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

The correct method to do a compression test is to crank the engine once and record the reading.   I.e. 60 lbs.,  crank again, record reading, i.e. 90 lbs., crank again record reading i.e. 125 lbs.  Crank again record reading i.e. 125 lbs. again.  When the gage no longer shows an increase and stays at 125 lbs.  that's your compression.   I just went through the process today working on my  1976 Yamaha RD400.   By the way, the engine should be at operating temperature when doing a compression test. 

trouts

the reason the compression is higher on electric start is because your moving the piston quicker and the air doesnt have time to leak by the piston rings, like borat said make sure the engine is at operating tempature.

Picked up an 8hp Snapper the other day in showroom exterior condition. I brought it out for a test and it was running solid, out distancing my 8hp Toro (125 psi cold) and 7hp Toro (110 psi cold).

The first compression test of the Snapper was 45 psi with the electric start. I'm not sure if that test was hot or cold. Earlier today I did a test again cold and got 45 with both pull and electric.

That test was such bad news I repeated the test after warming up the engine and got 45 with both pull and electric. This does not seem right but it's a new compression gauge.

I read some more on the net and some places mention reversing the spin on an engine to release a decompression mechanism. Does that apply to an HSK80?

I can hardly believe the low reading. If there's any snow left tomorrow I will have to try it again against the other machines.
trouts2

The first thing I think of when someone says "its working but the guage reads funny" is to suggest checking to see if the guage is measuring what you think it is. I.E. is it calibrated in PSI instead of Kilo Pascals or Kg per Square Cm. Many units I've seen over the years have dual ranges and I get caught reading one and thinking the other.

I've also had problems with pressure leaking out the sides of the rubber gasket when using a guage that you press into the spark plug hole while trying to pull the cord.

The gauge is in pounds per square inch. The gauge is probably accurate. I've got a few rigs here and the others test at reasonable values for their age. This week I got a 2007 5.5hp Ariens and it tests at 95 with the pull start and 160 with electric start (both cold). <BR><BR> This morning a wet test was done with the engine warmed up and 1 1/2 table spoons of oil into the plug hole which got a 55. The dry was 45. From my reading around the net it looks like the machine has worn rings and a valve problem. The readings seem horrible but the thing throws snow very well. ?? <BR> <BR> I want to throw some more snow with it to verify it's power but it started hunting badly. I'll try to clean that up this morning and get out there and give it another test. <BR>Kedawei

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.