throttle START ====> Roller on the notch mark on the cam cam can be unbolted and dragged forward to just touch the roller

Well, just because it sparks when you check with the plugs removed, does not ensure you have good strong spark while running under compression…Try those inline neon testers while running when it starts to mess up….Have you had a look at the coils? Old coils tend to crack, you can have a quick look pretty easily by removing the recoil and the little inspection plate on top of the flywheel…I just reread your post, where you indicate the engine had a recent “tune up”. Perhaps they messed up the ignition. You swapped the fuel pump back to the bypass cover, and plugged the intake manifold port. Does it look like someone had the intake manifold off? Perhaps your fuel pump is leaking past the diaphragm into the cylinder, but am guessing the fuel pump is mounted on the top cylinder.
The only other thing that comes to mind is that this era of 15hps used a regular lower seal instead of the usual carbon seal set up…The regular seals tend to rust up and come apart over time…Needless to say, there is no way of inspecting without pulling the powerhead, which I don’t want you to do until all other possibilities have been exhausted. The only other thing that might indicate a bad lower seal is the presence of water on the plug(s).

I would not use the J8C plugs, unless you plan on idling all day long, they are a bit too hot for high speed running. I’m sure the boatyard replaced the coils if they were cracked, kind of hard to miss that. But, they may have misrouted ignition leads under the flywheel, which may have eventually rubbed through….
The fuel pump is on the top bypass, so an issue there would be affecting the top cylinder….You mention that the intake looks untouched, so I guess issues there are not likely. What about the fuel pump pressure port that comes off the intake, is it plugged off securely? You could try decarbonizing, but that is probably not an issue unless you have been idling/trolling excessively.
Again, any evidence of water on the plug(s)?
I’m not sure about whatever lucas oil blend you are using, you mentioned using some generic stuff also, perhaps that is an issue.
Finally, this is a relatively low compression engine, there is probably no benefit from using the higher octane fuel…But, I realize you are using it mostly due to the lack of ethanol.
Finally, how does the engine run before the lower plug gets fouled? Idle OK?

• This reply was modified 2 years, 10 months ago by fleetwin.

crosbyman wrote:

see bleeder 2-1 initial file was wrong…

• This reply was modified 2 years, 10 months ago by crosbyman.
• This reply was modified 2 years, 10 months ago by crosbyman.
• This reply was modified 2 years, 10 months ago by crosbyman.

Good point, forgot about the drain system…..