470 misfiring

[nola mike]

I've been working on the floor of the boat this year. Finally got a new floor in and gel coated. Way more work than I wanted, but came out nice. Waiting for the new seats now. Anyway, finally got it out on the water for the first time this year. Went about 15 miles and it ran great. The last 5 could feel it hesitate. Didn't seem related to speed. Replaced my fuel/water separator (no water), checked the other 2 filters, both looked clean. Siphoned off old fuel and replaced. Went out today, same thing. Ran great for maybe 10 miles, then had the same issue. Later that day went out again (engine still warm) and immediately had problems. Have new plugs, wires appear in good shape, timing is good. Fuel vent is clear. Compression is 150 x 4. I have a Mallory electronic ignition. What are my next steps? How can I determine if this is fuel or ignition? I'm going to throw the timing light on there and see if I can see an ignition problem, but don't know how well that will work. I replaced the coil (I know, it's never the coil) with an extra I had laying around, haven't had a chance to run it yet though. I also have a spare carb (but can't remember if it needs a rebuild) that I'll throw on tomorrow. Other suggestions?

 

[boltonranger]

Vacuum in the fuel tank?

 

[nola mike]

Like I said, vent seems clear. I'll pop the cap next time it does it though

 

[boltonranger]

Oh sorry I missed that Mike.
But I agree that is probably worth it to open the cap when it's running poorly.

 

[nola mike]

Well, Damn! Was it actually the coil? 25 miles today, ran beautifully. I cleaned all the connections at the coil, but hell maybe that was it. More I think about it, more likely it's electrical since the problem only appeared when the engine was warm. We'll see. Cosmetics tomorrow! Teak and building the frames for the new seats...oh, and new fuel sender and Speedo pitot (working gauges! )