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Discussion Starter · #1 ·
1990 gsxr 750

FYI i just installed my factory 3.0 jet kit. I have K&Ns and a yosh slip on

After finally fixing my bike its finally running the way it should be, but only when its cold.

Here is what's going on.

at cold temps the bike screams through all gears no hesitation just runs straight through to redline. Once the bike gets to normal operating temps its a whole nother story.

Senerio: get on highway after a few miles of riding. bike pulls nice and hard to get to highway speeds. once i start cruising 75mph 6500-7000rpms the bike starts to spudder and then becomes almost unridable. sometimes it will clear up for just a moment and then start to surge kinda like its going to act normal, but then hesitates. I've also experienced what seemed to be a loss of atleast 2 cylinders when this happens. the bike will also stall out if the clutch is disengaged. this happens in ALL GEARS at ANY RPM once i've stayed at a consistant RPM like i stated above. Currently im running the FactoryPro recommended settings w/ a 150 main jet.

When i pulled over I reved the snot out of the bike i was getting small flames out of the canister and then it cleared out the problem just for a few minutes until i returned to the highway.

After going over Factorys Carb Tuning Guide i have figured that i need to change the Main Jet down to a smaller size and Lower the needle Clip one clip to start.

Am i going in the right direction?

any Information, Advice would really help me out at this point.

 

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it's tough to diagnose over the internet, but if the problem is directly related to cold/hot, and it runs really well cold, and like absolute snot when hot, then it's *probly* an ignition-coil on the way south. Could be both of them. Mine did this exact thing to me right after I installed the jet-kit/pipe on it and I chased my tail for a week before I finally gave up on the carb-tuning and looked at the coils.

If carburetion-changes don't seem to have any effect, then the coils are the likely suspect.
 

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Discussion Starter · #3 ·
coils huh....

the coils were just changed last season and as far as i can tell they all give off a good spark. I'll have to check them over with a multi meter according to the inspection guide in the manual.

anything else?
 

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Justmanson, My 86 750 does the EXACT same thing! I'm not sure what it is, but it only does it when it the bike gets really hot. The coils do sit right above the cylinder head, so maybe they don't function as well when extremely hot???
 

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put the bike on a dyno...not an inertia system(dyno jet)...I mean a real dyno with road load simulation. At least for diagnosing the problem....
 

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Discussion Starter · #6 ·
well i know the coils are sitting right above the motor, but if they are acting correctly then how could this be related?

also i don't have access to a bike dyno at the moment.
 

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Does this problem only happen at highway speeds? If you are just cruising around town, does your bike act up when hot? If your bike only acts up at highway speeds your bike might be fuel starved. Make sure all tank vent lines and fuel lines are not pinched.

If fuel starvation is not your problem, you then need to rule out if this is a heat induced electrical failure (i.e. heat causing coils or other electrical parts to fail). To do this, start up your bike cold and hit your coils and ignition box with a heat gun or hair dryer to heat them up. If this causes your bike to run rough, then it's an electrical issue. Alternatively, you can measure the primary/secondary resistance of the coils both when cold and hot and see if they are within spec. Also, check your pick up coils as well.

If this is not an electrical issue, then it's probably a jetting issue. Typically, when a bike runs well cold but rough when hot means the jetting is too rich.

Good luck.
 

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Good point. The stock fuel lines are so flimsy nearly anything will make 'em kink:( I recently switched to using some high pressure fuel injection line.......no way I'm kinking that, and it fits nice and tight on the petcock and carb:)
 

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The coil problem that Fastcat beat me to suggesting only happens when they heat up. The heat causes them to expand and open the secondary windings. Then, no spark. The second scenario that leaps to mind is the starvation that MackJack proposed. Could be related to a blocked vent not allowing fuel to flow. Go for a ride and loosen the gas cap if there is no chance of fuel blowing out. Good luck. Troy341
 

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i run clear gas lines so I can see whats in them .!!
 

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Thanks for the explaination 341.

Is there a chart somewhere that shows all the coil part numbers that we can use for our bikes? A couple days ago I saw a post on here where a guy used cbr1000 coils on his gsxr. What other bikes work for us?
 

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Discussion Starter · #12 ·
update....

I took apart the carbs and changed out the Main Jet to a 147.5 which was included with my FactoryPro kit.

I then moved the C-Clip on the needle one notch to lean out the bike.

For good measure i changed the spark plugs as well they looked like they needed to be changed.

I rode the bike today for about an hour and its purring like a kitten. once at full operating temp everything is still great.

ITS ABOUT DAM TIME! lol i've been waiting to ride for 2 months now....
:angry:

FYI my carbs have been synced and are near perfect according to the tool my mechanic used. the carbs were also cleaned and a new jet kit was installed. my carbs were setup to FactoryPro reccommended settings and this was the only change i've made to them so far. I'm hoping for a bright sunny day to pop its head out of this week.
 

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If yours does what mine did last fall, it will run pretty good after all your carb changes, then one day, out of the blue it will fall back into the exact same thing again. After chasing carburation for a long time, I realized it didn't make sense for the carburator tuning to just "go off" all at once, so I started looking at electrical stuff. Trimmed the ends of the (stock) plug wires, re-screwed the caps back on and that worked - for about a month, then same thing again. Deciding the wires themselves were the problem, I replaced the coils and wires with Dynas I got from A.P.E. Problem solved. For about a month. Sporadic and infuriating, the only constant was that it only happened after the bike was very warm. I started to suspect the signal generator might be bad, so I tested it in accordance with the manual. Didn't get any reading close to what it said I should get, but the bike would still run, so I figured I'd clean the connections and that worked. For about a month. So I called Rohm Performance Machine and asked if it was possible for the bike to run if the signal generator was bad. The answer is "yes". They'll run until the signal generator gets heat saturated, then it'll start producing wacky voltages and the bike will spit, sputter, pop, run on 2 or 3 cylinders, and belch flames out the pipe. So there you have it. The signal generator on your 90 and my 91 750s is common to just about any oil-cooled motor, save the 1052s and short-stroke 750s.

That post may have been long, but at least it was boring.
 
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