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All,

I checked the compression numbers on my 3 mowers, throttle wide open and decompression still functioning. All I did was remove the spark plug and pull hard several times to get these readings.

GCV 190 - 91psi
GXV 140 - 101PSI
GXV 120 - 70PSI

I installed new rings on the GXV 120 and oddly enough it has the lowest readings. It smoked before and now it does not.

Are these numbers good for each engine? Why the difference between the 120 and 140? Just the setup of the decompression timing?

Old post. But what kind of compression tester? You really need to use a small engine specific tester, it has a second schrader valve on the tip of the test (where it goes into the motor)

ham404 said:

Old post. But what kind of compression tester? You really need to use a small engine specific tester, it has a second schrader valve on the tip of the test (where it goes into the motor)

Click to expand...

In my opinion and observations of my Actron tester you are wrong about having two schrader valves. You only need the one that closest to the cylinder and it must of the light weight spring type. Having one at the gauge end just would throw off the readings as you must pressurize the hose every time the cylinder go through a compression. Adding a second one will also add restriction to opening up to pressurize the gauge.

It is also to note this compression number can vary quite a bit depending how well the valves are adjusted on these engines with ACRs.

I used a very basic compression tester with one Schrader valve. Had the tester forever so I am sure the is nothing special about it, for small engines or the like.

jsalis57 said:

All,

I checked the compression numbers on my 3 mowers, throttle wide open and decompression still functioning. All I did was remove the spark plug and pull hard several times to get these readings.

GCV 190 - 91psi
GXV 140 - 101PSI
GXV 120 - 70PSI

I installed new rings on the GXV 120 and oddly enough it has the lowest readings. It smoked before and now it does not.

Are these numbers good for each engine? Why the difference between the 120 and 140? Just the setup of the decompression timing?

Click to expand...

Bypassing the tester discussion for now and answering the original question
Piston rings seal by an oil film between the ring and the bore
The ring is pushed against the bore by air pressure that gets behind the ring ,
Getting this to work takes quite a bit of time so it is quite common to have a lower compression ratio right after re ringing an engine which will go up once the rings have bedded in.
A biger problem with two strokes than 4 strokes.

As for the number in theory it is atmospheric pressure x the compression ratio so roughly 15 x cr
Note STP would be 14.7 at sea level but because we are used to 1/4 hours , most people can multiply by 15 in their heads .
Remember atmospheric pressure changes all the time , remember all those H's & L's on the weather map and gets lower the higher you go .
This is one reason why engine makers do not publish compression figures , that and the variability between testers

When engines are made they start with a bore & stroke then over bore it to the next size the stroke it to the next size then bore to the next size etc etc etc
With high performance engines they will use 2 different pistons for each bore size, one for the shorter stroke & another for the longer stroke.
With mower engines, the prime factor is cost so generally the long & short stroke versions of the same bore will use the same piston so the CR's fluctuate as you progress up the displacements thus the actual PSI's will fluctuate.
The actual number is not all that important , what is important is plotting the reduction of this over time.
However no one bothers to take compression pressure readings of their engines when new

A very loose rule of thumb is 4 strokes you need 50 psi or better for the engine to run and double that for 2 strokes

Thank you very much for the in-depth reply. Very informative. Makes me feel better about my re-ring job.