1983 Suzuki GS650G High idle when warm

[ChrisEd]

1983 Suzuki GS650G High idle when warm
Hi all,
I purchased a 1983 GS650G as my first project bike, I have a GSF600 as my main day to day bike but wanted a project bike to expand my mechanical knowledge.

My original plan for the bike was to make sure it was mechanical running well before starting the restoration, I hoped to have completed this year but reached the peak of my mechanical knowledge and can’t seem to work out the current issue I am having.

A bit of background on the bike from when I first had it and where we are now, it had been a non- runner for about 3 years but was stored in a dry environment. The below is a list of the what I carried out when I first got the bike:
Spark plug change
Oil Change
Fresh fuel
New battery

This was enough to get the engine to turn over but not fully start, upon investigating the coils needed replacing. After a new set of coils, the bike fired up no problem but died shortly after making me believe it was a fuel issue. Just a note I did not clean the tank after removing the old fuel, probably not the best decision.

From here I bought an auxiliary tank to use while I cleaned the actual tank and replaced the petcock, I also decided to strip the carbs and replace all gets/gaskets. The only thing not replaced on the carbs were the slides and diaphragms, a bench sync was carried out and the carbs put back in place. i have not been able to do a fully carb sync yet due to the bike not being able to run the bike for long periods of time.

The bike now starts after doing this, however, after about 5 minutes when the engine starts to get warm the revs will just shoot up from idle and hang around 5-6000rpm.

This is where the bike currently sits as I have reached the peak of my knowledge and cannot figure out what would be causing this, I have checked for vacuum leaks and sticking throttle neither of these seem to be causing it. I have not yet checked anything engine side as I didn’t want to start stripping parts not knowing what I am looking for. I have seen many posts in which people mention cracks on the inlet rubbers, i have checked mine over and there seems to be no problem but i will be replacing these regardless at some stage.

Has anyone had any similar experiences before or possibly know what may be causing it? Any help/advice is appreciated, and I am happy to answer any more questions.

[Nessism]

You might want to check the Newbie Mistakes thread linked in my signature.

A high idle is often signs of an air leak. And spraying junk won't find it in all cases.

Your bike has O-rings behind the rubber intake boots which need to be changed. If the boots are hard they should be changed too. This is one of the rights of passage with all GS projects, along with rebuilding the carbs.

[Redman]

Welcome to TheGSR.

You have described a typical problem, and have described the specific symptoms of the intake air leak between the carbs and the heads.
Typically the o-rings in the boots up against the head. Or maybe the boots themselves have cracks. THose are the main culprits.If that doesnt completly take care of it, then another source I found can be the oring around the enricher ("choke") mechanism in each carb.

I cant fully describe how an air leak there causes the rpms to rise, but is a known situation and I have expereinced it myslef. I took off the carb boots (those screws were a pain). ANd the o-rings were so brittle that they broke and seemed more like ceramic or glass than rubber.

Look for Cycle O-Ring online ( a fine member here, who does this for us as an option to having to get OEM parts) for the boot orings and replacement boot screws. Note: he also has oring kits for carb rebuilds, which is usally all that is needed for carb rebuilds.

[Gorminrider]

a bench sync was carried out and the carbs put back in place. i have not been able to do a fully carb sync yet due to the bike not being able to run the bike for long periods of time.

The bike now starts after doing this, however, after about 5 minutes when the engine starts to get warm the revs will just shoot up from idle and hang around 5-6000rpm.

Air leak has already been suggested. I'm never sure about this as the only cure all the time where the bike needs to get hot before it leaps in rpm. here's Some things to do to confirm or offer different directions while you are waiting for the o-rings:

What exactly is your starting procedure? Do you fill the carbs with petcock to "Prime" function? Does the bike drip on prime? Do you use the choke* to start? When do you take choke* off?
Were you careful setting the throttle cable?
as in:
What happens if you turn the revs down with the big plastic knob that sets idle? Is the bike then rideable? I'm thinking a very "lean" or "advanced" bike will be without power. A spark plug check can indicate too.

Throttle cable binding/routing: Are you able to relax your throttles to full stop with all adjusting screws to "zero"? Does turning the handlebars to full stop right or left affect this?

Is the vacuum line to the petcock in good shape.? If you disconnect this vacuum line and plug it (with a small bolt) and turn the petcock to prime does this make a difference?