89 YZ250 wont start

[UnatralyAspirat]

I'm new to dirtbikes, been riding street and snow forever. I picked up an '89 YZ250 last week and rode it a few times and it ran fine. THe other day I went out and it wouldn't start. It is getting spark and appears to be getting fuel. I put a new plug in (old one looked fine) and still nothing. Drained the gas thinking it may have been old. Still nothing. I rolled it down the driveway and it ran for about a minute then died like it ran out of fuel and wouldnt restart. I pulled the carb and went through it, everything looks fine there. My next step is a compression test, what are good numbers to see? Its odd how one day it wouldn't start. The previous owner told me it was bored out and new piston/rings recently. What are symptoms of bad crank seals?

Now every once and a while when i kick it i get a fart through the exhaust but thats it.

Thanks,
Chris

[mxerncolo]

How does it seem to be running for that one minute that it does (if you push it). I had an '88 YZ250. It was a real drag to start, but ran pretty good. Needed starter fluid sprayed into the carb sometimes (think it was the compression).
jim

[UnatralyAspirat]

Jim,

I cant even push start it now. The one time it did run from push starting it seemed a little flat, and got flatter till it died. It died on the gas so I think something happened. It doesnt help that I only rode the bike twice before it happend so I dont really know what I'd call 'normal'.

Im gonna do a comp test and see what I get. then I wil start investigating

Chris

[UnatralyAspirat]

welp i did a comp. test and got a dissappointing 50 psi. so i pulled the head off and it was evident right away that something went down. the front of the piston was all dicked up. and i could see the edge of the rotary valve thingy in the exhaust port (sorry i dont know the lingo yet). i pulled the valve linkage cover off and found that the bolt that holds the cam to the rotary valve was loose, like almost out. so i think this allowed the valve to rotate too much and contact the piston/ring. the other kicker us there is about a 1.5 inch section of the top ring missing, gone... and a gouch in the front of the piston of the same width as the ring. this makes me think the rest of the ring is down in the crank case where it doesnt belong... this could turn into a pretty big project.

the cylinder has already been bored .020 over and needs to be bored again.. is this possible or do i need a new jug? where should i look for reman/re-sleeved jugs??? or would it be cheaper to find a used motor. i will probly end up splitting the cases to make sure the rest of that ring isnt gonna cause problems later on. so it will likely become a full rebuild... i'm not sure if this bike is worth it.

help me out guys

Thanks,
Chris

[CoalPilot]

Did you check your kill switch wiring? mine did that same thing. When i pullrd the tank off, I discovered the wire had rubbed off its casing and was shorting. I rewired it and it started right up and never gave me another problem....oh, and I found this out after installing a new stator, and coil, and CDI box...duh!

[CoalPilot]

It would be cheaper to find a used cylender than to resleeve it. You can go as high as 70 over on these. I had that issue once with a loose powervalve retaining bolt and seal. caught it early and it was fine. Use some locktite blue on the threads.