Occasional High Idle

Well I looked at the bypass valve this weekend. Dutch, I didn't have it in me to cut that pin off like you said. I was afraid I would end up with massive amounts (650cc's) of engine braking. I polished the end of it so it would interface better with that nylon plunger thingy in the carb and lengthened the spring free length from 22.7mm to 27mm. I ended up with increased engine braking and occassional high idles. But, not as high as before. The jury is still out.

The situation I ride in here in the Northern Michigan woods made riding with the carb not returning to idle unacceptable. The tight single line trails are not very forgiving with a 650cc dirt bike storming into the corner and the motor is at a high idle. I had to grab the clutch going into the corner and had lots of things to do already.

It is true that when you remove that nub off that diaphragm it makes the return to idle more abrupt, but it allows you more flexibility in your idle speed settings. You now can set the idle up a bit; I couldn't do that before. The fact is that the conditions up in these northern woods exaggerate this return to idle problem, perhaps your riding situation makes this not so critical. I had no choice; it was either fix it or replace it, (which I did later with a TM40 Mikuni).

How about instead of cutting off the entire nub, take off small amounts and check the results?

What pin? Did I miss something?Whats this about drilling holes in the slide? :)


It's hard to know what is right. If you removed the pin and got no high idle and I lengthened the spring and got less high idle and more engine braking I have to deduce that the bypass valve has a window in which it operates (Because we got the same results from oposite approaches.), like a PCV or EGR valve. I am going to try plugging the vacuum port. That way if I don't like the result it can be re-opened. I like your idea of setting up the idle to diminish engine braking. I should have never stretched the spring. Thanks for the info Dutch.


What setup do you run? Have you tried shorter peices?

I run mine stock. I've never had a complaint with it the way it is. The off idle hesitation is inherent an you learn to live with it or buy a new carb. I've never noticed the snatchiness others mention.

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