Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

After checking around the net and the library I have some questions about compression testing.

Is it correct that 60 is the general line for good and bad compression in snowblower engines?

Is 90 to 150 the expected for a new engine?

When you take a reading I think the cylinder would empty each cycle but not the gauge. If you go through several cycles of the engine then the needle seems to jump up at the first few rotations, then bump up a little more on the remainder, and finally hits a high point.

It seems that only one cycle would give a true reading but that does not seem to be the case when using the rope pull. The gauge moves more with a full pull and even more with a few pulls. When I use the electric start the readings are higher still.

What's the correct method of doing a reading for a small engine? With multiple turns of the engine the readings seem to be additive up to a max reading. ??
Dawei

I can confirm that turning the engine backwards will prevent the effect of any compression release mechanism.  These mechanisms open a valve slightly during the compression stroke of the engine.  This stroke would otherwise make pulling the engine more difficult.  Once the engine is running, the mechanism disengages allowing full compression.  Even at cranking speeds, however,  the compression does not need to be released during the power stroke of the engine.  In both the power and compression stroke, both valves are normally closed.  By turning the engine backwards, the piston travels up during the power stroke.  Therefore, turning the engine backwards will make the power stroke equivalent to the normal compression stroke, without the interferance of any compression release mechanism. 

This is why Briggs and Straton recommends spinning the crankshaft by the flywheel in the opposite from the normal direction with the normal spark plug installed. A sharp rebound (as it compresses the trapped air since the compression release mechanism is not active in reverse) indicates good compression.  The same method is sure to apply to a PSI test, its just a matter of finding a way to crank the engine backwards.

While this may or may not be a consideration, depending on the engine, I would try turning the engine backwards if low compression was determined cranking the engine normally- just to be sure.