Join Date: Sep 2005
Location: Base of the Rocky Mountains
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ATV: ThunderLemon 1000
2008 Mileage: Depends upon Fa
Some of those older cats have Mikuni carburetors and the PMS screw is most likely a Air adjustment screw. On the Keihin CVT carburetors the PMS Screw is an Fuel adjustment screw.
On a Keihin Carburetor approximately 2 turns out from lightly seated would get you close enough to start the engine. Clockwise to lean, Counterclockwise to enrich the idle mixture.
On most Mikuni carburetor the PMS works opposite, Clockwise enriches the mixture and counterclockwise leans the mixture. Usually about 1 turn out from lightly seated will get you close enough to start it.
The PMS screw on both Mikuni and Keihin carburetors is adjustable for a reason because there is no particular ideal setting, Both require fine tuning one way or another to get a smooth idle and good starting characteristics in cold weather.
Idle speed adjustment and Idle mixture adjustments are always external to the carburetor float bowl for ease of adjustment and both should be adjusted to maintain smooth idle and good cold weather starting as needed.
In your case if you cannot start it at all the plug may be so badly fouled it won't start no mater what PMS setting you have it set for. I have no idea if your carburetor is a Mikuni or a Keihin either and if those base settings above and a new spark plug does not get you running you may have additional problems. Once again your engine should start and run over a wide range of PMS adjustment. It may not run smoothly until you get the PMS screw tweaked to an ideal setting open or closed from base setting maybe 1/4 turn at a time until you find the sweet spot. The sweet spot on a Keihin may be anywhere from 1 turn out to 3 turns out depending upon your engine and mods.
From what you typed I'm not sure what you adjusted was the pilot metering screw. On the attached drawing, #7 is the main jet, #8 is the Pilot Jet & #9 is the PMS adjustment screw. This is typical of all Keihin CV carburetors. Only the PMS screw and a knob for setting Idle speed is external to the carburetor. Not shown in this drawing is the Start Jet which looks identical to a main jet but has a smaller number to indicate the size. The Main jet is always located directly under the center of the slide needle where the Start jet will be off set from the center. I have no drawing on a Mikuni carburetor. Maybe another user here has an older 300 with a Mikuni that could share a picture of his carburetor and supply the base settings. If not you can probably Google it on the internet.
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Last edited by Wyo_H1_Cat; 03-19-2011 at 12:11 PM.