Terrifyingly fast GS idle... HELP!!

[drblasto]

Yeah, I've just sorta been ignoring this problem with my 91 GS500, but considering the fact that I'm probably going to be selling it soon, I think I need to get something straightened out:

My bike starts really nice, warms up fast, idles at around 1200, which seems reasonable.  Until you drive it someplace.  Then it starts idling towards 1500, 2000, and above.

If you take it out on the highway for 5 or 10 minutes, then get off at the next exit, you will find the poor creature idling at a terribly fast 3000-3500 rpm (but never faster than 3500).  Park the bike for an hour or so, let it cool down, and it'll start and idle at 1200 again, until you drive it.

This is obviously not right.  Please help!  Tell me it's a simple- and cheap- problem.

-Jerrold

[Jared]

Do the rpms slowly return to normal or close to normal if you let it run?

Have you worked on your carbs?


And you do hav ethe choke off right??

Hanging rpms when returning to idle usually means a lean condition....

[The Buddha]

OK its sucking air from somewhere or has a lean mix from the factory.
Cool.
Srinath.

[Rema1000]

My '92 does the same thing, although only when it's hot out (like 90+ F).  I have no problems when it's only 80 degrees.  I had assumed it was due to the factory jetting, but I haven't thought to check for vacuum leak.  

Question: when you say to spray WD40 around and see if it smokes, where do you spray it (at the carbs, I assume?; or where the carbs meet the airbox?).  And how do you know that WD40 isn't just getting sucked into the air intake.

I suppose I could duct-tape a vacuum cleaner hose to the air intake, so that the bike is drawing air from 5 feet away...

[jabbajosh]

your idle may also be set too high

[Kerry]

Question: when you say to spray WD40 around and see if it smokes, where do you spray it (at the carbs, I assume?; or where the carbs meet the airbox?).

If I'm thinking correctly on this, you want to spray the WD-40 around the rubber boots on the ENGINE side of the carbs.

If air leaks in around the rubber boots on the airbox side of the carbs, so what?  You may be getting a bit of unfiltered air, but there really shouldn't be any EXTRA air.

But an air leak downstream from the carbs would definitely lean out the mixture.  That's where it makes sense (to me) to spray....

[yamahonkawazuki]

might not want to do the wd-40 thing on a hot engine. stuff can burn rather easily. course you probably already know that

[Rema1000]

Thanks, Kerry.  That makes sense.

might not want to do the wd-40 thing on a hot engine. stuff can burn rather easily.

...so propane from an unlit torch is probably a bad idea too?  That reminds me of checking for leaks in the natural gas line.  I was being so careful, using soap suds to check for leaks.  Then I had it profesionally checked, and the pro used a lit propane torch, running the flame over the fittings looking for the flame color to change, or get bigger.  "I thought you weren't supposed to do that," I asked.  "You're not.  Never do this," he replied.

[500rider]

I have the same problem but I haven't sorted it out yet. This is a common problem it seems with a lot of bikes (GS500's).  

Besides the air leak theory, others have suggested that it could also be caused by the carb balance or valve adjustment.  I'm planning to do both of those jobs early in the spring wo hopefully that takes care of it.  I am also going to put in bigger main jets (not because of the idle problem).

Rob

[drblasto]

No, I've never opened the carbs up on my GS, although I've worked on the carbs of an EX500 and a Honda C70

Yes... I have the choke off... thanks for your overwhelming faith in my mechanical aptitude.

One hint... occasionally, while I'm idling way too fast at a stoplight, I'l hear a mechanical "ping" or "clink" come from the engine, and the revs will drop a little, then idle back up to 3k.  I wonder if it might have to do with valves not seating properly?  I had a totally inept mechanic tell me to check the valves, but as I said, he's a totally inept mechanic.

Could it be valves?

It seems to be related to the temperature of the engine, as it never happens when I star the bike cold, but does happen (and continues to happen) after it's really heated up.  I would have to let it idle, in place for an hour or so to see if the revs drop as it cools off.  I'd guess they do, as long as the temperature of the block is decreasing, though.  As I said, it runs fast when the engine is hot, and slow when it is cold.  That's why I don't think it's carbeuration.

Next time I ride, and it's idling up to 3k, I'll try to turn down the idle, but the thing is, I doubt it'll start right, because it seems to idle totally normally until I get the engine really revved up.  If I turn it from 3k down to 1.2k, I think it might not even start.

Thanks again, keep those suggestions coming!
-Jerrold

[drblasto]

I think it may also be important to note, perhaps, that I have a Yoshimura can (don't know which previous owner put this on) and a mechanic told me that he thinks the carbs were re-jetted, but he wasn't totally sure.

-Jerrold

[GRU]

now i'm thinking that i have the same problem...i didin't start riding yet because of weather but did take my bike on a 10 km ride about a month ago...that's when i rejjeted the carbs, put on a pod filter and gsxr exhaust...i set the idle to about 1300 and the bike was running awsome. i rode it for about 5 minutes and didn't have any problems...then i went for a longer ride and about 5 or 10 minutes into the ride the engine started to increase the rpm's up to 6000....

i had a problem with my throttle cable and now i fixed it...now i have to go for a longer ride again and see if that was the problem...

[Kerry]

I would have to let it idle, in place for an hour or so to see if the revs drop as it cools off.  I'd guess they do, as long as the temperature of the block is decreasing, though.

I'm not sure if you were serious about this, but don't try it.  Idling in place will heat the GS air-cooled engine up very nicely.  When I have to idle in place (to synch the carbs, etc.) I direct a large-ish fan at the front of the engine to prevent any overheating problems.

Several folks have "wrinkled" the hinder part of their front fenders by allowing the engine to get too hot.  I would wager that most of them left the engine idling while the bike stood still....

================================

Meanwhile, back to the problem at hand.  I'll be interested to hear about your results after turning the idle down.  My (stock-jetted) bike will not start in the cold without the choke (and usually some throttle).  But if the choke is still on a mile or so down the road, the idle will go up to 3000 RPM or more.  I usually have to turn the choke off incrementally - so I can still pull away from stop lights without bogging down, for example.

Once the engine is warm and the choke is all the way off, my idle is often at 1500 RPM - so I turn it down a bit.  Yes, it makes for an interesting experience the next time I start the bike, but by now I'm used to doing the choke/throttle/idle "synchro dance"....

[Laura]

I think I have the same, or very similar problem. Once the bike is warm, it will idle at around 3500-4000 rpms. But it doesn't ALWAYS happen. Sometimes it is fine. At one stoplight it will idle really high, and then at the next stoplight it will be fine. Sometimes if I let the clutch out a little bit, that will be enough to get the rpms to drop back down. But sometimes it won't. I also have trouble switching from second to third gear when the bike is warm. I suppose this may be a separate issue. Kerry, would you mind explaining in a little more detail about spraying WD40 on the rubber boots? Thanks. Oh, and my bike has been rejetted.

Laura

[aslam]

I had a similar problem as is being described here, although much more seriously.  I would recommend adjusting the valves as one of the top priorities.

When I had this problem I adjusted my valves and rejetted my carbs at the same time, so I'm not sure exactly what fixed it.  Could have been either one or both, but I have read in other posts that poorly adjusted valves can perpetuate this lean condition phenomena.

ASLAM.

[The Buddha]

OK all of you... the bike is lean.
Air leak will also cause it to be gultess down in the low rpm's and it will also make more noise cos the noise is escaping through the same hole air is entering. Anyway 40 pilots and air screw to 3 will fix it nicely. But Check it for air leaks anyway.
Cool.
Srinath.

[Kerry]

Kerry, would you mind explaining in a little more detail about spraying WD40 on the rubber boots?

As I understand it, the idea is to:

1) Have the bike running at a smooth idle.
2) Spray WD-40 around the boots that connect the carbs to the engine block (see picture below).
3) Listen for increased RPMs.[/list:u]If the RPMs go up, then WD-40 is being sucked (through a crack or other leak) into the stream of air/gas mixture flowing from the carbs to the cylinders.  I'm not sure, but the WD-40 may even be more volatile than the gasoline.  Anyway, the extra "fuel" in the mix would cause increased revs.

Did I get it right, Srinath?  I'm having a hard time reconciling how both a lean mixture AND a rich mixture will cause higher revs....

EDIT: Changed link from sisna.com to bbburma.net.

[Diderich]

Gotta agree with srinath (always a wise move).  I know mine is lean from just off idle to about 3k..but i never ride there so it's ok for me.  If i do put the revs there, it pings as you suggest and stumbles.  Actually it backfires through the carbs is what it's doing on mine...i got a neat flame when i was balancing them.  When you fix the lean as described above, set the idle when it is good and warm, because I think you also have the idle set too high.  Mine actually idles at about 900-1000 when cool, but off choke and raises up to about 1200-1300 10 minutes later and stays there.

[Laura]

Kerry, you rock. I didn't know what you were talking about until you put the picture up. Thanks. I'm not going to blow anything up if I do this, will I?

Laura

[The Buddha]

The air sucking will actually not exactly make the idle go up. When the bike is ridden and you close throttle the idel will float down to ~ 3000 and sorta stay there... for a looooong time, and then abruptly drop fast and possibly misfire 1-2 times and die. That is idle hovering. The reason it sits at 3K is cos there is free air comming in wihtout the motor having to suck it through the carbs. That makes the thing stay high. The leanness of the mix eventually catches up with it and it cant sustain combustion. Hence misfire and die. If you spray wd40... the propellant in wd40 is the actual reason it revs up. It can be burnt and that substitutes some of the air with petro chemicals. It smokes due to the fact that the combustion is haphazard and a lot of un expected crap is produced. Lean mix will just stay at 3000, not misfire, not flutter, not respond to wd40, not make more noise and not have no power. Of course plugs will be white in both cases. In case I dont have the right explaination please tell me guys. I heard this from an old mechanic.
Cool.
Srinath.