So i just rebuilt some amazingly dirty/glazed carbs on my ladies '89 GS500 and got them back in the bike. The thing runs like clockwork now compared to before the rebuild. I've got a major issue though that is confusing the heck out of me and hoped one of you could provide some input. The manifold clamps are all tightened down well now (they were completely loose before the rebuild) and i've added an inline fuel filter to prevent any additional rust from getting in the carbs. Vacuum line is hooked up and I've triple checked all connectors for air leaks etc. but maybe I'm missing something. The bike starts up within 1/2 second no choke needed and idles amazingly solid around 1.5-2000 rpm with no hiccups (idle screw is fully out).

Here's the problem, after about 1-2 minutes warmup time the bike starts revving close to 5000rpm at which point i have to shut it off. I'm having trouble figuring out why this is happening. I took the bike out and rode it around and again it seems to run great although it is as if on cruise control since I can drive around with no throttle engaged. when i put it in neutral it revs above 4-5000 again. not good. I should note this also happens when both the choke and throttle are completely disengaged/disconnected from the carbs so it's not a pinched cable. I checked the sync on both carbs (at idle before it started revving up) and they are in sync.

I've also sprayed the manifolds and carb bodies while running and it seems to have no effect so i don't *think* it has any air leaks but maybe I'm wrong. I did alter the jet/air screw on the bottom of both since i had to take them out to throughly clean the carbs. I did also pull both carbs apart to clean them better (the manual suggested not to do this if possible). I reset the pilot screw to two turns out from a lightly seated position but not sure if that is correct. I should also add that when running the bike *without* the airbox it will idle and not rev up on it's own (at least for the couple minutes i let it run). Engaging the throttle immediately after starting the bike with the airbox on will throw the bike into this high revving situation but if i do the same with the airbox off it will peak and then the RPMS will *slowly* come down again to idle state.

The setting for the clip on the needle jet is one down from the top out of 6 positions (this may of been done because it came with an aftermarket pipe?). I should also mention that i did reuse the existing o-rings since they still seemed pliable but wondering if this could also be the cause of my problems. Any ideas from you guys would be appreciated.

Did you adjust the idle screw?  I would try and adjust that down and also make sure your fuel/air mixture screws are properly set.  Also you said the choke cable is out... is the choke getting stuck open on the carbs?

Hope this helps... but I've never worked on carbs so I really have no idea what I'm talking about .

Best of luck

johnbot: I have the exact same problem as you. My bike was running pretty good a couple of weeks ago but I knew my floats were out of adjustment. I had cleaned and done a re-jet but didn't get the float height set correctly.

So the past few days, I decided to set the float height and got them spot on pefect as I have seen documented on this site. Since then, I have the high idle like you. Mine will stay right around 3200-3000 and then gradually drop within about 1/2 a minute. It will come down to around 1350 - 1500 and sometimes level off at 1200-1250 but it's jumping.

The only thing I did differently was to make sure I was at 3 turns out on the air mixture screw and I put a cap over the vacuum barb on the back of the left carb (since I have eliminated the need for vacuum on the petcock.

If you figure this out, please post some detailed info. I have read all over this board and it keeps pointing me to an air leak in the carbs. Others have had success diagnosing their leaks and that is what I am heading towards but I sure would like to not waste any more time in the wrong direction.

Hopefully, some of the others will chime in on what they "have" done to get rid of a high idle.

I had a "hanging idle" caused by a missing vacuum cap on the petcock connection on the left carb.  Twice.  So I have a tiny hose clamp on there now.  I don't know how the cap could come off on it's own.  Maybe I knocked it off when I did a valve clearance check. 

i had an issue like this, it was the idle screw. i let her warm up real well and brought it down a little at  tmie. no issues since.

I had the same issue yesterday, turned the idle screw and it went back to normal.

also make sure that you put the float assembly back together correctly. The little metal tab attached to the float goes INSIDE the wire spring on the needle valve. I made this mistake when I cleaned out my carbs and it caused the bike to run lean and idle real high.

SOLVED For mine!!!!!! I decided to take a moment and look at my carbs (again) and see if maybe I missed putting something back together or if maybe, I createad a leak. Well, I pulled the tank carefully and inspected the hose for the tank vent and it was kinked (bent 90*) so I simply removed the hose and started the bike. I let it warm up and the idle did not climb. I rode around the apartment complex a couple times and it was stable. So, I buttoned down the tank and seat. Put my gear on and rode to Pepboys about 5 miles away and the bike ran absolutely perfect.

So, my assumption is: If you have done everything correct to the carbs and there are no leaks, the float height is correct, the adjustments are set for your bike, check that vent hose. I assume also that the suction from the carbs and maybe the vacuum pull on the petcock must create a condition where there is back pressure to the fuel in essence force feeding the fuel to the carbs causing the high revs. In my case, it would relieve pressure and settle down "some" but it did stay high. (PLEASE, someone explain if this a correct assumption of if I am just and "ass"umption.) 

To test, simply remove that hose and see.

I myself am going to be ordering some more flexible race / vinyl hose or something. Maybe that will help with hose routing and such.

Good Luck.

Was that the hose from the bottom of the tank?  That's a drain hose.  I believe the vent is through the gas filler cap.

Hmmmm, I would have thought it was part of a venting system. Yep, that hose.

Is your high idle issue still resolved after a couple of days running? I have two hoses in addition to the fuel output on the gas tank. Ones going to what appears to be CA specific emissions stuff and the other looks to be the drain hose but correct me if i am wrong. From what I know the vacuum hose going from the left carb to the secondary pet chicken is responsible for assuring fuel is being pulled into the float bowls not any hose going to the gas tank.

I still need to check my float height and the air screw. Could either of these cause the high idle condition? Or would they just make the carbs run rich or lean?  The only thing I don't have hooked up is the Vent Hose from upper T connection on the carbs as shown in the link below:

http://wiki.gstwins.com/index.php?n=Main.HoseRouting

I will be able to tell you more later today. I plan on riding it to a Dr.'s Appointment and later, I am thinking of putting the hose back on and trying to duplicate the problem. So far, it is starting and running fine but today will be the 2nd day. Spark plugs look fine so no rich condition that I can see there.

I don't have the same setup as far as emissions but the hose coming from the tank is the ONLY thing I removed. It is entirely possible that the hose was pushing on something and causing the idle to go high. That's why I am looking for other people's experience. I am just too dense when it comes to carbs and engine running stuff. (Trying to learn though)

Rode it around again today and everything ran fine. The idle seems a little low at start up and sometimes seems to want to stumble but no real high idle. I have not had the chance to put the hose back on. I am letting the bike cool down and the day to cool down.

This thread had me really hopeful because my bike is having choke/hanging idle issues but I think my tank overflow tube is straight.  Oh well, Saturday morning will give me some time to play with it.

I go out yesterday to put the hose back on and test to see if I could duplicate the problem. It was too hot to spend too much time on the bike so I didn't really get to do that. Maybe later this week. Mine is running fine except when I first start it up, it idles at around 800-900 until it warms up. Not sure if that is normal. Choke will bring it up to a solid 1450-1500 and when it's warm, it stays at 1200. Dunno if that is the proper range.

QuoteMine is running fine except when I first start it up, it idles at around 800-900 until it warms up. Not sure if that is normal. Choke will bring it up to a solid 1450-1500 and when it's warm, it stays at 1200. Dunno if that is the proper range.

Yeah, that sounds normal.

Okay. I put the hose back on the overflow for the tank but did not get to ride it last night. So, I decided to ride around a bit to see if everything was hunky-dory. Well, it wasn't. I was back up to high revs around the 3200 range. I tried to pull the hose off (not clamped) but couldn't get it to slip off. So, I rode back to the apartment and pulled the tank loose and pulled the hose off. I rode it around again for a short distance and no change.

This confirms that the drain hose has absolutely nothing to do with high idle and I am a complete idiot for thinking so, and everyone in the room is now dumber for reading it!     

Anyway. I decided to follow some advice I read earlier on here through other posts. I removed the tank completely and whipped out the WD40. I started the bike and sprayed a few shots near areas where I thought there might be leaks. As it turns out, there is a leak on the right intake boot. Well waddaya know!? I could have saved a boat load of time if I had just done that first huh?

A couple of other things that I noticed worth mentioning. When I didn't have the floats set correctly, the bike ran rather decently. The first few trips I took on it, were actually pretty smooth. I noticed flat spots on wide open throttle and the bike would not accelerate all the way through the range well. After I attempted to re-set the float height, I was still high and some of the problems started creeping in. I finally set the float height spot on and adjusted the air mix screws to 3 turns (I couldn't remember if that's where they originally were set after my jet re-do). The throttle seemed a lot heavier than normal and the high revs started up. Then, they settled down for half a day.

I now have the air mix screws set at 2 turns and the throttle seems a bit more snappy even though my idle is waaaaay too high.

So, all in all I am going to buy a new set of intake boots (just in case both are bad.) and a carburetor oring / float needle valve kit. (In case I need it later.) I think that should wrap up my re-condition on the carbs.

By the way: My apologies for the complete thread jack. I really thought there would be more responses and suggestions about really high revs. That particular problem seems so different that all the others I have read about.

Well... hanging idle generally means a lean condition (AFAIK) or a stuck cable.  There are only so many spots to check... and Buddha has posted several times what to look for (just do a search for hanging idle or check the FAQ section).  

I had a high idle which was caused by a poor exhaust seal (removed the exhaust and didn't get new gaskets), but even after I replaced them I had a hanging idle which was then fixed by adjusting the idle screw (and possibly tightening the carb intake boots).  After that there have been no problems for 2500 miles.

Anyway... I'm glad you've solved your problem.  Enjoy