Kohler CV740 - good compression, leak down failed??

[Turbodriven]

Kohler command pro 27. After warming up the mower I get about 160 PSI in both cylinders with my compression tester. But then, when I pressurize both cylinders to 50psi at TDC (poor man's leak down test) I get air hissing and escaping through the oil fill tube (cap off). Pretty fast actually, it even blows a little smoke/vapor out during this.

Have you seen this before? Is there another way this could be happening besides rings (or maybe head gasket?) on this motor? I didn't notice any noise through the exhaust or through the intake. So valves seem good. It's the good compression readings that have really thrown me off here .

Just for reference, its a Gravely 260Z with 1200hrs, and the motor is bogging under medium and heavy loads. Motor starts easy and sounds great at rest or light loads.

 

[Bertrrr]

I'd doubt any cylinder is leak free, if it runs good and not smoking there is no worry , why are you doing a leak down or compression test? It's not a head gasket leak or valves leaking , if so it would be noticeable when it is running - my suggestion to every situation is " if it ain't broke , don't fix it "

 

[Turbodriven]

I'd doubt any cylinder is leak free, if it runs good and not smoking there is no worry , why are you doing a leak down or compression test? It's not a head gasket leak or valves leaking , if so it would be noticeable when it is running - my suggestion to every situation is " if it ain't broke , don't fix it "

That's the issue, it bogs heavily under load. Mostly trying to get up small inclines or through medium thick grass with blades engaged. The mower slows to a crawl in those situations. I've been through everything, electrical (coils/etc) and in that process ignored the cylinders because I had good compression numbers from the start. But good compression numbers I guess don't tell you everything on these motors? Trust me, I'm confused as anyone. I don't understand how I can get good compression numbers AND have rings so leaky they won't hold even 50psi for more than a few seconds.

 

[Bertrrr]

If your rings are leaking you'd have lots of crankcase pressure blowing from either the oil fill cap, dipstick or any vent you have - would also be smoking like a train and plugs would be wet with oil residue .

 

[StarTech]

The poor man's leak tester can only give you the very basic info. It is how you do the diagnostics with either it or the better tester. You must remover the rocker covers as part of the testing. And using your ears you can telling if the head gasket are leaking into the push rod galley or not. Also with OHV engine you can use a modified leak down test to test the top to bottom piston stroke looking for oval or taper wear. Also the better tester will give you a percentage of cylinder leakage. Normally you will see up the 20% leakage even with new engines; although, it is usually around 10%.

In to do the modified leakdown you need a low pressure version and not one of those 100 psi models. This is why I like the tester I got from HFT as it tests at 12-15 PSI. At this pressure I am able to hold the flywheel still and also able to rotate it through the entire piston stroke.

 

[VegetiveSteam]

Anything over 150 psi compression is good on a Command twin. The air you hear when pressurizing the cylinders is very normal. What is your compression cold?

 

[Turbodriven]

If your rings are leaking you'd have lots of crankcase pressure blowing from either the oil fill cap, dipstick or any vent you have - would also be smoking like a train and plugs would be wet with oil residue .

It's not smoking a whole lot. Although it does suffer from the Kohler full throttle shutdown syndrome where it smokes briefly on startup if you don't let it idle before shutting down previously. And my plugs are not wet. Matter of fact, #1 cylinder is soot black (always) and #2 cylinder always looks golden brown/white after new or cleaned plugs.

 

[Turbodriven]

Anything over 150 psi compression is good on a Command twin. The air you hear when pressurizing the cylinders is very normal. What is your compression cold?

Haven't done a cold compression test. Kohler manual says engine should be warm. I can run one but I'm not sure what good those numbers will tell me??

 

[Turbodriven]

The poor man's leak tester can only give you the very basic info. It is how you do the diagnostics with either it or the better tester. You must remover the rocker covers as part of the testing. And using your ears you can telling if the head gasket are leaking into the push rod galley or not. Also with OHV engine you can use a modified leak down test to test the top to bottom piston stroke looking for oval or taper wear. Also the better tester will give you a percentage of cylinder leakage. Normally you will see up the 20% leakage even with new engines; although, it is usually around 10%.

In to do the modified leakdown you need a low pressure version and not one of those 100 psi models. This is why I like the tester I got from HFT as it tests at 12-15 PSI. At this pressure I am able to hold the flywheel still and also able to rotate it through the entire piston stroke.

I'll pressurize the cylinders again with the valve covers off to see if I hear the oil galleys making noise. At this point I already ordered head gaskets though (cheap) and am not scared to do a full rebuild with new rings and a hone job. At 1200 hours it certainly couldn't hurt. Especially if I'm going to keep this mower for a while. But maybe when I pull the heads the head gaskets will show me an obvious issue and I can wait till winter or beyond for the full rebuild.