Could be a big issue

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#1
i got a 93 yz 125 and just got the top end rebuilt and honed. Before that it had a standard piston in it and not the right size and still ran. it now has the right size and takes a half can of starting fluid and a lot of kicking just to get it started. 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. im probably going to see if it is the jets. I got the manual and will see if they are standard if not I will change them. but somehow I feel this is not the issue. someone at a shop (c and c) told me it could be seals could be damaged or worn on the clutch side of the bottom end. im hoping this is not the case and would like some insight before I tear the engine down. I feel stuck
 

High Lord Gomer

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#2
I would recommend checking the compression with a gauge. If you really don't have sufficient compression, everything else will matter little.
 

High Lord Gomer

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#5
Off the top of my head I would guess rings not sealing. Not sure if a powervalve hung open would result in lower compression.

How did the reeds look when you put it back together?
 
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#7
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
 
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#8
rob- it had too small of a piston in it at first. the original is standard bore and whoever had it sometime way in the past resleeved and bored it so it wasnt standard. this guy that honed it and everything had been doing it for years and years he works on all small engines and owns his own shop then took it to another shop to have it honed he then cleaned it well. he even checked the power valve and said it was doing ok. he saw nothing wrong with it but i will look at the power valve and as for the compression test with my finger in the spark plug hole (from where i was told how to do it) it didnt move my finger away. however when i had the original piston on it something happened and it lost all compression. it ran like **** when i had the old one in the new one (when i actually get it started) it runs amazing. but zero idle.
 
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