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My compression test

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anderbud

New member
1979 OMC 140hp. 181 4 cylinder Chevy Glastron Carlson CVX16SS
cylinder 1 110
2 100
3 95
4 120
Just purchased boat 1st time out seemed run fine, maybe
dissappointed a little with power
what kind of shape is my motor in ?
thanks
 
Most will say compression should not vary over 10 per cent from lowest to highest compression. 120 to 95 close to 25 per cent.
 
Ayuh,.... Ya got a rebuildable core motor, at those numbers,....

A good runnin' motor would have 150 psi, to 180 psi each hole,....
 
Run a quart of ATF through the carb and then leave it sit for half a day. Then take it out and run the heck out of it and see what happens.
 
How are you doing the compression test? I always check the compression three times through with a full charged battery engine cold 1-4 1-4 1-4 and use the high number for each cylinder. Crank the starter until the compression guage stops moving. Seafoam will clean up the rings and valves. Bring the engine up to normal temp and the at around 2000 rpms pour half a can of seafoam down the carb at a rate to almost stall the motor and quickly shut it down. Throw a line in the water and wait at least 1/2 an hour then point the boat at the big part of the lake and run it WOT for 20 minutes to blow the cobwebs out. If you not able to reach proper WOT rpms it may be over propped causing the engine to carbon up. Check the compression again and see if the numbers even out.
 
kimcrwbr1 said:
How are you doing the compression test? I always check the compression three times through with a full charged battery engine cold 1-4 1-4 1-4 and use the high number for each cylinder. Crank the starter until the compression guage stops moving. Seafoam will clean up the rings and valves. Bring the engine up to normal temp and the at around 2000 rpms pour half a can of seafoam down the carb at a rate to almost stall the motor and quickly shut it down. Throw a line in the water and wait at least 1/2 an hour then point the boat at the big part of the lake and run it WOT for 20 minutes to blow the cobwebs out. If you not able to reach proper WOT rpms it may be over propped causing the engine to carbon up. Check the compression again and see if the numbers even out.
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IMO, we should never operate any Marine Engine at WOT RPM for any longer than what's necessary while checking for correct propeller pitch and/or ratio........ 10 to 15 seconds should be long enough.



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RicardoMarine said:
IMO, we should never operate any Marine Engine at WOT RPM for any longer than what's necessary while checking for correct propeller pitch and/or ratio........ 10 to 15 seconds should be long enough.



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I run them at WOT for extended periods of time on a regular basis. If it comes apart, there was something wrong with it.
 
I suppose that we could all Roll the Dice! :D

I think that you'll find that most OEM manuals will suggest that wot rpm is for testing ONLY!


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The most economical is to run the engine at max timing advance. The engine will live alot longer also. It would help boat owners alot to know the proper curve for troubleshooting and to monitor on regular service intervals. But you need some kind of security clearance to get that info.
 
How did you do the compression test? I have always found it best if you loosen the plugs and then run it for a few minutes before doing the test. Reason being, sometimes but not always you can get a little carbon to break loose as the plug comes out. Often the loose carbon will get stuck under a valve which in turn will show up as low compression. So if you break them loose and then run a bit before doing the test any carbon that was loose should go out and not give you a false reading.

Engine up to operating temp when you checked? Some say that is important some not. It can change readings some times. Personally I look for readings to be consistent and I do not like burning my fingers on a hot engine so usually I check cold. Wide open throttle when checking? That can give you lower readings sometimes if the throttle is closed. What I am seeing from what you posted, maybe a little low overall and that has some variables depending on several things is the gauge correct, temp of engine, throttle setting etc. Right now the variation is more an issue than anything and the big question is what is causing that. Do you know how long it has sit before you got it? How many hours are on the engine? If it has sit for awhile and low hours then I personally would put a little time on it and recheck. If high hour engine that has been in service all along then it might be time for a rebuild.

Besides the low power complaint do you notice any other issues such as hard starting or poor idle. Have you done anything else to the motor besides checking compression? There certainly are other easy to fix (cheep) things that can cause lower power issues such as timing off and that should be checked through out the power range or fuel filters that are getting plugged or even old fuel in the tank. What rpm are you hitting at the top end?

Another good tool for checking out the condition of an engine is a vacuum gauge. It can show you things like late timing, bad valves, restricted exhaust, and most will also let you check fuel pressures up to about 6 pounds.
 
Update to compression test. As I tried to follow up with compression test after running the motor. Motor was not turning over as easy as should be. And not able to due compression test without running battery totally down. Pulled starter out and was going to have a load test done to it and found starter housing cracked. Installed new starter. Motor now turned over with ease. Did compression test after motor got to running temp and all 4 cylinders between 135-140
now tacking a close look at timing and carb.
Thanks for all your input
 
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