# Compression on Tecumseh HM80



## Roper 826 (Dec 31, 2008)

Hi,

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

Thanks,


----------



## pyro_maniac69 (Aug 12, 2007)

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


----------



## paulr44 (Oct 14, 2008)

pyro_maniac69 said:


> 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.


----------



## Roper 826 (Dec 31, 2008)

Thanks to all. Your replies are very helpful.


----------



## justin3 (Apr 10, 2007)

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


----------



## paulr44 (Oct 14, 2008)

justin3 said:


> 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.
> 
> ...


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.


----------



## Ray C (Dec 31, 2015)

*HM80 tecumseh timing and solving high rpm spark miss*

When diagnosing a 8 hp Tecumseh engine model HM80 – 155043A on a 130 ampMiller/Canox portable welder, the following timing and values were noted for future reference
Values are approximate.
Measured in degrees of crankshaft rotation from 0 at TDC

Degrees
-15 intake starts to open
-10 points open- spark
0	TDC INTAKE STROKE
15 exhaust closed
123 points close
180 BDC COMPRESSION STROKE
240 intake closed 
Around 240 exhaust lifts slightly and closes for compression release
350 spark
360 TDC POWER STROKE
485 Exhaust starts to open
493 points close
540 BDC EXHAUST STROKE
705 intake starts to open
710 spark
720 TDC INTAKE STROKE 
735 exhaust closed

Other notes
Compression(dry) forward pull- 50psi before grinding valves, 80 psi after grinding and increasing exhaust cold clearance to 0.018” to improve forward compression (but harder to pull start). 
Compression (dry) when pulling in reverse direction to bypass compression release 100 psi.
Cleaned carb and optimized needles at 1 ¾ turns
Be careful with the points and coil insulation!!!!!! The unit would have a steady spark cranking and at idle and would either not rev up or if it did rev up , spark would falter /miss at high rpm with no load and would lose spark when a heavy load was applied. Under high cylinder pressure, the spark would jump from the points cover plate to the wire that goes from the coil to the points.
Solution- tape and silicone insulation between wire and points cover- and motor runs perfectly now J
Another problem was that the plastic recoil starter broke. To solve the problem and make the system most reliable, I went to a simple pulley with a notch to use a simple pull cord that wraps on the pulley for starting. An ideal stamped steel pulley was from a water pump from an older car that is offset that fit nicely with the flywheel and cover ( just replaces the recoil assembly).


----------

