250x m300EX

[KeHall]

Ok Im about to loose it ive got the 87 250x with the 300ex top end ive tried moving the needle iv cleaned the carb checked the plug put the air box on and it still wont run right as soon as u touch the fuel it dies I was wondering if by some way the key is messed up on the magneto even if off by a fraction would couse prob right it will idol but backfires every once in a awile all i know is this $250.00 project is becoming a pain in the butt any help would b app

[bwamos]

I know you said the cam timing was right on your former post.. but I want to just tripple check with you to make sure it's all aligned correctly.


Cam sprocket is on the cam with the lobes pointing the opposite direction of the center timing mark.

On the flywheel there's several marks.
Theres a || mark. Ignore this one..
Then there's a |T you want the notch in the plug hole to align with the | next to the T. Not the T.

Now make sure the cam sprocket marks are in line with the gasket surface of the head with the center mark pointing up.

ALL slack on the timing chain should be on the carb side, as that's where the tensioner is. There should be no slack on the exhaust side.

Once that's all confirmed.. then we're going to have to lok at some other things.

Did you use a brand new head gasket? It almost sounds like your getting a head gasket leak. Sucking in air on the intake stroke.

All sealing points between the carb and engine are sealed. (You can use a little petroleum jelly w/ a little food coloring at the seal points to see if you have a bad seal. It will suck it through leaving a tell tale red streak on the inside. ;)

Your jetting should be close enough to run. So it's not jetting.

[KeHall]

we did not use a new head gasket the motor come in a box apart and the head gasket looked good but looks can b deceving as for the timing that is how its lined out now I guess I need to order a new head gasket and star ove rthat just might b teh best thing to do

[bwamos]

Yea grab a new headgasket. Th eold one may have been used. And once "crushed" won't reseal properly.

It really sounds like you've got an air leak if your timing is good.