Apparently sticky valves

[JimF]

I had a very hard time starting my motor last week, after cleaning the plugs, and then letting it sit I still could not start it. I check compression and had no compression in the front (of the boat) two cylinders! Now last spring after I bought the boat I had to remove the head and unstick two sets of valves (the middle two cylinders I think) this was after the motor had been idle for three winters. This time I squirted some PB Blaster into the spark plug holes, let it sit a while and then actually got i started, thought I believe it was only running on the rear two cylinders.

By running the motor for about an hour and revving up and down a few times, ti seemed like the valves unstuck themselves, because the motor now runs pretty smooth. A friend recommended a fall valve job, but is there anything I can do in the meantime? I do use Marvel Mystery oil. anything else to aid this ancient piece of iron. Typically the motor starts and runs great if I start it every day, but I am mostly a weekend sailor, so sometimes there is a layoff, and it is always harder to start.

Any advice will be appreciated.

[tenders]

As a rank amateur mechanic, I have convinced myself many times that a messed-up engine is running "smoothly" after I've been listening to it for a while and gotten used to its blatant roughness. There is probably a psychological term for this phenomenon, or at least a Sniglet.

I would check to make sure you have compression again on those two cylinders before you conclude anything. Two adjacent cylinders without compression are also indicative of a head gasket which has blown out between them, and in my experience (3 over the years, all for utterly stupid reasons) that's always been how head gaskets have blown.

Once you've ruled out the head gasket you can check that the valves are functioning by verifying that the engine runs more roughly once you pull off the ignition wire from the plugs in the cylinders of concern while the engine is running. If there's no difference, there's something wrong.

If it is the valves, once you're sure they're unstuck and properly working on a warm engine, probably the best thing you can do for them short of a valve job is giving them a long run with the engine hot and a double-dose of Marvel in the fuel. If there's rust or crud on the valves this should help work it down.

As for the tough starting if the engine hasn't been run all week: do you have a mechanical fuel pump? I've had both mechanical and electrical pumps on various boats and I think mechanical pumps are prone to making the engine behave like this.