, used with the Stage 3 induction kit, will guarantee a 12-15% power increase at peak(95bhp). To get 100bhp on the early bikes would take a serious amount of money - around £1,200 - so for most people it's not worth it."

"We do drop-in cams which increase mid-range power by some 6%, but you'll need to let us dial them in properly for maximum benefit. After the Stage 3 induction tuning and exhaust, a Vance & Hines ignition advancer will crisp the acceleration and give you maybe 1bhp in the mid-range. If you really want a healthy mid-range kick, fit a 771cc Wiseco big-bore kit. The bigger bore and higher compression ratio works really well on the early GSX-Rs. Fit a set of Dyna coils, because the originals will almost certainly be knackered and you'll cure all sorts of ills with the new coils alone."

1988-1989: J & K models

"Never was the best thing to tune. Great bike out of the box, but the short stroke motor just didn't work so well. The motor's very peaky, so you'll get better power increases with a Stage 3 Dynojet kit and individual K&N airfilters. The 750J doesn't like aftermarket road legal exhausts, and you've got to go for a full-race item. Again, we'd say use either a Yoshimura or Cobra system - they give decent results on this motor."

"You can cheat a bit and just stick a full-race system on by itself with no jetting changes. If the bike's well sorted, you could get a 10% power increase just with a pipe alone. Earlier GSX-Rs always tended to run a bit rich, anyway. Don't stick open cans on the standard 4-2 system, it'll cause serious problems. We were working with Heron Suzuki on the 1989 J-racers for Whitham and it took us an age to get them right."

1990-1991: L & M models

"If ever the GSX-R peaked, it was here. Everything works well together - 38mm carbs, much better head design, slightly higher compression. It's been properly tuned by the factory from the previous model. Stage 1 tuning is easy: a Yoshimura Duplex exhaust, Stage 3 Dynojet kit and individual K&N air filters. We've seen 116bhp from Ls and Ms with those mods alone: brilliant!"

"There's nothing actually wrong with the airbox design and, if you want to keep it for road use, stick a Stage 1 jet kit and replacement K&N filter in and you'll see 105bhp (3bhp more than a stock 1995 model). Either a Yoshimura or Cobra race can, used in conjunction with a Stage 3 jet kit, will give you an 8% power increase. Bung on a can by itself and that figure is still going to be around 6%."

"If you want big, big power gains, fit a full Yoshimura exhaust and a set of 38m,m Mikuni flatslides. these carbs just pump so much air and fuel into the engine that a 24bhp power increase at peak is totally possible. Again, fit a 5 degree ignition advancer to crispen the acceleration, and our drop-in cams should add another 6% to that power increase."

1992-1995: WN, WP & WR models

"Stage 1 Dynojet kit in the airbox, K&N replacement filter, Yoshimura slip-on can. That'll give you a guaranteed 110bhp with more to come, no trouble at all. Skorpion does a system for the water-cooled GSX-Rs and the results I've seen from them are excellent, Worth getting if you can afford it."

"These bikes are pretty peaky, so you don't need an ignition advancer as with the other models. They run much leaner than earlier GSX-Rs, so an open exhaust by itself is a bad idea. Run the engine on four-star - that goes for all the GSX-R series - but other than that, most of the tuning mods are the same as the M model."

"For handling, Pirelli Dragons are the tyres to go for every time. Try Ron Williams at Maxton for suspension problems. Anything he sells you is worth it."

Company: Specials (Glasgow)

Contact: Geraint Short
Where: Scotland
Phone: (0141) 954 4225

1985-1987: F,G & H models

"Stage 1 Dynojet is redundant on these models, because the airbox has to go if you want more power. It's too restrictive. Fit a Stage 3 jet kit and individual K&N filters. The carbs aren't too bad - work well on GS750s, actually!"

"For exhausts, it's pot luck on the early GSX-Rs. Some give good results, some don't. Stick with American race pipes and stay away from european systems as a rule of thumb. Vance & Hines and D&D systems give the best results we've seen."

"In the engine, make sure the valve clearances are spot-on. GSX-Rs are very susceptible to over-tight valves and it puts two power glitches in the curve if they are."

"Ignition advancers work well on these bikes. The standard ignition puts a glitch in at 6,000rpm and, combined with flatslide carbs and an open pipe, it can turn into an almighty flatspot. A 4º advancer will cure that. New coils can be good for 2bhp, and we'll stick 8-10bhp on to peak power with these Stage 1 modifications. There will be no increase in mid-range, though, because of the carbs."

1988-1989: J & K models

"These are much harder to set up, but I prefer the short-stroke motor. You can't get power without revs, and this motor really revs! Blueprinting is one of the safest ways of getting more power without stressing anything, and it'll tranform one of these earlier engines."

"With a Stage 3 Dynojet kit, you run the risk of compromising low-down (0-5,000 rpm) power. It gives good top-end gains, but the better the full-race system you have (Yoshimura, D&D), the more top-endy the bike will be. By removing the airbox, you then make that even more revvy. Balls-out people won't care, but those who want low-end power should fit a Stage 1 kit and a more road-orientated pipe, like a Micron. Fit an ignition advancer on the J, but not the K. We've had 108 bhp from a K fitted with just a Stage 3 jetkit and a full D&D race pipe."

1990-1991: L & M models

"Don't bother with ignition advancer on these - a Stage 3 induction kit is way to go. Same rules as on the J & K, but Stage 3 on these works with fewer problems and gives even better gains. Use it with a Yoshimura, D&D or Micron full system for maximum peak horsepower, or a Dynalite or Quill can if you don't want a full exhaust. But you're not going to have quite as much at peak with a can as with a full system."

"Ls and Ms respond well to a pipe alone with no carburation changes. They'll hold 9bhp over stock at peak, but the mid-range loses 3bhp in two places (where the over-tight valve timing makes a difference). GSX-Rs must have synthetic oil which makes it run 4 degree lower, and that will produce a little more power. Use carburettor heat shields to keep induction charge cool."

1992-1995: WN, WP & WR models

"There isn't a Stage 3 kit available for these. Stage 1 works well, but the main jet size is critical. It's a leaner burning engine than before, and lowdown response from 2,000 rpm is affected by the main jet. The thing is, don't go for too big a main jet: it needs to be in the 120-ish mark, otherwise it's just too damn big. We end up with 118-120s."

"After that, fit a K&N replacement filter which flows 20% more air and an open can. Again, Dynalite and Quill do the best in our opinion and give good power gains. I've had 114bhp from a WN just with a Stage 1 kit and an open can."