Posted by Hemimech [96.35.72.145] on Monday, August 04, 2014 at 23:40:11 :
In Reply to: how to run compression test posted by George in MIchigan [174.125.89.26] on Monday, August 04, 2014 at 22:29:42 :
Yes George you are correct. Ideally it's best to cut off the fuel to the carburetor also. That way you aren't pumping raw fuel into the engine & out to the muffler. A pair of vice grips with rubber over the jaws work nicely to temporarily crimp off the fuel supply. When performing a compression test always crank the starter for the same number of revolutions each time. You will hear the starter briefly bog down each time the cylinder being checked comes up on compression. So lets just say you pick 5 compression revolutions(20 crank revolutions). The first couple revolutions will fill up the compression hose & gage after that it should stabilize at a maximum achievable pressure. This pressure will vary depending on the engines compression ratio & the condition of the components. You will need to observe the speed of which the gage reaches it max value in relation to the other cylinders. Example: cylinder #1,3,4,5 maxed out at 135-140psi in just 4 compression strokes while cylinder #2 achieved just 95 psi in 5 compression strokes indicates that there is a problem in cylinder #2. Now isolating that problem is another issue, a leak down gage is the best tool for that. A leak down gage requires an air compressor capable of typically 100psi. The LD gage has a hose similar to a compression tester except it is open at the bottom - no Schrader valve. A LD test is done on each cylinder at TDC of the compression stroke. The Leak down tool has a gage that reads in % of cylinder leakage. A new engine would be at 5% or less while a high mileage one might be at 20%. 5 cylinders at 20% & one at 75% indicates a problem. The nice thing about a LD gage is that you get time to diagnose which direction the compressed air is going. Sound out the exhaust is a bad exhaust valve or seat, Air out of the carb is a problem with intake valve/seat. Air in the radiator is a bad head gasket. Finally excessive air out the breather or oil fill cap is a sign of worn rings/ scuffed cylinder bore or cracked piston.
If you have a compression gage with a removable hose you can do a poor mans LD diagnostic by removing the Schrader valve in the hose & connecting it directly to shop air that has been regulated to a reasonable level...say 50psi. Where the air escapes & how loud it is will help you diagnose the problem.