Newbie Help Please!

I have a stock 99 yz400f. The bike will only start and run while choke is pulled all the way out. Once warm if I push the choke in it will die, if I touch the gas with the choke out once warm, it will die. I have taken apart and cleaned the carb and accelerator pump. The only prob. I saw was the accelerator pump had quite a bit of rust in it. I cleaned it all out very well and still no luck. Is there anything special I should know about the diaphram inside the accelerator pump? It looks to be o.k. It shows some wear around the outside edges of the rubber and two tiny creases twards the middle of the rubber (where it meets the metal piece). Thanks a million guys for any help. Zuvee

There is normally a period during warm up when the choke is too rich, but without the choke, the bike is still too cold and lean to idle. This usually passes in two minutes of riding or less. It could also be as simple as having your idle speed set too low.

If it still has a problem, check the hot start valve to be sure it's closing, and the connecting hoses to be sure they aren't loose or leaking air. Then try adjusting the idle mixture (the needle screw that passes through the float bowl). You should have that set somewhere around 1 1/2 - 2 1/4 turns out.

If none of that does it, and your valve clearance is good, then you still have something in the pilot circuit in the carb obstructing the flow of fuel.

"idle mixture" Do you mean the bolt at the bottom, on the outside of the float bowl?

Yes. If you cleaned the carb, you saw a screw forward of the jet plug in the float bowl that is an adjustable needle screw.

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