YZ250 Rebuilt top end, still no compression


I have an 88 YZ250. Just rebuilt the top end and theres no compression. It sounds like the air is escaping somewhere inside the top end. Any help?

did you put the head gasket or orings back in?

yea i put all new gaskets on the top end. i took the air filter off and it feels like air is being pushed out instead of being sucked in. Is that normal? I don't think it is but i really dont know

could that happen if he has the piston bacwards?

If you installed the piston backwards you bent all three intake valves = no compression.

88 YZ 250 is a two stroke. Sound like you have a bad or broken reed valve.

how do the reeds effect the compression? makes no sense to me. once the piston makes the stroke and blocks the ports up the compression has no where to go besides the head, between the rings and the cylinder or out a port if the piston is backwards

Sorry thought it was a 4-stroke, anyway yes reeds can cause problems. Think of your reeds as a intake valve, if it does not close it will not run correctly. With the reed cage in your hand and looking through it from the intake side you should not see any light, if so there is the problem.

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