The symptoms are:
Erratic idling, LOTS of oil blow by on the Crank vent at high RPM (+4000 RPM)
#6 sparkplug it’s quite oily. Compression on #6 sometimes reads right sometimes read 20% below the others.
What can I do to locate the problem?
Thanks,
Pacha

 

Could be a bent valve (and therefore a bad valve guide) as well.

You need to do a leak down test to see where the trouble is.

Basically, with the engine warmed up so the rings are sealing as best as possible, you shut down the engine, set the questionable piston at TDC compression (so the valves are closed) and attach an air compressor to the spark plug hole and apply air pressure...

If the air comes out the carb, the intake valve is messed
If the air comes out the exhaust, the exhaust vale is messed (not likely in your case)
If the air comes out of the crank case, the rings are messed

Some air is going to come out... the amount is the determining factor.
If the air comes out the crankcase, squirt some oil in the bore & quickly retest. If the volume of air coming from the crankcase diminishes dramatically, you have bad rings.

Oil will not usually seal up a bent intake valve.

Since the compression varies, this is a sign that the problem is not just bad valve seals.... so I avoided that issue. It is, however, a good indicator that the intake valve is messed (not always sealing & sucking oil). A broken ring is usually indicated by a consistent low compression reading.

This is all based on the info you provided. If the info is questionable (e.g.- error in taking compression readings), then all bets are off. It could just be a bad manifold seal at an intake port.

 

Thanks Tom, I'll try that