81 ATC200 Low Compression

[NickN]

Guys,

I have rebuilt my engine from ground up. I installed new valve guides, valves, piston, and rings. Had the new valves machined to fit, cylinder wall honed. However, when performing a compression check, I only get 135 psi or so. Therefore, connected a leak-down tester and at 100 psi, I can feel a small amount of air leaking from the crankcase vent. Also, there is a small amount leaking from the top end from the valve adjustment cap hole (removed cap to adjust valves).

Is a very small amount from the crankcase vent normal, as well as from the valve adjustment cap if off? Just not sure why I would have low compression. I may need to adjust the valves but even with no tension on them, I may have some leakage past the valves. Won't know till I can stop air leaking the other two places.

I did not bore the cylinder. Only honed and re-installed original spec piston and rings. I may need to install over sized piston and rings.

Total, with 100 psi, I'm loosing around 20 psi in leakage.

Also, would this cause a very rich gas mixture situation? My spark plug is black within only a few mins of idling.

Any advice would be great. Thanks!

[Cornholio]

I think, It may be because the rings have not seated properly yet. So the pressure slips by and blows out the crankcase breather. Im probably wrong, but thats my thought.

[CodyRosa]

You got the same problem I have right now. You have to bore the cylinder not just hone it. Your cylinder is more than likely an egg shape. OR you put the rings in upside down.

[Dave8338]

... or have the oil ring over lapped OR have all the ring gaps lined up.