Hi,

Anyone know what the compression should be for a Tecumseh HM80.

Thanks,

anything above 40-50 lbs. is good for any 4-stroke engine

anything above 40-50 lbs. is good for any 4-stroke engine

An engine with a compression release, such as the HM80 cannot be truly judged by compression. It will give you an "idea." A compression release being present, plus deviations in the release ramp height prevent a baseline being possible. On an HM80, I would expect to see more like 60 to 80, but again, it's not an accurate test.

Without a compression release some 4-strokes can be as high as 170.

A leak-down test would be the only viable method (other than tear-down and inspect/measure) to check the piston, rings, cylinder and valves. You would need to inject 100 PSI into the spark plug hole, while holding the crank at TDC on the compression stroke and expect to see a max. of 10% loss as a rule of thumb, although some mfg. publish their own loss specs.

Thanks to all. Your replies are very helpful.

Paul is correct as usual with the compression release and how it effects compression readings, your best bet is a leak down test but ill add that if your HM80 is for a snow blower and it has a plug in electric starter, I have seen those starters spin fast enough to bypass the compression release as if the engine was actually running. You may get a more accurate reading that way.

Another way to test compression that I believe I learned from 30yeartech is to spin the flywheel counterclockwise with your hand, and if it hits the compression stroke and bounces back then it generaly has enough compression to run the engine.

-Happy new year as of 10 minutes ago :) I need sleep.

-Justin

Paul is correct as usual with the compression release and how it effects compression readings, your best bet is a leak down test but ill add that if your HM80 is for a snow blower and it has a plug in electric starter, I have seen those starters spin fast enough to bypass the compression release as if the engine was actually running. You may get a more accurate reading that way.

Another way to test compression that I believe I learned from 30yeartech is to spin the flywheel counterclockwise with your hand, and if it hits the compression stroke and bounces back then it generaly has enough compression to run the engine.

-Happy new year as of 10 minutes ago :) I need sleep.

-Justin

That's an old mower trick (spin backwards, observe "bounce"), but some things don't change and are tried and true...
Just remember to always ground spark plug wires please, don't need to read about any of our forum folks in the paper as having lost digits or a hand. AND SOME 2-strokes CAN run backwards.