Extremely low compression? Help!

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12/23/2018 6:46 PM

Recently bought a 1990 YZ80 to semi-restore and hang on to for nostalgic reasons since I had one when I was a kid. The bike appeared to be in really good shape for the yr with much of it still in original stock form (even original front tire). Im the third owner. Guy i bought off of said been sitting for some time and was hard to start. I cleaned the rats nest out of the airbox and cleaned carb. Could tell compression was really low by kicking it over so I tore it down and bored it once over and installed a wiseco piston kit. Now that everything is back together it still has no compression. Literally has 65psi doing a compresion test. Will not even start! Reeds look fine. What could i be missing that could pull the compression down that low?

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12/23/2018 6:52 PM

When you said you bored it over... you sent it to a shop with a piston and had it done? Like you made sure the cylinder and piston clearance was right?

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12/23/2018 6:57 PM

Check crank seals.

Paw Paw

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12/23/2018 7:00 PM

kb228 wrote:

When you said you bored it over... you sent it to a shop with a piston and had it done? Like you made sure the cylinder and piston clearance was right?

I work at a machine shop so I did the work myself. Wiseco states that it should be bored at the measurement that the oversize is, and that the piston is made to fit with correct gapping. I checked myself and had .003 piston to bore clearance.

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12/23/2018 7:05 PM

Paw Paw 271 wrote:

Check crank seals.

Paw Paw

Not totally sure how to do that . Any quick tips would be appreciated. Would that cause compression to be that low?

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12/23/2018 8:01 PM

Paw Paw 271 wrote:

Check crank seals.

Paw Paw

2stroke153 wrote:

Not totally sure how to do that . Any quick tips would be appreciated. Would that cause compression to be that low?

Never worked on a smaller bike. But to check them you usually pull the flywheel, stator, and its easy to do. The clutch side is the cover, case cover half (not the actual case) and the basket, hub, etc. and its easy to see as well.

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12/23/2018 8:14 PM

Take a look at this link, you need to do a leak down test. Harry Klemm talking about air leaks. http://www.klemmvintage.com/airleaks.htm

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