by Rick Lavoie, St Augustine, Florida, USA
April 1995
This is a two person job for safety reasons. Also be sure that your mag switch is cold and test to ensure that both mags are not grounded hot! You will be moving the prop and it is imperative that you know for sure that the engine will not fire or back fire!
First you will need to own or borrow a "cylinder pressure tester". Get one that has a built in "on-off safety valve". An air compressor is also needed.
1) Warm up the engine
2) Remove the top spark plugs
3) Rotate Prop until piston of #1 firing cylinder is on top dead center of compression stroke (you should feel air blow out of hole if you hold your finger there, be careful not to burn your finger. A tip is to put the adapter into the plug hole to prevent you from burning your finger.) Note the firing order stamped on the engine: you will test each cylinder in this order by rotating the prop 180°
4) Insert the adapter into the spark plug hole (if it is not already in)
5) Attach the tester to the adapter with the shut off valve in the OFF position. The other end is attached to the air compressor line. Set the air compressor regulator to about 90psi.
6) Have a friend firmly hold the prop in position, standing in a way that he can back off quickly if needed. Set the gauge on the tester to read 80psi, then slowly turn the valve from the off to on position. Move the prop slowly an inch at a time in either direction until you obtain your highest reading. Turn the safety on/off valve to the off position. Write down the reading for entry into your engine log book.
7) Move the prop 180° for the next firing cylinder (on my O-235 the firing order is 1-3-2-4). Move the adapter to that spark plug hole and repeat the process until all 4 cylinders are tested.
If you are obtaining a low reading, say 60psi or less, you can listen for escaping air (pumped from the compressor) and figure out where it is leaking from. If the air is escaping from the Carb intake, then suspect defective intake valve. If the air is escaping through the exhaust system, the you can figure on the exhaust valve causing the low compression. If the air is coming from the crankcase breather, then it may be the rings.
Hope this is of some help!