High Idle when engine is warm ***SOLUTION FOUND ***

I'm driving a '82 XJ750 SECA for 15yrs and had it parked for a couple years, (after accident) rebuildt it and also cleaned, disassembled the carbs. Started it and found out that when engine gets warm the idle will be stuck at 3-4K revs, it only will go down when letting the clutch come on.

This is what I did till now (without a good result);
-chg the airfilter
-chg the carb holders all 4, were in bad condition.
-sinc the carbs
-checked the choke and throttle cable, run without having it attached.

This what I didn't do yet (found some tips on the forum, have to try)
-no 'clunk test'
-didn't set the mixture adjustments to 2 1/2 turn from bottom.

I really want to ride my bike again, so if there'a anyone who can help me I would be gratefull. ;-)

 

Re: High Idle when engine is warm

Mine did the same, as a good starting point do the baseline 2 and a half turn adjustment of those pilot screws.

I won't say it's throttle cable / linkage related because she goes down when you engage the clutch and remains there.

 

Re: High Idle when engine is warm

Thanks, I'll give it a try tonight.

 

Re: High Idle when engine is warm

Definitely do the clunk test. That is a likely candidate. Another thing to check is for vacuum leaks. Take some starter fluid and spray around the intake boots and at the carb throttle shafts to check for leaks there. If you get a higher idle from that at the carbs, then the throttle shaft seals will need to be replaced.

A couple of other things to check is to see if the cable or throttles are binding. When you hit the high idle, between carb 1 & 2 or 3 & 4, try to push the throttle closed with your finger. If the idle goes down, you could be binding up somewhere in the linkage.

 

Re: High Idle when engine is warm

Hi Joefriday77,

I have checked the throttle cable, even without the cable attached the Idle stays high. Also pushed on the throttle lever but it stays high.
I have changed the intake boots with new ones.

First thing I'm gonna check or adjust are the mixture screws and see what effect that will be.
The so called 'clunk test' will be done after the mixture adjustment, because that means I have to disassemble the carbs :?

let you know

 

Re: High Idle when engine is warm

first turn idle adjustment screw down, then could make mixture screw adjustments then need to synch carbs, if rev and idle stays high for awhile than slowly comes down one or two cylinders are stealing fuel due to vacuum higher in those , thats why must be synched.And must use yics tool or make one to isolate each carb,they are easy to make.

 

Re: High Idle when engine is warm

Lean mixtures burn better hot. That's likely why your idle goes up.

Look for air leaks, plugged pilot jets/passages, pilot mixture, float level.

 

Re: High Idle when engine is warm

Update;

I've done the 'clunck' check and found one piston a little bit sticking, could not find the reason. Onces removed and installeb back into the carb it slided perfect.
Than set the mixture adjustments to 2 1/2 turns from full CW, 2 adj were set at 3 turns, re-adjusted and did a testdrive.
The revs still rising but not as strong as before, even when the engine is at idle it rises towards 3000-4000 rpm. Only get it down when release the clutch a bit.

An other thing I noticed was that 2 of the four exhaustpipes where hotter than the other 2 (see pics)



What els can I do?????

 

Re: High Idle when engine is warm

Nice!
Be patient, you're walking on the right way. 2 and 1/2 turns is just a baseline to start with. Do the synchronization again and check the plugs. Try to make them look tan. When they look tan and the synchronization is good you should have a proper idle.

If your exhaust developed that nice color recently, that should mean that your settings are way too rich so you need to make her lean. But check the plugs first.

Don't worry, just keep tuning. It needs a bit time to set everything correctly even when you're doing the right thing.

By the way, do you have any mods? (pods, open headers, god knows what else)

 

Re: High Idle when engine is warm

He,

What do you mean with 'check the plugs. try to make the look tan'?????

There are no mods on this bike, original as it can be

 

Re: High Idle when engine is warm

Firefox,
Blue pipes mean lean mixtures. You should start from scratch. Get the carbs back off and run a torch tip cleaner wire through the passages. Make sure your float levels are at the proper height. Someone suggested squirting starting fluid around your carbs. I would not do that in case of a backfire you could start a fire on your bike. Use an unlit propane torch and just flood the carb boots with alittle propane to see if the idle picks up. If it does check your boot gaskets and see if one didn't seal right. The other thing that happens alot is the throttle shaft seals dry up and crack which lets air in and that could cause a lean condition. Just remember if the engine has any vacuum leaks it will screw up your mixtures. If it continues to be lean major damage can be done to pistions and valves being overheated.

 

Re: High Idle when engine is warm

I agree with MN-Maxms blue pipes mean lean she is running hot, ask any snowmobiler when seals go and pulls in extra air goes like heck but then puff burns hole in piston. I know as happened to me.
I think i had my carbs out 3 times recleaning, setting float hieghts and synched .
NEVER USE STARTING FLUID for test -to flammable!

 

Re: High Idle when engine is warm

Ok I'm convinced, I'll take the carbs out and take them apart.
what should I check when disassemble the carbs??
At the cylinders who are running hot, is there anything that I can find/notice that caused the lean mixture?

How far do I have to disassemble the carbs, also take out all of the nozzles?

 

Re: High Idle when engine is warm

Hey guys, stop for a second. If he's able to set the mixture at idle there is pretty likely that she wont have any leaks anywhere. High idle at warm can mean a lot of things, but if you can fix with synchronization and idle mixture tweaking that should mean that your carb is in proper working order.

Or at least, that's my experience: a bit of off sync and a lean mixture setting can cause a high idle. High idle, doesn't necessarily an indication of air leaks. It can be a wrong setting.

If he's not running the bike just make her warm to be able to tweak with the sync - mixture it wont really harm the engine.

 

Re: High Idle when engine is warm

Ok ok don't get upset, I'll give it I try to set the mixture adj. and sync the carbs again.
when that doesn't work, I disassemble them and start all the way from scratch.

Turning the mixture screws CCW means a richer mixture?

 

Re: High Idle when engine is warm

Sorry about that, I just don't like the idea to remove everything again unless you have other options to try Let her idle for a while when she's warmed up and check the spark plugs. I'm assuming all the 4 will be different.
There is a link which is a good catalog for initial spark plug reading: http://www.dansmc.com/Spark_Plugs/Spark ... talog.html

You have to check the insulator and based on that information make small adjustments. Less than 1/4 turn at a time. It's time consuming but you should try.

And yes, CCW means richer mixture.

 

Re: High Idle when engine is warm

Friend of me gave me a Colortune device and going to try if I can adjust my carbs to the best.
He insured me that this would be the best way to adjust the carbs.

Anyone any experience with this?

 

Re: High Idle when engine is warm

Colortune is the best way to get proper idle mixture.

But the sync has to be right first.

Should probably bench sync them if you can.
(take them off and reset the butterflies mechanically)

 

Re: High Idle when engine is warm

I am very interested in the outcome of your work. I have exactly the same symptoms (I'm driving an xj750MK), but am inept at repair. If you solve the problem can you let everyone here know

Mine starts fine and drives fine , but will idle between 3-4k once it gets hot, which causes the exhaust to make some popping noises. The only way to remedy the situation is to kind of ease half out off the clutch.

 

Re: High Idle when engine is warm

Mine was doing the same thing.

Replaced the throttle shaft seals to correct the problem.

YMMV