Motorcycle Repair: 1982 Honda z50r, psi reading, punch mark

Question
Hello

I have no (20) compression in the cylinder. Should I replace the piston and rings, just rings or should I be checking something else first.

Thanks

Darren


Answer
Darren, the first thing to do is to adjust the valves! Very often they are overlooked and with only .002" to begin with, the clearances tend to tighten up and then the valves either burn around the edges or they just don't close enough to trap compression. Generally, however, tight valves are a slow process and compression will be more towards 80-100 psi. Check the valve clearances first at the T mark when both valves are closed (compression stroke), NOT when both are slightly open (overlap stroke)!

In all likelihood, a 20 psi reading indicates massive compression loss due to broken rings, seized/scored piston and cylinder walls. You won't know until you tear down the top end for inspection. It will probably be fairly evident.

The other, outside possibility is that the engine was apart and reassembled with the cam timing in the wrong phase and the valves are bent from contacting the piston.
Cam timing needs to be set when the piston is at TDC (T mark on flywheel) and the punch mark on the camsprocket is aligned with the notch in the cylinder head opening.

If the bike sat for a really long time, I have seen the valves get rusted, corroded from being opened to atmosphere in a moist climate. If the valve adjustement attempt yields having to screw the valve adjuster screw wa in then the valve is stuck open. If the adjuster screw has to be backed WAY OFF, then the valve head could be burned off around the edges.

Without knowing the history, these are all of my best guesses...

Bill Silver
www.vintagehonda.com