Mikuni CV carbs - Pilot Screw: How Do They Do What They Do?

[mixelpix]

I have a '79 xs 400 which uses Mikuni CV carbs. For the longest time I thought that the lil' pilot screw actually controled a flow of fuel mixture through the little passageway it opens and closes that comes from inside the carb.

I now think that hypothesis is wrong.

Maybe I am crazy, but is the princple of operation instead controlling a vacuum of air which pulls up the diaphragm, the slide it is attached to and thereby richens the fuel/air mix while the throttle is otherwise closed (open only as slightly as the throttle stop is set to)?

If I am correct, then it would indicate that the function of idle is also affected by the smoothness of the slide, the pliability of the diaphragm as well as the relative clip position of the needle. Would also explain the need for the three position choke a little better...

Thanks for the clarification.
-Patrick "Using The Ol' Noggin', Or Is The Noggin' Using Me?" Kennedy

[mixelpix]

sorry, make that:

"Is the princple of operation using the pilot screw to control a vacuum of air which pulls up the diaphragm, the slide/needle it is attached to, thereby opening the main nozzle and enrichening the fuel/air mix while the throttle is otherwise closed (open only as slightly as the throttle stop is set to)?"

[randy]

Patrick,

Your original premise is correct. The screw controls fuel fed from the pilot jet to a small hole in the intake tract located just past the throttle plate. The screw setting determines how much fuel is dribbled out of the hole. Turning the screw clockwise (in) allows less fuel (lean), turning it counterclockwise (out), more fuel (rich).

HTH

[mixelpix]

guess i need to look again

hmm, maybe I need to better ventilate my garage...

So it is a fuel delivery passage. Is vaccuum pressure involved in the delivery? I ask cause my diaphragms don't have any leaks but do seem a little tired and my idle still breathes - ever so slightly now, not enough to knock out the engine anymore, but I was thinking that maybe there was more involved in the idle system besides the choke, balance, pilot screw and throttle stop screw settings? Oh, and a working petcock diaphragm/connected vacuum hose...

Maybe I am looking too hard, but I couldn't figure how the fuel gets up to the delivery passage to drop down into the throat... anyone have a good cross section illustration?

Thanks, -p

[John]

Pat, click on the members tab at the top of this page, and locate the member Skids. Click on his web site. he has some good carb info there. You'll find the answer.