Compression can't accurately be tested on engines with a compression release, but typically an engine with one will have between 60 and 90 PSI.
To check to see if a compression release is working there are 3 ways I can describe. I will ignore the type of engine (OHV / OHC / flathead) in order to explain all 3, as not all 3 can be used on all engines but then at least you'll be well informed. Modern engines may use the comp. release on either valve, old engines only used it on the exhaust (emissions reasons current versions).
1. Remove the rocker cover / side valve cover, and the spark plug. Turn the engine over by hand slowly, after observing the intake valve close, the compression stroke occurs and it is during this stroke you would expect to see one of the two valves "bump" (open a tad) in order to release some compression.
2. Remove the cylinder head. Follow instruction in #1 above and expect to see same results.
3. On older flat head engines, it may be possible to observe the valves through the spark plug hole and see the same results (a valve bump on the compression stroke).
There is also one important aspect to consider, and that's valve (cam) timing. The easiest was to check to see if the cam is in time (if the timing gear on the crank spun a bit or the cam boss is damaged and let the cam move away and jump a tooth or the timing belt jumped a cog) is to have the head off. What you do is to bring the engine up in correct rotation direction to TDC at the end of the exhaust stroke. Both valves should be open a tad. Rock the crank back and forth with the piston achieving TDC on and off and you'll see one valve close, the other open a tad, and again with the piston at TDC both should be open about the same amount.
When a mower blade strikes a tree stump, curb or other hard object that shock-load can affect timing gears and more (usually bends the crank). After all, there's a lot of inertial in the rotating assembly (crank, flywheel etc.) and when suddenly stopped or slowed dramatically the energy has to go somewhere.
Leak-down testing.
1. First and foremost, the crankshaft MUST be able to be held steady, and it can take quite a bit of force to do so. It helps to have 2 people, one to hold the crank with a wrench or other method, the other to perform the test.
2. Remove the spark plug(s).
3. You will need to determine TDC of the compression stroke, but if you have no method (such as observing the rocker arms or valves) if you bring it up to TDC on the exhaust stroke and attempt to do the test you'll know it, as all the air charge will rush out of the intake and exhaust. 4. Depending on the gauge you're using you'll need to calibrate (set) it before the next step. 5. Connect your test gauge into the spark plug hole cylinder of choice (with that cyl. on TDC as indicated above). 6. While having the crank held steady, charge the cyl. using the tester (they come with instructions) and note the reading. A maximum of 10% loss is usually the limit - anything more indicates a problem. 100% loss means you're on the wrong TDC.
Kawasaki has us perform this test for them on occasion, and also has us repeat it at BDC of the power stroke, in which case you do not need to hold the crankshaft (as both valves are closed and the piston ain't going to move the crank). This test would be to check the lower cylinder for advanced wear. Usually the top of the cylinder at the apex of ring travel is where the most wear occurs.
http://www.gregsengine.com/cylinder-leakdown-testing.html
This an excerpt from a Kohler service manual:
(manual found here on page 20, although Kohler service manuals are free at kohlerplus.com)
http://www.kohlerengines.com/onlinecatalog/pdf/sm_24_690_06.pdf
CYLINDER LEAKDOWN TEST
A cylinder leakdown test can be a valuable alternative to a compression test. By pressurizing combustion chamber from an external air source you can determine if valves or rings are leaking, and how badly. Cylinder leakdown tester is a relatively simple, inexpensive leakdown tester for small engines. This tester includes a quick-connect for attaching adapter hose and a holding tool. 1. Run engine for 3-5 minutes to warm it up. 2. Remove spark plug(s) and air filter from engine. 3. Rotate crankshaft until piston (of cylinder being tested) is at top dead center (TDC) of compression stroke. Hold engine in this position while testing. Holding tool supplied with tester can be used if PTO end of crankshaft is accessible. Lock holding tool onto crankshaft. Install a 3/8 in. breaker bar into hole/slot of holding tool, so it is perpendicular to both holding tool and crankshaft PTO. If fl ywheel end is more accessible, use a breaker bar and socket on fl ywheel nut/screw to hold it in position. An assistant may be needed to hold breaker bar during testing. If engine is mounted in a piece of equipment, it may be possible to hold it by clamping or wedging a driven component. Just be certain that engine cannot rotate off of TDC in either direction. 4. Install adapter into spark plug hole, but do not attach it to tester at this time. 5. Turn regulator knob completely counterclockwise. 6. Connect an air source of at least 50 psi to tester. 7. Turn regulator knob clockwise (increase direction) until gauge needle is in yellow set area at low end of scale. 8. Connect tester quick-connect to adapter hose. While fi rmly holding engine at TDC, gradually open tester valve. Note gauge reading and listen for escaping air at combustion air intake, exhaust outlet, and crankcase breather. Condition Conclusion Air escaping from crankcase breather. Ring or cylinder worn. Air escaping from exhaust system. Defective exhaust valve/improper seating. Air escaping from intake. Defective intake valve/improper seating. Gauge reading in low (green) zone. Piston rings and cylinder in good condition. Gauge reading in moderate (yellow) zone. Engine is still usable, but there is some wear present. You should start planning for overhaul or replacement. Gauge reading in high (red) zone. Rings and/or cylinder have considerable wear. Engine should be reconditioned or replaced.
