extremely high idle issues

I have been searching these forums for weeks now and have not found anything to help with my specific idle issue. I have an '81 xj550 maxim that idles between 3.5 and 4.5 thousand RPM once the engine has warmed up. Until the bike is at operating temp it idles great. I have tested for vacuum leaks and to the best of my ability have found none. The voltage is low coming out of the CDI and at the pulse coils but I'm not sure if this could cause this specific problem. I plan on replacing these parts as soon as possible, and sooner if it is confirmed that replacement will solve the issue. Any ideas? I am not sure what other information you might need and I am more than happy to provide. Thanks for any help regarding this.

 

You're most likely seriously out of sync or have improperly set floats.

TCI and coil issues won't cause a high idle. Replacing those won't "fix" anything; if the bike runs on all 4 cylinders doubtful either of those is at all problematic.

You need to check and adjust the valve clearances; and properly service and adjust the carbs.

But start by backing off the main idle knob once the bike is warmed up. Then you'll need the choke to cold start it and it WON'T idle (without the choke) until it has warmed up.

 

35-Hundred to 4-Grand is NOT Idle.
To make that power takes MORE Fuel than the Pilot Circuit can provide.

AIR and Main Jet Supplied Fuel is feeding the Plant.

When you get all the prelims set, ... Get the Bike to IDLE with the Throttles Closed or as close to closed as you can begin with by Bench-syncing with:
• 3X5 Card
• Business Card strip
• Resume' Paper
• 35mm Film Strip

Make sure the Diaphragm Pistons aren't sticking letting extra Gas loose.

Remember, ... Tuning requires patience.
Be methodical.
Don't do multiple adjustments.
You won't know what you did that changes things.

 

My high revving issues turned out to be the intake carb boots on #1 and #4. Leak testing with combustibles didn't show much for me either, because it was already "idling" so high the I could tell no determinable difference. My boots were cracked and very stiff, but I didn't think they were leaking because the tests didn't prove an obvious raise in the idle. After several times taking the rack on and off, cleaning and bench syncing, the carb end of the #1 boot actually broke completely through and stayed on the carb. After that DUHH!!! moment, I bought a whole brand new set from Chacal, and have had no issues since. Just something for you to look into, so check your boots. Small cracks on the outside won't hurt, but if you find ANY cracks on the inside, it is likely some of the outside cracks do go all of the way through and is letting gobs of air in (just like rolling the throttle open will open the buttery valves letting in more air) and that much air will activate the main fuel circuit and rev the engine high.

 

Did you split the rack and change throttle shaft seals?

If so, when you re-installed the butterfly's did each one close with a *tink* (snap shut and seal completely)? Any binding on the butterfly's will also make idle temps rise when warm and keep you from getting a good sync on the go. Personally I think this issue and throttle shaft seals might be the two biggest high idle issues for these trusty steeds.

Either way it sounds like you need to start back at square one. Don't give up hope, most people here can pull a rack of carbs with their eyes shut and can diagnose problems listening to their bikes via walkie talkie .

 

i just literally had the exact same problem (same year bike and everything) i got my float levels all sorted out and today it started up and stayed at a good idle (12-1500). i would suggest you do the same check out my other thread for an idea on how to make a carb stand so you can check your float levels http://xjbikes.com/Forums/viewtopic/t=43163.html. and feel free to PM with any questions. like i said same problem and its now fixed!