Question...yz400

(1999 yz400)Hi there, my bike is running fine it seems but to start it I have to turn the idle way up or it won't start it sounds like it wants to start but won't idle, once i turn the idle way up it will start fine but then in 1 sec it revs right up as the idle is turn up high. Then I turn it back down to normal idle. I just putted up and down my street seems to be running fine but really havn't fully check it out I brought it back into the backyard after that........i tested starting it but still had to turn the idle back up to get to start and idle........

what are the reasons its doing this? only mods to carb was boyeenseed quick shot, power now. I belive before I installed the carb back in i adjusted my fuel screw to 1 and 3/4 turns out.

any suggestions to whats going on?

That's usually from a lean condition, and will cause what's called a hanging idle, too. Give it a quick look to see if there's anything loose around the carb mount, etc., then try backing out the fuel screw some and try to get it set right.

thanx gray i'll try that asap i know i did make sure the boots were tight because i knew that can cause problems but i'll check again and adjust the fuel screw...

Something to watch for is the carb mount loosening due to shrinkage. The clamps for this have little spacers in them to limit how tight they can be so that the rubber doesn't get cut or distorted by over tightening. If the rubber gets old and shrinks, the clamps won't put enough pressure on it to keep it from leaking.

A clue this is happening is that you can rotate the carb in its mount with the clamps tightened. Another test is to use a carb cleaner spray, preferably one with alcohol and ether in it, such as Berryman's B12 to spray around the suspected leak area. If it's leaking, there will be a change in the idle speed.

well i adjusted the fuel screw to two full turns out and checked the boots and they were tight, now I couldn't get it started at all........yesterday i had it running but only starting with idle turned way up now i can't start it at all :bonk:

Try turning the idle back down a little?

Double check the cam timing, just for fun.

ok i'll try the idle adjustment i know my cam timing is correct i even took a picture of it before i put all back together.....If i had itrunning fine(not starting fine) yesterday wouldn't that mean cam timing is ok? or can cam timing just mess up your starting? but still run ok??

ps im getting an aftermarket fuel screw its a Pain in the ass to adjust looseningthe boots and rotating the carb everytime.........btw ive noticed on ebay 3 fuel screws all our for kehin fcr carbs but they say yz426/450 is the fuel screw on the yz400 different? the part numbes are the same according to dirtbike parts finder for the yz426 and yz400 stock fuel screw

Not sure, but I don't think they're different. Someone here will know.

well we are back to yesterdays case.....i adjusted the idle screw down to low point, then kicked, adjusted it up a bit and kicked and it sounded like it wanted to start so i kept kicking and moving the idle up a bit it was getting better then it finally started and ran but the idle had to be high again....as soon as it starts and run its winds right up then i turn the idle down so it sounds normal idle speed then about 30secs it just dies........ive got the fuel screw to 2 full turns out right now.

ps. a few kicks there were backfire popping sounds

Sounds like a partially clogged pilot jet. Remove and make sure clean.

well same thing is going on, but its not dying it will stay at idle once ive starting it and turned the idle down i can rev it as well and it seems fine.........i had the carb off this winter and cleaned it all up, blew compressed air through everything, would it still be clogged i guess it could have gotten something in from the tank? i havn't riddenthe bike yet this year.........

When you had the carb apart did you remove the fuel screw. Perhaps the o-ring fell out if you removed the fuel screw. I do not know if this would cause your problem but it sure sounds like carb issues to me.

That would do it, alright. Good point.

hmm i actually can't remember if i removed the fuel screw im pretty sure I didn't, but it looks like im takingthe carb off the bike again i'll try cleaning out and i'll inspect the fuel screw........

thanx for the tips, yes its definately a carb issue....

My '98 YZ400 did the same thing. I cut a small screwdriver down so I can adjust the pilot screw with the bike running at low idle to make the adjustment. Stock is 1 3/8 turns out according to my '98 manual so maybe try using that for a starting point. I think there is a common thread in this forum that covers pilot screw adjustment.

i actually did that too, cut down a small screw driver but i couldn't seem to get it to turn i could only turn it with the carb tilted and a non cut screw driver, guess i don't have 'man fingers' strong enough to turn the screw at that angle ?

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