Other riders have had this problem:

When the idle is set to 1200rpm, the engine when revved to 5 or 6k rpm HANGS, slowly comes back down to idle, or even begins to race. Giving a little clutch brings the revs back down.

One line of advice seems to be to check out how well the carb boots are sealing, as it could be due to an air leak. Checked my seals with visual inspection with the engine running and couldnt find one.

Now I went and shimmed my needles with 2x3mm washers each and replaced the old stock air filter with a new K&N air filter with restrictor.

Performance has changed - the engine runs smoother.

However, I still have this rev hanging problem. What I dont understand is how this problem DEVELOPED, and how adjusting the idle throttle stop could possibly give me this rev hanging problem...

I mean when the stop is set too low, I can keep some throttle on when at a stop, but thats no way to ride around.

Can anyone mechanically inclined explain this with carb logic please?


Fin

That's classic running lean.  You're getting too much air or too little fuel somewhere.  What's your jetting?  What exhaust are you running?  How thick were your washers?  What is the setting on the idle air/fuel mixture screws?  Have you checked to see if the jets are clogged? 

Egaeus, I'm running the stock exhaust and stock jetting (plus washers now). I'm not sure what the exact thickness of the washers were, maybe 1mm at the most. Adding the washers has definately improved the running of the bike, yet this hanging/racing problem remains.  With the idle adjust set too low, the bike runs great, accelerates well, sounds fine, decelerates fine. Only when I set it to idle at 1200rpm, do I encounter the revving probs.

What I havent done yet is pull the carbs and overhaul them. I dont understand how, when I originally purchased the bike (used), it would idle and rev fine after it warmed up. The problem has sort of cropped up over about 6 months of riding.

Its looking like I will have to pull the carbs, I just want to rule out anything less work intensive before I do so...

-Fin

Sounds possible that your jets have become somewhat clogged.  You might try a good carb cleaner fuel additive.  That will sometimes work.  However, cleaning the jets isn't too difficult either.

Sounds like the idle mixture is too lean. Yes, could be the pilot jets clogging up a bit. Where are the fuel screws set?

FWIW, this is how to adjust fuel screws. Your range of adjustment is from closed to about 5 turns out. I would expect you to be somewhere around 3 or 4 turns out.

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With the carbs synched, the engine warmed up to operating temperature, and the idle speed set, adjust the fuel screws so that when you rev the bike in neutral and release the throttle, the revs quickly drop to exactly idle speed again. If, when you release the throttle, the revs hang up a few hundred rpm above idle speed, then drop to idle, the idle mixture is probably a bit too lean. Turn the fuel screws out. Use a minimum of half turn increments until you know you've just about nailed it. You'll drive yourself up the wall trying to tune the thing in eighth or quarter turn increments if you're a mile off. In extremely lean cases the idle will hunt between the proper speed and something above it. If, when you release the throttle, the revs drop below idle speed then pick up, the idle mixture is probably a little bit too rich. Turn the fuel screws in. In extremely rich cases the engine will die after revving the bike and releasing the throttle, unless you've turned the idle speed screw way in, in which case it may act like the idle mixture is a little bit too lean. In slightly rich cases, the engine will respond well to throttle blips when cold, but will die or dip slightly below idle speed when hot.

Is the bike hot when the idle is set?

Scratch, I'll ride the bike for 10-15 minutes before twidling the idle adjust.

Another observation I made when I had the tank off is that when the engine races, from the outside the throttle linkage doesnt move.

Obviously when it races, something is moving inside the carb to let more fuel/air mixture in. I assume the butterfly valve is connected directly to the thottle linkage on the oustide of the carb- so if the butterfly isnt moving, then maybe the slide is popping up when it races??

I know from working with 2-strokes, as you lean out the mixture, the rpms go up. Cause the fuel/air mixture stays set on the gs500, I dont see how the engine revs unless something else is changing- what could it be? Maybe a bad transition between jets? The slide or the butterfly valve moving? Also if it was running too lean- wouldnt it be backfiring/ popping the exhaust?

Too many unknowns!!! I'll just have to pull em out 

I would pull the carbs for disassembly and cleaning.  While you've got them out swap out the pilot jets with 40s (if your bike is an 89-00 model) or whatever the proper size is for the newer carbs. 

I'm seriously thinking about starting a Srinath-style carb service.  Do you think ppl would be willing to spend $40 + shipping to have their carbs cleaned and re-jetted? 

-M

Quote from: finmac on April 22, 2006, 12:37:03 PM
One line of advice seems to be to check out how well the carb boots are sealing, as it could be due to an air leak. Checked my seals with visual inspection with the engine running and couldnt find one.

Sometimes visiual isn't enough.  Look again at the screws that secure the bands around the rubber boots at both ends of the carbs.  After making sure those are tight, spray some WD40 around those areas where air might be getting in, while the bike is running.  We may need this dynamic test.  If the rpm's drop, then the boots are not sealed completely.  It is simple to test.

Finmac,

I'm having the same problem. I think new pilots will help, I'm gonna give this a shot tomorrow.

Also, float level may be too low (=lean).  And, try setting your idle to 1100, you will have to keep your rpm's up with the throttle in the mornings, but that's been normal for our bikes (that may or may not be acceptable).

Adjust your mixture screws out a bit, it will cure all.

I'm struggling with the same exact problem and am fiddling with the mixture screws.  I have them 3.5 turns out now which seems like alot to me and I'm still having the same problem.  I've also noticed a bit of a flat spot around 6-7K.  Is this characteristic of lean and I should turn my mixture screws out to 4?.  I think i remember hearing someone saying that theyre screws fell out at 4 turns while riding.  I just finished cleaning and rejetting my carbs (40 nonbleed and 127.5), have two washers (I believe they're #4's though) and the K&N drop in filter (unrestricted though).

I've been messing with my GS's jetting for a while now.  It's about 95% (at least that is what I consider it).  There is still a slight hesitation off the bottom.  I am about done with fooling around with it.  I am 3 1/2 turns out on the screws, 40 pilots, 122.5 mains.  On top it runs good.  Mid-range is good.  Just that stumble, right out of the hole.  Raised the float height a skosche, still about the same.  Does anyone out there have perfect carburation?   I've talked to a few others and so far, not perfect, from anyone.  And I have never had a bike (over 30 of them over the years), that used more than 2 1/2 turns out on the air screw.  I've never had this much trouble jetting before.  Should I just accept that this is just how it is going to be?

Quote from: corndog67 on May 13, 2006, 07:43:06 PM
I am 3 1/2 turns out on the screws, 40 pilots, 122.5 mains.  On top it runs good.  Mid-range is good.  Just that stumble, right out of the hole.  Raised the float height a skosche, still about the same. 

Everything you've done is trending toward a richer mixture.  In my experience, when I've went up a size on the pilot jet and an extra turn out on the mixture screws I got a rough idle and a stumble.  IMO, if you want to keep the 40 pilots you should try leaning it out some on the mixture screws--or vice versa--ditch the pilots and stay out on the mixture screws--see what works best.

Shimming under the needle can also cause off-idle stumbles.

I dunno if finmac solved his problem... (he was last active on April 28th). I am still having the same problem after putting 40's in. It will idle great but then as soon as i touch the throttle it will race up to 4k and just hang there.