engine idle creeps up

[79GS850VETTER]

I was all excited that it could be such an easy fix....but...no such luck. I put the heavy dark foam weather stripping on and my problem persists. I took it for a run today. At start up and a short warm up the idle is great. After about 5-10 minutes of highway riding the idle was up around 2500. A blib of the throttle brought it down to normal for an instant then it creeps back up to 2500. If I wait about a minute it will come down to 2000.

I adjusted the idle at that point to 1500 and it runs ok but it will not idle at all when the engine is cold to warm. After that it idles at about 1500 but if left idling for a while it will drop to 1000.

Its workable but I know this is not how the bike use to run so I know something is not right.

[Nessism]

Did you change the intake boot O-rings lately? If not, that is a highly likely source of the problem. If that's not it you might want to tweak the fuel and or air screws to richen up the mixture at idle. Where are the screws set now?

[81gs1100]

All great things to check out, however I didn't read if anyone had mentioned the petcock leaking gas or air into the signal tube to the #2 cylinder. A bad petcock can leak fuel down the tube, or air which will mess with your idle. Pull your plugs after letting it idle high for a few minutes. See if #2 is running leaner or richer then the other cylinders.

Also when was the last time the valves were adjusted? If the problem only happens when the motor is warm/hot, maybe the valves are a bit tight affecting the ablility to accept and keep a proper air:fuel ratio charge in the cylinders.

[rideOn]

my rpms varied until i replaced the intake o-rings and re-worked the carbs. now she's good.

[Matchless]

Intake leak or carb sync, or some combination of the two. Have a look at both.

Your bike is a '79. Replace the intake O-rings as a matter of sanity maintenance, or the symptoms will drive you nuts.

As Robert says if everything is physically correct and you have not resychronised the carbs after taking them apart......

[79GS850VETTER]

The carbs were just sync'd so it should not be that. However I did notice some oil that appeared to be seeping from the upper head gasket. (hard to tell for sure because the wind blows it around) I am wondering if air could be getting into the cylinder causing it to lean when the engine is hot.

[nutsandbolts]

got a 78 gs 750 with the exact same problem. New boots, orings, box rubbers, sealed ends of boxes, have been tweaking the air screws. No carb sync though. Sounds like we all have the same issue, interior orings and carb parts, carb sync neded- rebuild pending.

[Jagir]

+1 on the petcock question.
My bike had a manual (not a vacuum) petcock installed and the (now useless) vacuum line that ran from the carb to the petcock was leaking. I ran the line back to the carb and plugged it onto a protuberance on the carb body and zapstrapped it there. This solved it for me.

[Rick65Cat]

got a 78 gs 750 with the exact same problem. New boots, orings, box rubbers, sealed ends of boxes, have been tweaking the air screws. No carb sync though. Sounds like we all have the same issue, interior orings and carb parts, carb sync neded- rebuild pending.

My bikes idle will creep up to about 1700 once its hot after an extended ride. At idle when its just off the choke and warm enough to drive, its about 1100 to 1200.

As a side note, its been said that air leaks will cause it. What about if you are using the *FACTORY* hose clamps on the rubber boots. Mine sorta bottom out just as they are tightening up. Are there recomended replacements? Or just go buy radiator hose clamps?

[Ponderosa]

New..

My bikes idle will creep up to about 1700 once its hot after an extended ride. At idle when its just off the choke and warm enough to drive, its about 1100 to 1200.

As a side note, its been said that air leaks will cause it. What about if you are using the *FACTORY* hose clamps on the rubber boots. Mine sorta bottom out just as they are tightening up. Are there recomended replacements? Or just go buy radiator hose clamps?

I bought new clamps, they are cheap, auto style are a little too wide..

[nutsandbolts]

I am using the factory clamps which do seem to be all bottomed out. I might order news ones if I can get them. I mean the bike is 32 years old after all. It would be worth knowing if the new box boots with old clamps could still be leaking.
One more thing to consider I guess those doing the same.