Leak down testing and compression testing, OMC 225 1976 Chevy 305

[supremei]

I have been doing some testing, there's a misfire I am trying to find. I thought maybe a bad valve. I did a compression test (Cold) all plugs out, throttle open. It tested between 148-158 on all cylinders, not bad, I think. Then doing a leak down test on all cylinders TDC each one, the reading is 40%-50% leakage.The air is coming from the valve cover vents. Pull covers make sure valves are closed and try to locate, it's coming from the block. When testing one cylinder the air is coming out both sides of the block or heads. So I am guessing that the rings are worn. I am running about 15 psi for the test. Why would I have that kind of compression test readings if the rings where bad. Is it time for a new motor? The boat does about 25 mph at 3300 rpm , the motor doesn't seem to weak. New carb , new manifolds.

 

[Howard Sterndrive]

well, air should come out - the ring gaps allow it through. It's all about the rate of leakage. Are you using a regulator? and if so - on which side of the regulator are you measuring 15 psi?
With your compression results, I might look elsewhere for the miss.

 

[Howard Sterndrive]

The boat does about 25 mph at 3300 rpm ,

that's not WOT though right?

 

[supremei]

3300 rpm is not wot, but can't get the boat to move much pass 25-28 mph at 4200 rpm.

Regulator, Yes, about 15 lbs air pressure and 45 % leakage

I was also testing to see if I had a bad head gasket, but there is a lot of air 40% plus, which makes it unable to test other items

Thanks

 

[southkogs]

What size boat, what prop and do you have selectrim?

 

[tpenfield]

I had a cylinder that was getting good compression, but 80% leak. Compression and leak are 2 different things. . . The leak test obviously more revealing than the compression test.

15 psi is not very much to be doing a leak test with. If your gauge can support it, go more in the 50 psi range. Some folks go 100 psi, but to me that is a bit scary if the engine should decide to rotate abruptly.

 

[supremei]

Prop is a 14Rad. 16 Pit. 23ft. aluminum pilot house. 4300 lbs

I will try the higher psi on the leak down,

Thanks guys