Erratic Engine Idle on ZXR 750

[Tnate]

Exactly what the subject heading says. I have a 1994 ZXR 750, and the engine idle is very temperamental. When the engine is first started, the idle is fine. When the engine has warmed up, the idle hangs at around 2500rpm plus or minus 500rpm. When the thermo switch clicks the fan on, the idle returns to its normal 1500 position (even though the temp doesn't drop), but as soon as the fan goes off, the revs jump straight back up.

Any ideas what could be causing this erratic activity? Is it just a matter of getting the carbs tuned, or is there a bigger/smaller underlying problem?

Thanks for the info
Tnate

[mike-s]

does the idle change when you put the lights /highbeam on?

if so it could be a problem with the electrical cct? just at a guess as it's a bizzare problem.

[Smitty]

strange one
but has to be either carbs or electrics
I would get the carbs looked at first
and
then if nothing, get the ignition chased down by
a workshop or autosparkie

hth

[Bionic]

The idle will drop when the thermo kicks in because its making the alternator work harder.2500rpm I recon is a bit high.

[ZZRCHIKKY]

get a new bike !!!!

[We Be Toyz]

How does the bike start when hot - do you have to crack the throttle some in order to get it to fire?

I had a similiar experience with my '94 - only my idle didn't go quite as high, and I found that my problem was the idle jets were too rich. I set them (remember the idle mixture is fuel rich since the jets are in front of the carbs) to about 2 turns out, synch'd up the carbs and it has been fine since.

I'm running a jet kit & Hindle full header system with otherwise a stock motor. If you are running something more extreme, you might end up at 1-3/4 out, or if you are stock maybe 2-1/4. Tuning carbs at times seems to be more a Saturday of fun rather than an exact science.

[Strika]

Older ZXR's can suffer from worn carb slides, which cause the slides to hang up a little and fast idle it until they either have enough vacuum to pull the back down (like when the alternator cuts in) or enough load to slow the revs to the point where the slide will drop back down.

Also it would pay to check throttle cable stiction.

These are items that are easily seen to and are not going to cost money if you can pull the carbs off yourself. Otherwise if it is electrical, it off the the auto elecs unless you're electrically talented!