I have read hundreds of times about this test.
Could someone take a minute to give it to me laymans terms, I understand it is done inplace of a plug, per cylinder, but what gauge, how do you do it etc.
What numbers are good, better or bad?
Please be specific.
Thanks in advance.
Bryan

Thanks for the info, I am catching on....
What about "leak down"????
Do you just leave it in for say 24 hours and then read it?

Just trying to get smart.
Thanks again.
Bryan

O.K. I'll try to give this one a shot...
Let's start at the beginning. First you need a compression guage. This test should be done with the engine warmed up to get best results. Next, pull all the plugs out. Disable the ignition system ( pulling out the lanyard is the easiest ) , then screw the compression guage into a spark plug hole. Hold the throttle wide open and crank the engine over. Observe the guage, you should get a good amount of compression on the first stroke of the piston ( you can tell when the piston pumps up by looking at the guage, the needle will rise and the guage itself will shake a bit) You should see at least 75 lbs on the first hit. Then let it crank maybe 2 or 3 more times. Record your readings, then continue to the remaining cylinders. The guage should read about 150 lbs on a stock engine.

Now, if you really want an accurate test, try a leak down test. It is a little more complicated to do, but it is a whole lot more precise. This test will give you a percentage of how much pressure is passing through or bypassing the cylinder by either rings or valves. I have seen engines with good compression have bad or marginal leakdown numbers.

OOPS... FormulaFastech posted while I was typing the above out. I type real slow...

A leakdown test is done with a leakdown guage. To do this, you need a compressor. You have to get the piston to top dead center or to the top of its' compression stroke. Screw in the guage, connect the guage to the compressor, then read the leakage on the guage. You will always have some leakage. If you ask 10 people you will probably get 10 different answers on how much leakage is acceptable. I always go by 10% maximum. While the air is in the cylinder, you can determine where it is bypassing by listening to either the top of your carb or your tailpipe or through your oil fill cap. Wherever you hear air leaking is where your problem is. On almost all motors, you will hear a little air escaping while listening to the oil fill cap. This is normal. A little air is always lost through the rings. The carb and the tailpipe should be silent, if you hear air then it's time to pull the heads and check the valves.. This is where leakdown can tell you what is going on in the motor. A compression test is too inaccurate. With that piston being pushed up the cylinder that quickly, it will easily force enough air into the compression guage while only a little is slipped past a bad valve. You will never be able to pick up a slight problem with compression tests.

I think I will start with the compression check...
I have NO issues or problems, I just always wondered what you guys were talking about, it is always the 1st piece of advice given along with a leakdown, which sounds a bit over my head at the moment! LOL
Thanks, I appreciate the time for your slow typing.
Thanks again
Bryan

I think you can find a differential compression gauge, also known as a cylinder leakage tester, in Summit and Jeg's catalogs. Because the leakdown test is the only approved method of verifying aircraft engine cylinder condition, you can find these gauges at aircraft supply houses like Van's, Sporty's Pilot shop, and Aircraft Spruce. Of course, if you've got a buddy that is an A&P mechanic at a local airport, just borrow his or have him help you do it.

Becareful when doing the leakdown test that the motor is at top dead center. If it is a bit off, it can rotate the crank when the cylinder fills and it will take of unexpectly... For checking valve leakage it can be down at bottom dead center, but you wont get accurate cylinder wall wear reading.. (it may not leak past the rings at the bottom of the cylinder). I also take a rubber mallet and give the valves a whack to reseat them, sometime a piece of grit can keep the valve off the seat, that is if I find air escaping past one of the valves..

Good luck... learn as you boat, I think that's what they call it..

Hey guy's, I have been wondering about this as well.
How long do you wait until you hit the 10 % leak down #, and know what is good or bad. I assume that if there is a problem one or 2 cylinders will be way off. I am looking at buying a leak down tester to check mine over the winter, so if I have a problem I can fix it now before it blows and leaves me without a boat all summer. I don't have any problems with the way mine runs, just trying to prevent any major problems.
Thanks!

I just did a search for a leak down tester.
Snap-On $288.00
everyone else $70-$99
WOOOOOOWWWWWWW, I know Snap-On makes good stuff, but that is as bad as boat parts.

28Eliminator;

It only takes a few seconds for the guage to get an accurate reading. As Mr Gadgets said, just make sure the cylinder you are checking is at T.D.C. because the motor can spin over from the air pressure you are puting in the cylinders.
I have a Moroso leakdown guage. I think I paid $75 for it. As usual, Snap-On is overpriced.