1st. Install a battery charger to insure constant cranking rpm.
2nd. Tie the throttle back so no fuel in the cylinders.
3rd. Use a different gauge with a different schrader valve. The wrong type of valve will give low results. A tire schrader valve used in a compression tester gives the wrong readings.

Crank each cylinder 5 times.
get back to us.

There is no sense to disassemble when a cranking compression, running compression, and leak-down will tell you what is wrong.