1988 Formula 311 420's B/W Trs about 500 hrs. Gil manifolds

I opened up my boat yesterday to find one motor fine and the other was locked up. I took all the belts off to make sure it wasn't an accessory locked up and put a wrench on the crank and could move it a little. I pulled all the plugs and #5 was wet. Cranked it over and out came a bunch of water. I did a compression check and #5 had 225 psi, just like all the other cylinders except #7 and #3 which only had 160.

I know water didn't sit in there all winter because it cranked for a little bit and then locked up, so it was obviously in the exhaust and as soon as that valve opened it just filled the cylinder

I pulled the manifold on that side and pressure tested it and found a pin hole right in the #5 vane. I know it didnt freeze because when I drained that manifold it was full of antifreeze. (I meant to drain it when I winterized it but I guess I missed one)

My question is why do you think those other two cylinders have lower compression, and what should I do next? Im gonna replace all four manifolds.

Also I'm looking for 4 Gil manifolds in good shape, preferably fresh water, or headers for reasonable money

Thanks
Chris

You may just have rust on the valves from the moisture in the exhaust, this will cause improper sealing. You could pull the rockers off and smack the valves with a hammer a couple of times, this will sometimes bring the compression up. I might also suggest a leakdown while you have them off, that will tell you if it's past the valves or not.

Isn't the 200+ a little on the high side? Also I am assuming the motors are cold? If so they get them warm before doing the test. The higher numbers could be that you still have water in the piston.

My question is why do you think those other two cylinders have lower compression, and what should I do next? Im gonna replace all four manifolds.

I'm going with the rust on the valve seat theory and/or compression rings. Squirt some Marvel Mystery Oil in the cylinders and let it sit for a few days...should work a "miracle" on the rings and seats. Run it and re-check.

Don't freak...you're probably ok.

Chris, I posted in the tech section before I saw this. I agree change the oil,bring it up to temp,and recheck.You should be O K. Swap the exhaust from the other motor,after you pressure test off coarse,and run it. Good luck.

If you need just one set of factory 420 exhaust, I have a set.