Yzf 400 won't start need help!!!!

[Yzf 400 rm 250]

Hey guys I have a 99 yzf 400 that I bought cheep cuz it didn't run at the time. It has a brand new cylinder and head new plug the carb had been cleaned so you would think it would run... No it starts but won't stay running I've have had it idling but when I have it gas it would bog until high throttle so I adjusted the fuel air screw and now it won't idle! A little gas drops out the overflow so I'm lost I just can't figure it out!! Please help!!!

 

[dirt bike dave]

Have you checked the valve clearances?

If the valves are set too tight, the valve's won't close all the way, and the bike will be hard to start and it will run poorly. If you do get it started, the clearances will get even tighter with heat, making it impossible to re-start.

FWIW, the early YZ 4 strokes have a reputation as difficult to start (never owned one myself).

 

[Yzf 400 rm 250]

How do you check for valve clearance?? I've never had to do that.

 

[High Lord Gomer]

My experience has been just the opposite...if/when the valve clearance is gone, once it warms up they will develop more clearance and be easier to start. They are the hardest to start when completely cold.

The YZ400 has a fuel screw, not an air screw. Most 2 strokes have an air scren that is to the rear of the slide and it restricts the amount of air through the idle circuit. On the 400 it is a fuel screw at the forward portion of the carb that restricts the fuel flow through the idle circuit. Count how many turns it takes to screw that all the way in. it should be 2-4 turns from lightly seated / screwed all the way in.

As I recall, the 400s also had trouble with the accelerator pump delivering too much fuel when the throttle was twisted so you have many things to check.

To start I would suggest taking the carb completely apart and making sure that all of the jets and orifices are clean and not gummed up or clogged in any way. While it is off the bike set the float level and check to see how long the accelerator pump squirts fuel.

Checking / setting the valve clearance involves removing the valve cover, checking the clearance between the cams and buckets and replacing the shims under the buckets as necessary to get the proper clearance.

 

[BSWIFT]

The info from Dave and Gomer is spot on. Get yourself a manual and you will correct all the problems discribed. The fuel "squirt" issue was common and is easily corrected. After you are sure the valves are within spec and the carb is clean, make ONE change at a time so you can tell if it made a difference or not BEFORE you make additional adjustments. Making too many adjustments at once can lead to big headaches.