lean running 400

my buddy has a '98 yz400 that was running fine until it sat for about 6 months and now it won't run without the choke on. i have cleaned the carb very well and the jets were clogged but now they are all good, cleaned and changed the needle and filter. it starts first kick on the choke and runs great choked but as soon as you turn off the choke it starts popping and then dies and will not idle without the choke on. it has a full exhaust, powernow, and a 178 main. i think it is running out of fuel but i am stumped, any ideas would help.


Has your fuel screw fallen out?

no the fuel screw is still there... any other ideas???

Check your hot start valve, It might be leaking air past the seat. Plug off the tube going from the cylinder to the valve and start it up. If it's OK, than fix the leaking hot start valve.

i think it is running out of fuel but i am stumped, any ideas would help.


Rather than running out of fuel, it sounds like it's getting too much air, on the engine side of the carby or extra air is getting into the carby as suggested above. Are there any leaks in the intake hose(can't remember what it's called) that goes from the carby to the engine, are the hose clamps tight?

Make sure the pilot jet isn't clogged. It doesn't take much for it to run crappy (past experience). Take it out and look into it carefully. I just had a small piece of junk barely noticable and it caused the bike to not idle and pop really bad.

When you do push in the choke does the bike backfire on decel? Like the other post stated, check the hot start valve they can corrode and allow air to pass. Also, VERY IMPORTANT, and a lot less trouble, is to make sure there are absolutely no exhaust leaks. (1) at the head and header pipe and (2) at the mid pipe where it mates with the muffler. This is where my 400 had all its problem and all the jetting tinkering didn't do a thing. My symptoms were like you described, except I had backfire issues as well....I replaced both gaskets and all was well with the 400 .

Does the fuel screw still have the 0-ring?

Does the fuel screw still have the 0-ring?

i haven't had time to work on my buddies bike until this long weekend and so i read this again...

is there supposed to be an o-ring in where the fuel mixture screw goes in???

What needle did you change?

And I forgot to add did you get the slide put back in correctly. I remember some issues with the slide going in wrong?

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