Compression testing.

[propane1]

So I want to do a compression test, just for the heck of it, on my 1982 Case 446, with a B43M Onan engine.
What is the correct way to do this ?
Does this engine have decompressioners ?
If so, what do you do about that ?
I have done compression testing before, but the testing methods varies between testers on how to do it. So, what is the way I should do it for this engine.
Thanks muchly


Noel

[Spike188]

From a B43 manual,

To check compression:
1. Run the engine until thoroughly warm.
2. Stop engine and remove spark plugs.
3. Remove air cleaner and place throttle and choke
4. Insert the compression gauge in one spark plug
5. Crank the engine and note the reading.
in the wide open position.

Refer to SPEClFlCA TlONS section for compression
pressures. There may be variations due to temperature,
atmospheric conditions and altitude. These pressures
are for a warm engine at cranking speed (about 300 rpm).

One piece missing in this is a safety tip on ignition wires. I prefer removing the power to the ignition coil while running this test or attaching the plugs to the plug wires and placing the plug on the engine in a place where it will naturally ground. Engines around here get a compression test when they will not run so step #1 and 1/2 are skipped.

Spike

[propane1]

Thanks Spike.

Noel

[Eugen]

Noel, the Kohler engines have the automatic compression release, so you can do a regular compression test on the Onan. Spike gave you the correct procedure. If you don't have patience and you want to know where you stand worst case scenario, you can run the test cold too, though it's not by the book. If you want to get funny you'll start asking yourself when or if your compression gage was calibrated. Does it show you +-2 psi or +-10? Nonetheless it's an entertaining endeavour to do it. I like doing it, at least for the entertaining value of it. If you measure anything extreme then the real fun starts.

Let us know what you read, so we can all make wild guesses!

[DavidBarkey]

From a B43 manual,

To check compression:
1. Run the engine until thoroughly warm.
2. Stop engine and remove spark plugs.
3. Remove air cleaner and place throttle and choke (wide open)
4. Insert the compression gauge in one spark plug
5. Crank the engine and note the reading. ( 5-7 revolutions)
in the wide open position.

Refer to SPEClFlCA TlONS section for compression
pressures. There may be variations due to temperature,
atmospheric conditions and altitude. These pressures
are for a warm engine at cranking speed (about 300 rpm).

One piece missing in this is a safety tip on ignition wires. I prefer removing the power to the ignition coil while running this test or attaching the plugs to the plug wires and placing the plug on the engine in a place where it will naturally ground. Engines around here get a compression test when they will not run so step #1 and 1/2 are skipped.
( I use a jumper wire to operate the starter solenoid with the key off so no power to coil and on pulse pumps I remove the pulse hose to the pump. )

Spike

Agreed , in ( ) is what I added

Dave

[propane1]

So I did a cold test yesterday and both cylinders were at 80 psi or so. 4 pulses.
Today after throwing some snow, figured engine would be warmed up. Left cylinder at a little over 80 at 4 pulses almost 100 at 7 pulses. Right cylinder still around the 80 mark at 4 pulses and still around 80 at 7 pulses.
Any way. Worked good today.


Noel

[Eugen]

Noel, what do you mean by pulses? Also, a fresh battery helps, so that it cranks fast and strong. Did you open the choke and throttle completely?


Also, repeat the test after you put a table spoon of engine oil through the spark plug hole. If the test reads significantly higher, you'll know the rings/cylinder is the culprit for the compression loss.

[propane1]

Each time it had a compression stroke that would be a pulse. Means it turned over 4 times or 7 times. Is what I think. Throttle and choke set to wide open.

Noel

[Eugen]

Ah I see! I don't know if 4 is enough, but if the engine cranks fast and strong the needle of the tester should stay at a value and no longer go up until you release the pressure from the valve.

[propane1]

Well I was taught years ago you only do 3 pulses. But any way, that’s why I asked what the method is to do a compression test. Because everybody’s methods are different.
So put the oil in and spun the engine over a bit to spread out the oil. Did the test. Left cylinder now a little over 110 psi and the right cylinder is 85 psi.
Any way. I was just curious, engine works fine. It’s a half good engine.

Noel