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Pray tell, what would be symptomatic of air leakage around the idle mixture screws? I've replaced many other parts and it dawned this morning, while trying to start the beast, that perhaps that item needs attention.

Bad O-rings.

That's what I'm thinking. Don't know why I didn't think of it earlier!

Air leaking past the O rings will not be linear, meaning that sometimes the engine will respond to small adjustments of the screws and other times nothing will happen for half a turn or more as different amounts of air gets sucked passed the screw.

Don't forget to put a tiny bit of silicone grease on the O ring so that it doesn't get damaged on the threads during instalation.

Thanks for the note about silicone grease. And thanks for the observation about non-linear behavior - that accurately describes what I've seen recently. I have the O-rings on order from Bike Bandit.

On my way home from work, I bought some silicon grease. When I got home, allowed the bike cool down a bit then removed each idle mixture screw - one at a time - and lubricated the dry O-rings while I wait for the new ones. The O-ring on my left carb was allowing a wee of fuel to escape - thereby allowing some air in, as well. Hopefully the grease will help a bit until the new parts arrive.

It is very difficult for me to believe that air can work its way around all those fine threads and an o-ring with a tear in it or no o-ring and cause idle problems. I am pretty sure the o-ring is there to keep the mixture screw from rotating on its own and changing the setting. Before you install new o-rings, install the screws without the rings and see if you can tell any difference with and without.

When the new O-rings arrive, I will do as you suggest - and let you know what happens. I am not have any problems with the bike idling - it stumbles in mid RPM under light load. And fuel is escaping from the one idle mixture screw (not enough to form a drip), so air could be sucked up as well.

George Ryals wrote: I am pretty sure the o-ring is there to keep the mixture screw from rotating on its own and changing the setting.

I thought that is what the spring is for.

Greased old O-rings - not much better. Strange, starts OK, idles firm and smooth when warm, pulls under load to redline, stumbles under light load cruising between 3,500 and 4,200 rpm.

Could it be the needles (new 3 months ago) needs to be lifted a notch?