My sons 98yz250 runs fin for an hr then just stops and we have to put a new plug in and it go.s again?

Question

my sons bike yz250 run well for about an hour then cuts out and won;t start until we put a new plug in it the plugs not the problem as we can use them in my other sons bike no probs yz125 we can go through 4/5 plugs a day what could it be when running it.s not hard to start first kick

Answer 1

I had this problem with my yz. it turned out to be the magneto. dont ask me how but once i changed it the bike ran fine and i didn't have to change plugs anymore.   But before you do that make sure the gas/oil mixture is right. it could be that it is too rich in the 250 and just right in the 125. Good Luck, hope this helps.

Answer 2

You have some good answers here. Having the wrong fuel mixture can foul out a plug pretty quick. If your plug is the wrong one, then it can explain why it fouls out in your 250, yet doesn't in your 125. Compression would also explain this, well the lack there of actually. I would suggest looking for some information about the plug specs first, then if it turns out your plug is the correct one, then you need to look elsewhere.

You can used a compression tester to see what yours is like and compare it to what it's supposed to be. If that is ok, then the spark isn't hot enough to burn off all the oil. Good luck.

Answer 3

When was the top end last rebuilt, Ceck the coil.

Answer 4

I'm going with barry's answer - top end rebuild (cylinder, piston & rings).

Low compression won't fully burn the oil in the pre-mix and eventually foul the plug.

Throw in a new plug and zing-za-zing-zing-zing, away you go.

The other bike probably has better compression.

Good compression would allow the engine to run with a semi-fouled plug.

Answer 5

change your gas/oil mixture.your running to rich and it foul's the plug.don't go to lean either or you'll sieze the motor.also check the plug wire.sound's like a bad connection maybe!

Answer 6

Maybe the wrong plug in the first place?

My advice: NGK Spark plug BPR8EIX

They cost more then most, but are well worth it.

Answer 7

could be a carb problem. It sounds like it is fouling the plug.