chadster54321 said:
I even put 40:1 mixture with 110 octane racing fuel in it and it still dumps oil out the bottom of the exhaust and the end of it. it feels like it has no compression either.
Using 110 octane gas is NOT going to help any with starting issues. If there is a general lack of compression, the higher octane fuel won't help anything at all and could make things worse.
I didn't understand your comment about it having the wrong size piston in it before. I am wondering if it has the wrong size piston in it now.
Did you rebuild the top end, or did you have it done at a shop?
The easiest thing to check first would be the power valve. I am not familair with that year of YZ but many of the YZs have a cover over the power valve linkage that can be removed and still run the engine. With the cover removed you should be able to see the power valve operate as you rev the engine.
A compression check is always a good thing. I would have recommended a compression check before you rebuilt the top end so you would have something to compare it to, but it is too late for that now. Do one now, before you tear it apart again.
How did the bike run before the rebuild? If it ran OK, then I would say that the problem has something to do with the rebuild. It is very unlikely that your crank seals would have gone bad as a result of the rebuild.
It is possible that a ring was broken when the cylinder was put on. This would certainly result in poor compression. If you haven't run it much you might get away with just new rings. It is also possible that a broken ring could score the cylinder wall and force a bore job / re-sleeve.
Rod