Opinions on my 1958 35hp Lark

[eclipseturbors]

So, I started a thread awhile back about my knocking 1958 35hp Lark. After getting some great opinions and advise, I went ahead and shelled out $100 to have the fuel system rebuilt and tuned. The motor starts without a choke and I've even pulled started it with a weak battery. It ran great, but the knock is back after being gone for a full day on the lake without any noise. I spoke with a gentleman who tunes these engines for a living and he mentioned that if there was no play in the pistons, it could be from the crank end play. Also the way I had checked the pistons wasn't the way he'd done it. He said to have one person rotate the crank and watch the dowel or screwdriver move. I may have not seen the play just by moving the piston back and forth even with the head off. So, is it worth pulling this motor apart and rebuilding? The boat will accept a larger motor, but the cable steering in it is all new from the previous owner, so I don't thing a newer motor will work... or will it? I also have a spare parts motor for my lark. Any opinions are welcome.

 

[HighTrim]

Well, very hard to diagnose from that description alone. Could be something simple being mistaken for an internal knock? Maybe a video would help?

What is compression?

If the knock is from crank end play, or from rods, rebuild now would be easier than AFTER she lets go, that is for sure. A rebuild on these is fairly simple, with a few tools, a service manual for your first time, and some friendly advice from the forum.

I test piston play by rotating flywheel until piston is just starting to go in. Then insert dowel/screwdriver, and push in. The play should be minimal, if any on that motor.

 

[F_R]

It would be hard to imagine a true "knock" going away for a full day then coming back. Besides that, those motors are far from being knock free even when brand new---that is if we are comparing it to a nice quiet car engine or something. If you don't detect any looseness with the dowel/screwdriver test and the crankshaft doesn't have any fore/aft movement, and compression is good and equal, I doubt it has any problem. I'm not sure I've ever been able to detect any excessive crank end play by ear alone. Those motors are all roller bearing construction, and the bearings just don't get loose and knock. But they do break, seize, come apart, etc., and when they do, believe me you will know it. Pistons have a shocking amount of taper and sloppiness at the head end, but that is normal

 

[Tim Frank]

You would have been better to have continued your original thread.
Try to ensure that you isolate the "knock" to an internal source.
As suggested, there are a lot of noises that emanate from these old engines that sound expensive and serious, but aren't.

 

[eclipseturbors]

I didn't want to rehash an old thread, so I started a new one. I'd more describe it as like valve noise... but we have no valves here. I plan to run out to the boat this weekend and check the play again. I have nice even compression. The motor doesn't miss a beat or smoke. I would figure for the noise it makes, there would be some obvious movement or play. When I had the head off, I couldn't feel anything. If our Oklahoma weather holds up, I'll get a video of it running. Maybe it's just old and I'm paranoid? I think I would be foolish not to keep the motor even if it does need a rebuild.