'82 GS750e Persistent hanging idle, not an air leak?

[Gort]

Hi everybody, long time lurker since before I bought my '82 GS750e from an older gentleman who could no longer ride. In fact, the community here is what convinced me to pull the trigger on the listing when it came up. As an amateur wrench on some newer bikes and cars I thought it would be a fun project, and thus far it has been!

When I got the bike it wouldn't start, and had a laundry list of minor issues. It is stock with the exception of a V&H 4 into 1 the previous owned put on. So I started following the incredible advice on offer here, and heeded the refrain to go through the existing knowledge.

1. Cleaned carbs, full dip and ultrasonic, replaced gaskets, checked float heights, bench sync, fuel screws 2 turns out

2. Replaced o-rings via cycleorings

3. New intake boots and airbox boots

4. Adjusted valve clearances, replaced valve case gasket

5. Replaced rusted out gas tank with clean OEM tank, replaced petcock with new OEM petcock (vacuum style with screwdriver turn, no lever)

6. Replaced boot between airbox & air cleaner. Ran silicon gasket maker around the seam of the airbox which is of the plastic variety with two molded halves with a seam running around the center on all sides. No identifiable holes, punctures, or issues.

7. New air filter

8. Changed oil and oil filter

9. Replaced spark plugs

The bike now starts easily, idles at 1100 until it gets warm, then surges to 3000-3500 rpm and hangs there. I've already got new OEM intake boots and airbox boots, and I've doublechecked they're installed properly and tried spraying both water and carb cleaner to test their seals with no response to either.

I checked throttle cable routing, and it's good with a little play, no binding. I've checked choke cable routing and it also seems to be good, although I did replace the OEM lever with an aftermarket, it doesn't appear to be holding it open or anything of that nature.

I've taken the bike out a couple times to see if there are any other symptoms, but she runs beautifully up through 5th, with great power and throttle response. There is some popping from the exhaust when I go off throttle and decelerate. Aside from that the only issue is the idle hanging around 3k. Edit: There is also significant, noticeable, blueing radiating out from the exhaust flange on #1 that does not match 2-4.

I've got a Carbtune, but as the old girl sits at 3k while warm that seems to be out of spec for syncing per the manual as well as the advice on Bikecliff's site, so I haven't yet tried it. That said, plugs 1 and 2 are too light and look lean, 3 is a little better, and 4 is a chocolate and looks right. From what I've read the aftermarket exhaust shouldn't be making it run lean enough to require rejetting, and the fact that there's disparity between plugs, and it doesn't hang until after warm doesn't seem like that would be the problem? It makes me think this might be a sync issue, but I don't know enough to say.

At this point I've scoured every old post I can find, and can't seem to make any headway on what to try next. Any advice would be accepted with extraordinary gratitude. Thanks!

[Nessism]

Try 3 or 3.5 turns open on the pilot screws to see if that helps. And muddle through the carb sync. I suspect the idle will settle down after the butterfly's are properly adjusted. And don't forget to put a fan in front of the engine when doing the sync.

Good luck

[Gort]

I'll give that a shot and report back, thank you for the guidance

[timebombprod]

Lower idle when you get the rest done the bike wont idle as high as it does when it gets warm, make sure. I have my mixture screws a bit more than a full turn out from stock for my mac exhaust

[Nessism]

Lower idle when you get the rest done the bike wont idle as high as it does when it gets warm, make sure. I have my mixture screws a bit more than a full turn out from stock for my mac exhaust

Not true. A creeping idle when hot has nothing to do with the base idle speed setting.

[salty_monk]

As Nessism says if it's definitely not an air leak then you need to open up the screws. It's rare that you need a bigger than stock pilot jet with any setup & yours definitely not. You could consider going up half a step on the main jet (which also affects that circuit too) but I've run your setup on a 1000 with stock jets before. I'm betting 3.5 turns out will cure it....

[timebombprod]

Not true. A creeping idle when hot has nothing to do with the base idle speed setting.

Helped me for sure with my idle surge and hang

Edot: misread, but still a big part of my surge was when my bike warmed up fully and the idle adjustment knob was too high

[Killer2600]

replaced petcock with new OEM petcock (vacuum style with screwdriver turn, no lever)

Interesting, my '82 GS750E has the lever as does the Suzuki parts microfiche of the OEM petcock

The bike now starts easily, idles at 1100 until it gets warm, then surges to 3000-3500 rpm and hangs there.

That's the same behavior that occurs when you start the bike with the choke. You didn't mention if you used/needed the choke to start the bike cold. My idle mixture screws are at about 2-1/2 turns out with a stock exhaust so I would recommending going that many turns if not more out on the mixture screws.

[Gort]

Interesting, my '82 GS750E has the lever as does the Suzuki parts microfiche of the OEM petcock



That's the same behavior that occurs when you start the bike with the choke. You didn't mention if you used/needed the choke to start the bike cold. My idle mixture screws are at about 2-1/2 turns out with a stock exhaust so I would recommending going that many turns if not more out on the mixture screws.

You're not mistaken wrt the petcock. I replaced the rusted out tank that came on the bike with an '80, so same fit, different petcock.

I do need maybe 1/4 choke to start the bike cold, take it off entirely within a minute or two which is when it idles happily at 1100 for a few minutes, then jumps up to the aforementioned 3000-3500. I've tried getting it warm enough to start off choke, but still idling within spec, then turning her off, disconnect the choke cable, and start back up again to verify it isn't still engaged.

I've got some time today so I'm going to adjust the mixture screws and see if I can get her synced. Fingers crossed my next post will be about a success.

[Gort]

Success!

Try 3 or 3.5 turns open on the pilot screws to see if that helps. And muddle through the carb sync. I suspect the idle will settle down after the butterfly's are properly adjusted. And don't forget to put a fan in front of the engine when doing the sync.

Good luck

Just an update, this was dead on. Turned the pilot screws out to about 3.25 and played with the idle screw and that got it down to about 2k. Did the carb sync, 2 and 4 were pretty off. Got them synced up, reset the idle adjustment screw, and now it's a steady 1100 idle when warm. Thank yall so much for the advice and assistance! Excited to get back out for a ride tonight once the sun starts setting and it's not almost 100f degrees outside.

[eil]

Nice to see a successful resolution!

[ironbiker]

Where are these screws,my gs1000 is doing the same thing,driving me nuts...have a picture of location? THANKS MUCH!

[ironbiker]

Found the screws..backed them out...WALLA! -------->Fixed problem