I am working on an 87 Trail Boss 250. We bought this roughly a month ago. The starter wasn't working, but we could pull start it. It took quite a few pulls to get it to start. After a couple times of starting it, it stopped starting. I put a new starter on it, and rebuilt the carburetor (rebuilding carburetors isn't my specialty so I hope I didn't mess something up). We put it back together and still couldn't get it to start either buy pulling the cord, nor using the starter which the new one works fine. So I looked through the threads on here and saw that maybe there was a compression issue. I tested the compression and it was at 80psi. Based on what I have seen here, it seemed like that was way to low. So we took the engine out and redid the upper engine, had it bored out and put in a new piston. I put the engine back together and tested the compression at we are now at 118psi. So that seems to be in the tolerance of what I have seen posted. But it still won't start. I also replaced the ignition high tension coil because the resistance on the old coil seemed high at 10 where the repair manual I have says it should be at 6. I also tested the pulse coil and stator coils and they tested within limits. We still can't get the engine to start. I know fuel is going to the carburetor but it really seems like it isn't getting fuel to the cylinder based on sound because it doesn't act like it really wants to fire, it just is turning over. Any suggestions for what I can look at or do to get this to start? I am really not sure what else to do.

Thanks so much in advance
Richard

 

First off if compression is just 118psi on a fresh bore then it's still too low.Somethings wrong somewhere.. Piston to cylinder clearance is tight.. .0011-.0021 no matter what year model on the 250. That's why I set them up at no more than .0015 clearance if possible.Some other things to check are for any leaks.
A leak down test is another thing to do on an old engine if it wasn't completely rebuilt. Crank seals can leak even if it has good top end compression and the engine wont run. Basically you have to have good compression,should be around 125-135 psi or more on a fresh top end,a good bottom end compression ,NO leakage from crank seals,head gaskets,base gaskets or engine cases. Good spark,good fuel supply and good air flow to the carb. Simple engines but everything has to work properly together or you're just wasting money on a partial rebuild. On older 2 strokes if people just wanted cylinders bored for take out,I'd do it,but with a warning on what else it might need..