high compression

When that happens to me, this is what i do;
I put the piston at the bottom and remove the rocker gear. I pressurize the chamber with the LeakDown tester. Then I tap the ends of the valve stems, while watching the gauge, and listening to the sound. If you do this to one of the good cylinders first, you will get to know the sound that a well-sealing valve makes, and how the LD gauge responds.For this test, you can use full compressor pressure up to something like 100 to 120psi.

What I am looking for is ever-climbing pressure, and what I am listening for is the very specific sound that a well-sealing valve makes.

And here is the why of it. Sometimes carbon can build up on the seat or the valve face, especially if your carb is running fat.Then it can hold the valve off the seat, and the compression suffers. The LD test should have caught it, but when not using full compressor pressure, it sometimes sneaks by.The tapping on the valve stem allows the spring to smash the carbon to powder, and the air pressure blows it outta there.
..... DO NOT TAP ON THE RETAINER.......
If you dislodge the keepers, the spring will send the retainer into outer space, and half your face too,if it gets in the way.
Now there is a chemical alternative to this. You pour this stuff down the carb throats in a very specific way, and then let it ferment overnight. Next mourning you start it up and clean it out by going for a spirited drive. I think there are at least 3 long-time suppliers of this wonder chemical, but I'll be durned if I can remember the name of it.
In any case it's a carbon cleaner.