Originally Posted by gfackrell
Long story short, I bought a long block for my 20 ft boat. Put it in and it was very weak. It was supposed to be around 275 hp, but obviously wasn't.
I did a compression test with it still in the boat, and got very poor compression numbers. I then pulled it out and checked to see if the cam timing was on, at the engine builders recommendation. It was. I then did a leakdown test and these are the results I would like help interpreting.
cyl 1 - 11%, just a tiny bit past the exhaust valve, the rest past the rings
cyl 2 - 15%, a little past the exhaust valve, the rest past the rings
cyl 3 - 25%, a lot past the exhaust valve, the rest past the rings
cyl 4 - 18%, a little past the exhaust valve, the rest past the rings
cyl 5 - 23%, a lot past the exhaust valve, the rest past the rings
cyl 6 - 26%, a lot past the exhaust valve, the rest past the rings
cyl 7 - 15%, a little past the exhaust valve, the rest past the rings
cyl 8 - 11 %, a little past the exhaust valve, the rest past the rings
I could not find any leakage past intake valves on any cylinder, but every single exhaust valve was leaking, some pretty bad.
My main questions is this:
Is ANY leakage permissible past the exhaust valves?
Way too much loss and too much difference between cylinders. But first how was this tested?
The engine should be brought up to operating temperature before testing. This closes the ring gaps to operating clearance. If you don't do this then a thick engine oil should be introduced before testing a 30, 40 or 50 weight oil so it can seal up the rings without being blown out by the test pressure. Keep in mind that an operating engine is also using cylinder pressure to push the rings outward against the cylinder walls which is what actually makes the seal. Cranking or static testing cannot duplicate this exactly, so the pressure loses appear higher in these tests than they really are. If these things aren't done the leakage rate will look excessive. Yours is high on some cylinders and certainly the differential between cylinders is way out of bed. I look for no more than a 5 percent differential between cylinders. The test should be done with each cylinder at TDC and the crankshaft locked so the pressure cannot rotate the crankshaft, this is both from a safety and unified test process standpoint.
However, I start getting concerned when the loss exceeds 10 percent when following the process I just outlined and the differential between the high and low cylinders exceeds 5 percent.
I'm be less than pleased with leakage past the exhaust valves that you're seeing. This can indicate poor seat conformability between the valve and seat or it shows weak springs. Intakes are more flexible than exhausts simply due to increased size they conform to the seat better. Exhausts need better attention paid to them when doing the seats and may need a bit stouter spring than the intake.
Boat motors work a lot harder than road vehicle motors so extra cautions are in order when it comes to cooling and lubrication as well as timing and mixture. It is mighty easy to toast the exhaust valves and certainly piston and bore wear are greater issues but that said at 20 hours unless the timing and or mixtures are way off you shouldn't see valve damage. Since you are; my list of suspicious charters would immediately turn toward ignition timing against RPM and mixture ratio's. If these things get out of line; the motor becomes toast pretty quickly. Detonation is much harder to hear from a boat motor because of all the other noise, if you're running into this problem it can do a lot of damage before your aware that things are amiss. Sparkplug color is not always a good indicator on boats as they tend to run quite cold when fired up which may carbon the plugs making them appear that the mixture is richer or the timing less advanced than it actually is. Boat motors are lot trickier to diagnose than car/truck motors in these instances.