Low compression on rebuilt ct90 k1 engine

Bertlin08

Bertlin08

New Member
I have recently rebuilt my 1969 CT 90 k1 engine, I board the cylinder 1mm over with a matching piston kit. I lapped the valves and set my timing and valve lash correctly, but I'm still only getting 50-60 PSI. I tried putting a bit of oil in the cylinder but it didn't noticeably improve compression. When I hold my hand over the oil dipstick hole I can feel a ton of blowby. If any one has any thoughts or ideas it would be greatly appreciated.
 
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Seems like the lapping didn't do it. Perform a crude leak down test on the head in your hand with a little brake Kleen carefully sprayed in each port, and watch if it leaks past the valve. It will. Bring two new valves with the head back to your buddy at the machine shop.
 
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I went and did a leak down test and there was indeed a very small amount of brake clean seeping past the valves. Ill be getting the new valves as you suggested. thanks for the suggestion.
 
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Sorry for the late update, Just found the time to start working on the issue again. I got the head back from a valve lapping with new valves. the seals are good but I'm still getting very low compression, like 40/50PSI. I found the oil passage way gasket crushed. the cutaway for the gasket in the cylinder seems much to shallow and is making it expand out not letting the cylinder head fully seat. If anybody has any suggestions for fixing the issue or any insight on why this is happening I would greatly appreciate it.
 
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When I installed the rings I was very careful with the orientation, as well as making sure the ring gaps were staggered while in the cylinder.
And when I tested the compression I did not touch the throttle, I don't know if that's a bad thing or not.
 
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Oh, Though when I tested for compression I didn't have the carb attached. When I say low compression I mean like 20 PSI. I really think that it has something to do with the oil passage way gasket getting crushed between the cylinder and cylinder head. you can see in the first photo what the gasket looked like when I took it out, and the second shows it fully seated in the hole. Has anyone seen anything like this, if so how did you fix it?
 

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Sorry I don't know the tolerances, and I don't have a bore gauge. Before the top end rebuild a got a new to me cylinder, I sent it to a machine shop to bore it out to fit a 1mm over bore piston that came with a set of rings. I checked the ring gap in the cylinder with a feeler gauge. it was well within tolerance, and the piston seems to be pretty snug to me.
 
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Bertlin08 said:
Sorry I don't know the tolerances, and I don't have a bore gauge. Before the top end rebuild a got a new to me cylinder, I sent it to a machine shop to bore it out to fit a 1mm over bore piston that came with a set of rings. I checked the ring gap in the cylinder with a feeler gauge. it was well within tolerance, and the piston seems to be pretty snug to me.
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You might be able to measure the piston clearance with a feeler gauge.

As far as the oil seal the counterbore for that seal should be the correct depth as long as the cylinder has not been altered.
The seal looks pretty chewed up possibly expanding onto the head gasket.
Incorrect seal?
 
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late to the party and not much to ad ,but your compression guage might not be reading correctly and could be off by a huge amount ,if you have a no name cheap guage I would only concider it a rough guide at best .You can verify it to some degree by trying it on your car or some other piece of equipment that is running properly . I used to have a guage that read about 50lbs short ,it was a great moneymaker for me as I would bring it when buying a dirt bike or boat motor and I could show the seller his stuff was junk .
 
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