Idle Revving

So, got the bike running relatively well at the end of last fall. Didn't have time to reaally fine-tune it, but it was doing well.

The only problem I was having was with the idle revving up really really high when I would come to a stop.

From the few times I ran it, I thought it may have been the temperature. It was pretty cold but not freezing. It *seemed* as though once it had warmed up it was OK, but that might have just been me mucking with the bike.

I would back the idle screw out some and it seemed to help, but maybe only by making the revving less extreme.

But basically I couldn't figure it out.

Is this common? I feel like I've heard of this.

It's still freezing but I want that d#mn bike running as soon as the weather's better so I figure I'd better get started.

was the choke on during the ride? also did you check for a vacum leak around the carbs

If the choke is not on, check to see if the throttle cable is binding or damaged.
On my bike, with the same problem, I cured it by syncing the carbs.

Several potential reasons for the revving at idle.

1. check throttle cable for visible slack at the bellcrank which is the butterfly shaped piece that you hook the throttle cable into on the carburetor

2. weak return springs... not normally an issue on a Mikuni assembly

3. throttle shafts binding... loosen small 3mm screws and retighten as necessary to align the throttle plates

4. air leak... can be in carb holders (should put some new holder on any old, used bike) or in caps on vacuum ports or in the vacuum line that connects to your petcock.

5. choke plunger pads are hard and not sealing. This allows air through the enrichener circuit and at idle will cause the revs to go up. To check this, jiggle the knob when the idle shoots up.

All good suggestions guys. Thanks. I'll check it all.

I was playing with leaving the choke open just a little bit to change the idle a bit when it wasn't perfect...

The carb holders (I assume we're talking about the rubber boots on the engine and air-intake sides) seem to be in good condition, but that doesn't necessarily mean anything. What is it? Spray them with something? Isn't WD-40 flammable?

I think the throttle cables are ok. I had them pretty snappy. I'll double-check.

What are the vacuum ports???

Is it #1 on this picture?...

kzrider.com/media/kunena/attachments/leg...ages/carbmystery.jpg

Not sure what you mean by the vacuum line either.

These all seem like possible suspects.

This could be directly caused by two possible problems. Number 1, air leak. Number 2, unbalanced carbs. I would check for an airleak first by fogging the manifold boots with ether or propane or something similar while the bike is running. If no leaks are found then I'd bet your carbs are simply out of balance. Unbalanced carbs can cause all sorts of strange problems, but they most generally manifest themselves with a wandering idle.

Interesting idea... never exerienced wandering idle due to unbalanced carbs... as for spraying carbs, that will work fine for external leaks; carb holders in particular but if the leak is internal as in the orings around mix screws or choke plunger pads worn, the spraying won't make a lot of difference. If you hit each of those areas very directly, you can get the idle to bog some with WD40 but sometimes you won't.

I'll fog away. Too bad I tried to start it in the cold the other day and my petcock lever snapped off in my hand.

Any idea on that picture I posted earlier? If those ports are supposed to be capped off or something, maybe that's my problem. they're just wide open...

The ports in the pic are overflow tubes, the tubes should have a open line going down past the swing arm.

ok, so both #1 and #2 in that picture are overflow ports?

OK, I'm opening this discussion back up, if anyone feels like commenting. I've got a few new details and hopefully they will help.

The bike has the same exact revving as it did at the end of last season so I'm in the same boat and it has nothing to do with the temperature out or anything.

Big Hint #1: The idle revving up toward out-of-control is highly dependent on the position of the choke.

• There's one point at which I can get it to stay stable, somewheres in the middle.
• Open it more (or put it all the way closed) and the idle starts increasing slowly toward very very high.
• Put it slightly lower and the bike stutters down and dies.

Big Hint #2: The idle changes without any interaction with the throttle. I believe this eliminates the throttle cables as the source of the problem completely, yes?

I'm going to fog the carb holders and choke plunger pads and mix screws, possibly tomorrow. If that doesn't do anything, I'm going to eye-ball-sync the carb butterfly valves and give it a go.

Any and all comments welcome. Thanks!

Well:

1. Sprayed WD40 all over the mating surface of the carbs and the rubber boots. No change there. The rubber actually isn't all that hard either.

2. Thought a leak in that feedback tube that runs from the engine-block back into the air-filter housing was causing it so I duct-taped the sh/t out of it and still no change.

3. Took the carbs out and synced them by hand/eye to what I believe is a pretty accurate set of carbs (it is only a twin). Installed them and still no change.

The idle revs up pretty darn quick if the idle is set (via the choke or the idle screw) higher than a certain point. I have no idea why and I'm at a loss here. There is a point where I can get it to not rev up and the idle wanders all around but anywhere higher and it heads toward 6000+ rpms.

Any other ideas out there on how to trouble-shoot this?

Thanks all. :dry: