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Discussion Starter · #1 · (Edited)
Ok guys/Gals heres the issue, just aquired a 97 FZR600r with low mile but has not run in 6 years for about 300 bucks, completely stock except for the v&h exhaust.(Its going to need new bodywork). I got the bike home and it would fire up and run but when the bike was put into first gear and the clutch was let out the bike would stumble from about 2000 rpm to about 4500 rpm like it wants to die out, then she takes off like a bat out of hell. if you slowe down the bike wants to stumble out. The following day i went to start the bike but it wouldnt start, just backfire, so i pulled the carbs off and looked at all the diaphragms, carb 2 had a rather large hole in it, ordered a new for about 175.00 put that in and she fires right up. I then figure that since its sat for about 6 year that it could use a good carb cleaning. so after cleaning out the carbs and everything I reset the air idle screws to where they were when i got the bike 4.5 turns out (factory is 3, I cant figure that one out) put the carbs back in and it fires up but still has the same symptoms of bogging/stumbling up till 4500 rpm or so, ive checked all the jets (they are stock)and ive checked the float hights, they are all the same (but i dont know what stock hight is). I just cant for the life of me figure out what the hell is wrong. Does anybody have any ideas where i should start looking or how to fix this issue? Keep in mind that the carbs are good and clean and that the bike idles just fine. and rev's just fine in neutral and when in gear. Thanks for the help.

drink scotch whisky...
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couple of things


This sounds like a tough one but here are some things I did when I had some stumbling issues with my '98:

- Check spark plugs. Make sure they are clean and gapped to .030". I noticed that some of the CR8E plugs I bought came gapped too small (like .027") and while fine high in the rev range they would not fire cleanly at low rpm (possibly due to not enough cylinder mixing, just a wild guess). If you track the bike, live in a hot area, or use anything but premium fuel then I would go one heat range colder and get CR9E plugs to reduce the chance of knock. To be sure what is going on in the cylinder, clean all the plugs and ride around for 30min as you normally would. If they are black then go to the next step about A/F ratio adjustment. If they look grey, go to the next step about A/F ratio adjustment. If they look perfect, I can't help you.

- Check your A/F ratio. You mentioned you set the "air idle screws" to 4 turns out. These are the mixture screws that adjust rich/lean, not idle. The idle screw is on the left side of the bike behind the choke lever and that is what sets the throttle stop, effectively raising or lowering your idle. You set your mixture screws way out at 4 turns which means you are probably really rich, hence the stumbling at low rpm. How do you check? Fire the bike up and let it idle till you get some heat in the motor and the temp gauge is registering anything. Set the idle screw (throttle stop on the left of the bike) down to 900rpm. Blip the throttle up to 5k or so and quickly let it go. If it slowly lowers the rpms and hangs above the 900rpm idle you set (say hangs at 1800rpm for 2 or 3 sec before settling back down to 900rpm) then you are too lean. If the rpms lower quickly as they should but drops below your 900rpm idle setpoint (say quickly drops from 5k to 600rpm) and stumbles for a second till it pulls back up to your 900rpm idle setpoint, then its too rich. My money is on too rich at 4 turns out on the mixture screws. If you have stock jet sizes and you are rich per the process above, you should be closer to 3 turns out. Adjust the mixture screws 1/2 turn at a time and repeat. Even if it revs clean in neutral, your rich condition could only show itself under load as you ride it. Another note, once you are done with this process you should reset the idle screw back to about 1200rpm.

- Once you make sure the mixture screws are set right you should sync the throttle bodies. This would not point to your stumbling at a specific rpm, but once you solve it you should sync the carbs to make sure your mixture adjustments didnt throw the vacuum differential off.

I would start there. If you check the mixture, clean and read your plugs, that may not solve things but will at least rule out a couple things. Good luck man!

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