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

This afternoon I had the Snapper 826 out and an old Toro 724. The Snapper is the one with the wet dry of 55/45. The toro is 110/psi and I think that was cold. The Snapper out throws the Toro which is strange given the readings. ??

Again, reading through the back Abbys guide posts one poster said all small engine compression tests were useless because a decompression mechansims was used to reduce pressure so electrict starts could turn engines. Never hear of that before. The readings from other snowblowers seem to be reasonable so if it's a decompression issue then it's kicking in for only the Snapper.
dawei

hey trouts,

is the snapper ohv, if so they could use a governor style system that keeps the exhaust valve open until the the engine fires and rpm is lifted then valve works normaly (creating compressiom). they use it for easy pulling and on the bigger motors so the starter doesnt work so hard.

Niper99,
The valves are in the cylinder beside the piston so an L head. The engine is a Tecumseh HMSK80 155527S which I think is a specific version of a generic LH318SA.
Trouts

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.