Low compression on one cylinder

[nlan]

I have a Roadster, and after reading low compression on one of the cylinders (150-150-60 ) I decided to check valves/pistons.
Note that after adding a bit of oil in the low comp. cylinder, compression went from 60 to 100psi.

Valves seem to be in reasonable condition.
Piston seems to have been allowing some blowby, as I can see carbon residue below the second ring & onto the oil rings.
Inspected piston for wear, seems ok, lathe marks still there

Put the second piston ring in the cylinder, gap seemed reasonable. Same when I tried in another cylinder. It also looked fairly square with the naked eye.

Any suggestions? Should I just go ahead & buy new rings & valves and hope they will cure the problem?

Cheers
Nikos

[tolsen]

Re: Low compression on one cylinder

Have you got all pistons out? Are rings moving freely on the offending piston? How were gaps in piston rings lined up? Were gaps sufficiently far from each other thereby providing a labyrinth? Were rings gummed up? Can you tell any difference between cylinder bores? Have you measured ring groove clearances?

[nlan]

Re: Low compression on one cylinder

You're right, it could be the compression rings not being aligned properly. Unfortunately I didn't take note of that when I took the piston out. Rings move freely on it.

I only got one piston out, the offending one. The others are impossible to take out unless you take the whole motor apart, or mod it by drilling/cutting.

Rings had a lot of carbon deposits on them, especally the oil rings. Ring gap was similar when I tried the 2nd compression ring in two cylinders.