High Compression?

[Kurt]

So, I've been having intermittent issues with being unable to get my engine above 1700 rpm in neutral or in gear. I haven't dealt with this as the issue comes and goes. I'm in the process of doing a bunch of work to the motor so I naturally wanted to check the compression values prior to doing the other stuff (clean carb, replace wet exhaust that is about to burst, installing fresh water cooling system). When I purchased this boat (1976 C&C 33) one and a half years ago, I had a mechanic check the compression. He check it cold and all cylinders were at 110. I think I read somewhere that cold compression checks can result in low readings. So, I bought a compression gauge and did the check. I have: #1=130; #2=120; #3=120; #4=130. Should I be concerned about this? Does this possibly indicate excessive carbon build-up in the cylinders? If so, will this burn off once the motor is operating optimally?

A note on how I did the compression test. Engine warmed up. All plugs removed. Coil lead removed. With the gauge in each cylinder, I had a helper crank the engine in about 3 short bursts (stopping between each burst) until the gauge reached it's maximum reading on each cylinder. The reading would climb progressively with each cranking burst, but after about 3 series of cranks, the gauge would reach a maximum reading after which additional cranking would not result in a higher reading. I don't know if this is how to do it, but I did it this way in an effort to find the maximum compression value for each cylinder.

Any thoughts would be appreciated!

[marthur]

In think you have checked your compression correctly. And, compression numbers for a motor that have been warmed up are usually higher than the numbers for a cold engine, so I wouldn't worry about carbon raising the compression.

I am sure that a lot of A-4 owners are jealous of your high compression!

[tenders]

I'm guessing that someone removed your head and used one head gasket rather than two when they put it back on. The fact that the compression is so consistent across all four cylinders suggests to me that it isn't random carbon buildup--I think that would manifest itself with different readings across the cylinders.

How serious that is, I'm not sure. I doubt it's the source of the problem and strongly suspect it isn't worth taking the head off.

What does "replace wet exhaust that is about to burst" mean exactly? Your problem DOES sound like a partially plugged exhaust. The back pressure is preventing the engine from aspirating properly and it's limited in how fast it can rev. There might be some moving bits in there that periodically get out of the way and enable the engine to run normally from time to time.

[Kurt]

Thanks for your thoughts guys! Tenders - what I mean by the wet exhaust that is about to burst is this: The rubber exhaust hose going from the exit of the water lift muffler to the exhaust exit on the transom appears to be deteriorating. I can see a spot a couple feet downstream from the water lift muffler where the metal/wire reinforcement is rusted and popping out of the rubber hose. I am definitely suspicious that this could be causing elevated exhaust backpressure and contributing to my problems. In addition to checking the compression, I have taken off the water jacket side plate and cleaned some rust chunks out of there with a magnet, but it looked great in there. Actually surprising for a raw water cooled engine. I also pressure flushed the block and manifold and, after some black muck came out, all runs clean and clear.

[tenders]

> this could be causing elevated exhaust backpressure

I'd put a decent amount of money behind the idea that replacing this will not make your problem worse.