Strange, repeatable, fast idle problem.

Maniac · October 05, 2011, 05:24:13 AM

Ok, my cousin has been using the 2005 GS (no fairings, we've converted it to a naked since the fairings were trash) for about a month now after having the carburetors rebuilt and adjusted. It runs great, other than being a stubborn mule early in the morning, but that's pretty typical. He's started using it for a 60 mile commute on the highway, and it's had no problems with prolonged highway cruises.

However, it's got a strange, easily repeatable, fast idle problem. 99% of the time the idle is great, right around 1300 RPM when warm. However, if the bike is hot and is shut off for any reason and then restarted before it cools down, it'll idle at 3500 RPM (it does eventually return to the correct idle, but only after being ridden a bit). Basically we've only ever seen this issue after pulling over for a refueling stop.

I have no idea what could cause this, and have just decided it's one of those 'weird things the bike does'. I'm kind of thinking the vacuum slides are expanding slightly from the heat and sticking partially in the up position, but heck if I know. Any thoughts?

noiseguy · October 05, 2011, 06:36:10 AM

This is all tied in with the floating idle issue that these bikes have. Read up on that.

I have a theory that the idle starts to run away as the bike hits operating temps that are slightly higher than normal, but I've yet to verify it conclusively. Mine idles fine around town, but if I take it on longer runs on highway/interstate the idle starts hanging when I pull off back into town. I think it's running a bit hot on the interstate (prolonged 5-6K at 70 MPH) and that it then cools as it's run around town at lower RPMs and speeds.

Maniac · October 05, 2011, 08:15:31 AM

Yeah, that's kind of what I'm thinking too. Sitting stopped at a gas pump, I suspect that the carburators get heat-soaked and either one of the smaller jets gets vapor-locked or something expands and gets stuck (the slides, maybe).

Like I said, once you get on the road, after a mile or so it starts working fine again.

I can't imagine there is anything really wrong with it. I mean, it starts right up when cold, and once it warms up it pulls pretty good, and it's getting good mileage. It's just got this wonky idle at times.

noiseguy · October 06, 2011, 06:25:42 AM

I've about given up on trying to fix the issue. I've no vacuum leaks, carbs are in solid condition and rejetted richer, the valves are at factory spec and the carbs are balanced. I'll be dragging the bike out next week (heat has finally broken in SE FL) and at that point I'll look at everything, again, to see if I can find a culprit. I'm not optimistic.

I've found I can bump the idle back down by engaging the clutch while brakes are on and it's in gear... if the idle's getting out of control doing this lugs the engine back down to around 1200 and then it tends to stay there once you release the clutch again. I'm sure it's doing great things for the clutch...

tialloydragon · October 06, 2011, 03:20:48 PM

I have the exact same problem, and have dealt with it the same way as noiseguy; by letting the clutch out. It hangs around 3000, I let the clutch out and it drops to where it is supposed to be.

The only thing i haven't be motivated to do is to play with the air/fuel mixture screws any more than I already have (mine are drilled out, and I bought the ones that can be adjusted by hand.) It just doesn't bother me enough to f@ck with it and mess up its current equilibrium (and I'd rather me riding it than fiddling with it.)

I have some time this weekend to fiddle, though, so I might give it a go.

rkjjeep · October 07, 2011, 01:45:57 PM

Others will chime in and tell you this is a complex issue with many related and unrelated causes, and they will be correct, BUT - most of the time this issue is caused by a partially obstructed pilot fuel mixture system in the carb combined with the fact that someone has cranked the idle up a bit to compensate for the dirty pilot system. In YOUR case, I would first adjust the idle to just over 1000 with the bike fully warm. That might help. If you still have an issue, the right next step is to remove and clean the carbs. A thorough cleaning is best - you might get away with just removing and cleaning the pilot jets.

Maniac · October 07, 2011, 02:01:25 PM

Quote from: rkjjeep on October 07, 2011, 01:45:57 PM
Others will chime in and tell you this is a complex issue with many related and unrelated causes, and they will be correct, BUT - most of the time this issue is caused by a partially obstructed pilot fuel mixture system in the carb combined with the fact that someone has cranked the idle up a bit to compensate for the dirty pilot system. In YOUR case, I would first adjust the idle to just over 1000 with the bike fully warm. That might help. If you still have an issue, the right next step is to remove and clean the carbs. A thorough cleaning is best - you might get away with just removing and cleaning the pilot jets.

Right now it idles at ~1300 when warm. I don't think it needs to be cleaned, since we just had that done a few weeks ago, as well as new seals/gaskets put in the carburetors. They are about as factory fresh as they can get.

Guess he'll just have to live with it.

rkjjeep · October 07, 2011, 04:39:03 PM

OK - But TRY setting it at just above 1000 when fully warm. See if that makes the problem go away. Can't hurt to try.

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Strange, repeatable, fast idle problem.