86 Nautique Engine problems

OK, so i've been having this problem for a long time and have tried a lot of stuff to fix it, but at this point i'm just confused and don't remember exactly when the problem started.

I have a 1986 Ski Nautique with a Ford 351 V8 engine.

The problem is that 2 of the cylinders don't fire every time and it causes the engine to stall out and maintain around 3200 rpms under a load. The cylinders that are missing are right next to each other on the distributor cap. The way i determined this was putting a timing light on each plug wire and watched the light cut out.

Here's what i've done to try and fix it, or maybe one of these things is the cause....at this point i'm not sure.

1) Tried electronic ignition, but went back to the original points thinking that might have been the cause of the problem
2) New distributor cap
3) New Plug wires
4) New spark plugs
5) New coil with internal resistor
6) Rebuilt Carburetor

Please help.... I have spent a lot of time and money trying to figure this out and have not had much luck.

Any Ideas???
Thanks

Well what I would do @ this point is check the compression,it could be several things but you should start here,it may be a burned valve ,blown head gasket,blown intake manifold gasket,all the cyl should be pretty close in the readings How many hours are onthe boat?

how did the plugs look that came out of those non working cyls? oily? dry? carbon caked? or good?
sounded like a cap problem, cracked maybe, but you replaced that...

Definately sounds like you have a internal problem, nothing electronic.

I'm not sure about the old spark plugs, i think they looked ok.
Yeah, i read online somewhere about a cracked distributor cap causing this type of problem and so i just replaced it and the plugs yesterday, but they didn't help.

Leo, another thing i forgot to mention was that (although i forgot to check this the last time i ran it) the timing light would fire normally, without cutting out, if i had the sensor up next to the cap, but would show the problem down next to the plug.....thus that is why i replaced the plug wires, however that didn't fix the problem.

And just to be clear, the 2 cylinders that aren't firing, are not dead all the time. They fire for a couple seconds and the engine revs up and sounds good, then they will miss a couple times and this cycle just repeats over and over.

Sounds like maybe the carburator? You are getting a spark but no fire.

I have heard that rebuilt carbs come back in worse shape. But you probably had the problem before the carb rebuild.

yeah, the carb rebuild was an attempt at a solution, but didn't change anything.

quick question, did you buy the boat in tn?

Compression test results?

Two plug wires crossed?

Timing set at spec?

Lean or rich condition? add propane if it is lean.

The timing light not firing everytime is normal. Do not use that as a scope. There is a reason scopes cost thousands of dollars.

How does it idle? Just off idle? Higher speed not underload <Answer these questions for sure!!

Just wanted to let everyone know i fixed the problem. Turns out the mechanical advance was stuck. I took a look at the mechanical advance and pushed the weights in and out a few times and when i put it back in, IT WORKED GREAT!
Thanks for the help though.
Man i'm glad to have this problem fixed. It's been troubling me for almost a year.

Thanks again.

Todd

dude, I was just abut to say something about the weights. Mine were messed up and I am waiting for the parts right now. I would put that electric ignition back in if you still have it!