Hot engine rpm’s get stuck

[anesbitt267]

update on bike, everything is working good did a lot of maintenance. But I've noticed when driving around for 20-30 minutes sometimes less when I clutch in the rpm's raise. Figured I was in the throttle but nope. It will raise to 3500-5500 range depending on if I rev match on the stop or don't use the brake to keep it down. It's very inconsistent. I noticed my throttle cable can't be adjusted anymore at the handle and return cable seems to be pulling back correctly. I was wondering if it could be the throttle cable needing adjusted at the throttle body or if the throttle body is getting warm and working improperly. If you guys got any leads I appreciate it.


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[The Buddha]

Easiest to hardest -

Eliminate the throttle cable first. A closed throttle should be fully closed. Period.
Then you set the idle rpm when cold at what ? 1500 ? Try lower, maybe 1200, it should hold a comfortable idle, nothing more when its ready to be off choke. And get an idle plug reading. They should look grey/chocolate. Not black, not ashy white, just darkish brown/grey.
Then you have the carbs in synch ? Make sure they are. Again they should be.
Then you may need to open air mix screw a 1/4 turn. Especially if the plugs read too light.
Usually cold idle 1200, hot idle 1300 is a good range to shoot for. However some people (like me) prefer a much higher cold idle, like over 1500. I would then like to keep hot idle also @1500. Rpm governs oil pressure and that few 100 IMHO would help avoid the old cop car syndrome (tons of wear from idling).
Cool.
Buddha.

[Bluesmudge]

Sounds like idle is set too high. You should set it to 1200 rpm after the bike is fully warmed-up. That means after riding for 10 minutes.

The bike should require choke to start.

If the throttle snaps back with a hard and quick snap then it shouldn't need adjustment but it wouldn't hurt to adjust it at the carb so that its in the middle of the range up on the cables. There should be ~1/4" of free play measured at the grip.

[anesbitt267]

Sounds like idle is set too high. You should set it to 1200 rpm after the bike is fully warmed-up. That means after riding for 10 minutes.

The bike should require choke to start.

If the throttle snaps back with a hard and quick snap then it shouldn't need adjustment but it wouldn't hurt to adjust it at the carb so that its in the middle of the range up on the cables. There should be ~1/4" of free play measured at the grip.

Yes sir, but the idle is fine it will idle normally but when I tap the throttle it get stuck. The throttle snaps back and has about quarter inch of play but only happens when it's warm. Could the throttle body springs be getting stuck when it gets warm?


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[Bluesmudge]

Yes sir, but the idle is fine it will idle normally but when I tap the throttle it get stuck. The throttle snaps back and has about quarter inch of play but only happens when it's warm. Could the throttle body springs be getting stuck when it gets warm?


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I thought you said that when it gets warm the idle raises to 3500 rpm? Let the bike get warm and then set the idle to 1200 rpm instead of 3500 rpm.

[anesbitt267]

Yes sir, but the idle is fine it will idle normally but when I tap the throttle it get stuck. The throttle snaps back and has about quarter inch of play but only happens when it's warm. Could the throttle body springs be getting stuck when it gets warm?


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I thought you said that when it gets warm the idle raises to 3500 rpm? Let the bike get warm and then set the idle to 1200 rpm instead of 3500 rpm.

The idle screw is set and I've checked for vacuum leaks and got nothing. It will raise rpm once I touch the gas and like I said go anywhere from 3500 - 5000 rpm even if I tap the throttle. I have to let off the clutch to drop the rpm and then it will idle normally again. I've checked all the basic things so far. Also when riding the bike when I clutch in the rpm raised when it's up to temp (completely off the throttle). Choke it off. Maybe something is binding in the throttle cable but when the rpm's raise its like I'm on the throttle slowly giving it gas. Also maybe the throttle shaft seal needs replaced or carbs rebuilt. I'm not sure I'm stuck. It still rides but it's very annoying


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[chris900f]

So, assuming you have checked and adjusted the cables: An intermittent air leak at higher temps points to the carb-to-head seal.

Try riding the bike (somewhere safe) until you induce the problem, pull over, get it in neutral, then while sitting on the bike
just reach down around the tank with both hands (both sides) and lift the carbs gently from the bottom. Try pushing them towards the head, wiggle  etc.

If this makes any difference you can focus your efforts on the carb-to-head boots, the O-rings between the boots/head,
or the torque on the Phillips screws that secure the boots to the head. Those screws should be tightened with a manual
impact driver--they can't really be secured properly with a screwdriver without stripping the heads and will loosen up over time.

[Bluesmudge]

I think chris900f is dead on with his diagnosis. You have an air leak that is not significant enough to effect the bike at idle, but once you increase the vacuum (by adding throttle) enough air is sucked into the air/fuel mixture to make it run lean. Confirm this by shutting the bike down right after the rpm's first start raising and then pulling the spark plugs. I bet they are pretty white, showing its running lean.

I can't remember, what year is your GS500? Unless its relatively new, its probably due for new carb-to-head boots and o-rings.



One other, but less likely, possibility: Check the carb's CV diaphragm to see if it has any damage or pinholes. Damage to that diaphragm can make the throttle slide misbehave.

[mr72]

The problem with any diagnosis that equates to "it's lean" is that a lean idle will stall, not surge.

The reason the idle stays high is because for whatever reason the needle doesn't go down into the main jet, which means the slide is staying high for some reason. It will be very rich in these conditions unless you also have the throttle plate stuck open, normally by setting the idle speed too high.

If you have vacuum leaks, you might wind up trying to set the idle speed really high just to get it to idle, and the reason you wind up with it hanging like this when it's hot is because the leak goes away when stuff expands and re-seals leaks.

[herennow]

Josh, I understood that an air leak between the carb and the cylinders can act a bit like the butterfly opening, in essence allowing more air volume to get sucked in and thus letting the engine rev faster. It would run Leaner but it can run quite a bit leaner with no load, before stalling.
Although a leak that bad would probably affect the whole throttle range....

[mr72]

I doubt it. That air doesn't go past the jet, so it doesn't pick up any fuel. With a closed throttle, that would be insanely lean. Even just a hairline crack in the seal at the intake boot would let in like 10x as much air as the pilot air opening, I'd guess.

if you could mechanically hold the slides down and then try opening the throttle to get it to rev I'm sure you'd find it bogs right off and won't make enough power to overcome pumping losses to turn the motor at 3500 rpm.

If there are intake manifold o-ring leaks, which are very likely, then you would certainly have to compensate by jetting up and adding pilot enrichment.

Anyway, bike idling at 3500 rpm when hot, IMHO 99% is because the idle is set to 3500 rpm when it's hot. Lower the idle to 1200 rpm and then you'll find the real problem is with cold starting procedure or extremely lean pilot mixture, likely due to clogged pilot jets.

[Bluesmudge]

I doubt it. That air doesn't go past the jet, so it doesn't pick up any fuel. With a closed throttle, that would be insanely lean. Even just a hairline crack in the seal at the intake boot would let in like 10x as much air as the pilot air opening, I'd guess.

What you are saying makes sense to me on a carb with a purely mechanical connection to the throttle slide, but with a CV carb won't an air leak create additional vacuum which will raise the throttle slide? OP is saying that their bike runs fine hot at idle, but then if he gives it gas then the RPMs go up but won't come back down which makes me think something is preventing the throttle slides from dropping back down.

This is assuming OP is correct and the bike truly does idle in neutral at 1200 rpms when the engine is hot. But if the bike starts easily and the RPMs slowly climb until they run away to 3500 rpm, then yeah the idle is just set too high. A lot of people don't like how cold blooded the GS500 is so they set the idle too high.

[The Buddha]

Blue - a vacuum leak will cause a loss of vacuum and that would cause the slide to not open as much. Its not an air leak, we refer to it in rather weird terms, its a vacuum leak. There's no high pressure till past the intake valve and only when valve is closed.
Cool.
Buddha.

[anesbitt267]

I think chris900f is dead on with his diagnosis. You have an air leak that is not significant enough to effect the bike at idle, but once you increase the vacuum (by adding throttle) enough air is sucked into the air/fuel mixture to make it run lean. Confirm this by shutting the bike down right after the rpm's first start raising and then pulling the spark plugs. I bet they are pretty white, showing its running lean.

I can't remember, what year is your GS500? Unless its relatively new, its probably due for new carb-to-head boots and o-rings.



One other, but less likely, possibility: Check the carb's CV diaphragm to see if it has any damage or pinholes. Damage to that diaphragm can make the throttle slide misbehave.

Thanks you guys have been lots of help, it's an 06 I bought off some hick and he said it wouldn't idle so he adjusted the screw. I think some issue was masked when he messed with the idle screw and I'm sure I have a leak somewhere from what you guys say. It will idle fine but I tap that throttle it'll rev up and I have to give it a load to return the idle back to normal. All the seals look fine but it wouldn't hurt to replace the boots and clean the jets. Once I find time I'll give it a go. Just don't like the thought of messing up the carbs considering I'm new to anything carb related.


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[anesbitt267]

Blue - a vacuum leak will cause a loss of vacuum and that would cause the slide to not open as much. Its not an air leak, we refer to it in rather weird terms, its a vacuum leak. There's no high pressure till past the intake valve and only when valve is closed.
Cool.
Buddha.

Also something I might add, I do believe my valve cover gasket is leaking if that can cause said issues


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[The Buddha]

Blue - a vacuum leak will cause a loss of vacuum and that would cause the slide to not open as much. Its not an air leak, we refer to it in rather weird terms, its a vacuum leak. There's no high pressure till past the intake valve and only when valve is closed.
Cool.
Buddha.

Also something I might add, I do believe my valve cover gasket is leaking if that can cause said issues


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Nope. You have to have a leak in the air flow through the engine. Exhaust leak can cause it too, heck yea, if it happens when hot but not cold definitely figure checking the exhaust too.
Cool.
Buddha.

[mr72]

I doubt it. That air doesn't go past the jet, so it doesn't pick up any fuel. With a closed throttle, that would be insanely lean. Even just a hairline crack in the seal at the intake boot would let in like 10x as much air as the pilot air opening, I'd guess.

What you are saying makes sense to me on a carb with a purely mechanical connection to the throttle slide, but with a CV carb won't an air leak create additional vacuum which will raise the throttle slide?

OP is saying that their bike runs fine hot at idle, but then if he gives it gas then the RPMs go up but won't come back down which makes me think something is preventing the throttle slides from dropping back down.

Not in theory. A leak downstream the throttle plate will increase air pressure or reduce vacuum, reducing the pressure that lifts the slide. The bike will stall.

This is assuming OP is correct and the bike truly does idle in neutral at 1200 rpms when the engine is hot.

The problem is that eventually the slide will fall and the idle will return to some low idle speed, even if the throttle is still open due to high idle speed setting. It will idle low-ish because it's very lean.