Runaway idle! What's the cause?

jimw · Posted: Mon Aug 11, 2008 6:05 pm Post subject: Runaway idle! What's the cause?

My bike is a single cyl. 250 w/ Mikuni (BS34 I think) carb.

Sometimes when I goose the throttle from idle the revs keep on climbing and I have to hit the kill switch. Doesn't happen every time. The cable isn't stuck. What could cause this? I have a theory, but I'd like to hear experienced input.

Thanks.

unclejack857 · Posted: Thu Aug 14, 2008 6:56 am Post subject:

Possible carb. slide sticking?
Jack

jimw · Posted: Sat Aug 16, 2008 6:41 pm Post subject:

That was my theory, actually. That or a weak spring on top of the slide. Looking at the slide off the bike, the slide falls, but somewhat slowly maybe - if doesn't slap down if that's what it's supposed to do. It doesn't look dirty around there though.

But say the presence of vacuum made the slide continue to go up, say because of a weak spring above it. But the closed throttle plate ought to be killing the vacuum, so that couldn't happen. Still scratching my head on it.

Johnny5 · Posted: Mon Aug 18, 2008 8:17 pm Post subject:

Have you checked for air leaks around the carb boots?

jimw · Posted: Tue Aug 19, 2008 4:14 pm Post subject:

Not yet. Might try spraying something in the area and see if it changes speed. It has a metal tube vacuum takeoff molded into the rubber intake. I don't like it, but visually it looks okay.

I've leaned the mixture since then and it hasn't happened since, but the idle is a bit floating now, doesn't doesn't come down as fast as I would like when the throttle is released.

Also, the distance between the airbox and the intake is too great - although both are original and the carb is original, but for better fit they should be at least 1/4" closer together. From the fische, it looks like all the parts are present there though.... going to take it to a pro to get the brakes done and will have him look at the carb too, I guess.

Johnny5 · Posted: Tue Aug 19, 2008 4:47 pm Post subject:

Possible plugged pilot jet?

jimw · Posted: Tue Aug 19, 2008 7:57 pm Post subject:

The jets were spotless, along with the general appearance of the carb innards. Led me to not blow out any of the passages, which maybe I should have.

Johnny5 · Posted: Wed Aug 20, 2008 7:48 am Post subject:

If the pilot jet wasnt plugged, then the rest of the carb was probably fine.

Does your carb have the rubber diaphram on the top of the carb? Sometimes these get little holes in them.

jimw · Posted: Wed Aug 20, 2008 12:41 pm Post subject:

I glanced at the rubber for cracks and tears. Didn't check for pinholes. A new diaphram is over $100. If it had a small hole, I might try making a little patch. The worst looking thing was probably the floats. One of the corners looked rough, but not like a hole exactly. I could check the level pretty easy. Getting the float pin out looks like the kind of thing that turns into a problem though. The factory manual for this is terrible - it says like for example "remove the pin and then remove the float", instead of saying how to remove the pin Smile

Johnny5 · Posted: Wed Aug 20, 2008 12:46 pm Post subject:

As long as the float...well, floats, it should matter what it looks like or how banged up it is. Unless it's rubbing on something, prohibiting it from it's range of motion.

I know what you mean about the manual. "See engine. Remove engine. Put engine back in after taking it all apart."

I have an extra set of carbs for my bike that I was planning on selling, but for the cost of those rubber diaphrams, I just might keep the carbs incase the diaphrams in my bike get a hole.

jimw · Posted: Wed Aug 20, 2008 1:17 pm Post subject:

The price I related is an online dealer's price for a brand new one from Yamaha. It might have to come as part of an assembly, can't recall. I think it was $132.

reserected · Posted: Fri Aug 22, 2008 5:02 am Post subject:

hey jimw. after reading above you and johnny5 have it pretty much covered...one thought though,i had an almost inperceptible problem with the enrichment circut (choke on most bikes ) staying open even with slack in the cable. just a tiny bit of "sticking" caused a little fuel to creep in. not enough to flood (that would be too obvious lol ). just enough to mess with the brain after checking all the usual causes. good luck Exclamation

jimw · Posted: Fri Sep 26, 2008 1:31 am Post subject:

Just a quick update/revelation on this carb problem, in case it might be useful to someone.

Everything I read on carb adjustment said start a turn or so out on the mixture screw and adjust it for highest idle speed. On this Mikuni bs34 carb that apparently doesn't apply. The runaway idle was in fact what I had the carb set to do. The revelation came when while it was running away, I backed out the throttle screw to slow it down, to what is in fact its idle. I found good info on this carb in one of the groups that clued me into this. On the bs34, the way to adjust it is to turn the throttle screw out as much as you can (closing the throttle plate) and the mixture screw in as much as you can, while still maintaining a good idle. There is critical interplay between the mixture screw and the throttle screw. Runs great and sounds like a motorcycle now. "bum bum bum bum bum bum bum" Very Happy