Engine Question

You need to block the engine from rotating though, as soon as you put air pressure into the cylinder it will start to rotate and the valves will start to open. If you back the rockers off the engine will rotate to BDC and stay there. DO NOT put a lot of pressure into the cylinder 10 or 20 PSI is plenty to check for air leaks.

Pull the ignition coil lead, jack your carb butterfly valves full open, run the compression check on # 7 again. Then add a 1/2 teaspoon of oil to the number 7 cylinder thru the sparkplug hole and run your compression check again on the #7 cylinder. If compression improves you have a ring issue, might be stuck (doubtfull if you've run the engine for any length of time) if it doesn't improve its a valve issue, either the intake or exhaust is not sealing properly and the head will have to come off. Leakdown won't tell you anything at this point, with 1/2 the pressure of the other cylinders you obviously have a leak. The only question is if its rings or valves, and in order to fix it the engine has to come apart....You can back the rockers off #7 and put 10 or 15 psi into the cylinder thru the sparkplug hole and listen to the exhaust, intake or the block itself (you will have the valve cover off to loosen up the valves) you will hear air whistling in one of those locations and that will tell you a little more about where the leak is coming from.

Had this boat sat for a long time before this issue was found? It's possible the rings are stuck on that one cylinder but without some more info its hard to tell

Chris,
We have repeatedly tried to explain the compression not caused by a manifold problem and to find a new mechanic. Besides him saying a 87 should not be running, now he wants to replace all those parts? Where did you find the back yard hack? (sorry Gary    )

So, here's my update: Got my boat from the mechanic yesterday. I figured I will put the project on pause and pick it back up in the spring. On a warm test my numbers were: 1) 125 2) 130 3) 130 4) 140 5) 140 6) 130 7) 75 8) 130. He is sticking with the leak in the manifold as causing the issue with number 7. Is this a brackish/saltwater thing? He often refers to this when talking about the enging. He says I need to replace the carb, water pump, impeller, and fuel pump. I feel like I could probably do those things. He says then the boat will run, and doesn't think I'd notice a difference in performance even with #7.

My skills are pretty inconsistent...I don't trust myself, but I'm probably a little better than I think. I'll take a look for the videos. I just don't want to do further damage...

A little bit of lovin' and that Ford small block will be purring again. Don't give up yet. That boat should have a lot of life left in her.

JQ

Yikes! Any so-called boat mechanic who says an 87 351 shouldn't still be running and is not worth reviving clearly needs a new line of work.....I'd be peeling rubber and spraying gravel to get my boat away from this guy ASAP.......

I agree on a new mechanic but quit knocking back yard mechanics Pete, Ken Warby's record still stands almost 37 years to the day-----

Okay, just talked to the mechanic. He almost seems determined to bury this boat. He's convinced that an engine from 1987 shouldn't still be running and is not worth reviving. It seems like people on this site would see it otherwise.

He reported that the 7th valve is running at 75. The others are doing fine and 120+.   He thinks there's a leak in the manifold that is causing this.

He says it's going to need a new carb, new fuel pump, new water pump kit. I can't believe that the damage would be so permanent with the number of hours (440) and the fact that it's sat for 4 years...

I think I may want to get a second opinion...

This was the only info that I got so far. This is the second time he's doing the compression test. The first time was cold and brought numbers at around 120, but the #7 was at 70. I asked him to do it again with the engine warmer and he did. We haven't spoken directly, so I was just taking what was on his phone message.

The engine is PCM 351- 1987 SN2001. I don't have pics because it's out of the area right now. Thanks guys!

I'll add info. when I talk to him.

We're going to need a little more information that that.

With a compression check, the general rule is that there should be no more than a 10% difference between your lowest and highest cylinder. There is also a low threshold that would indicate the engine is in need of a rebuild.

What were the numbers for all 8 cylinders? What motor are we talking about? Boat model and year?

Some pics and a little more info would go a long way in getting you the help you are looking for.