Mercruiser 260 weird misfire

[Southtowns27]

Ok guys, I'm a professional mechanic and I've got a weird misfire issue that has me scratching my head. This is a little long but just want to supply as much info as possible. Boat is an '86 Wellcraft 260 Aft Cabin with a Mercruiser 260 (Chevy 350). Engine has maybe 150 hours on it since rebuild. Has the Edelbrock marine carb on it with the stock Thunderbolt ignition. I only run non-ethanol gas with Stabil Marine 360 in it. A few weeks ago I finally got the boat out of storage. It fired right up with no issue. I cruised it out the creek to the lake at about 1500 RPM with no problems. As soon as I tried to accelerate it up to plane it would break up and not accelerate. I tried this a few times with it even completely shutting down a couple times. If would fire right back up though and run fine at 1500. One thing I noticed is that the tachometer would jump from say 2200 up to 5-6000 when it was misfiring. I replaced the pickup coil in the distributor, cap and rotor, fuel pump (it looked old anyway), water separator/filter, and plug wires. I pulled a couple plugs and they looked really good. Zero change with the problem. I was patient with it though and tried to gradually work it up to speed slowly. Over the course of about an hour I was finally able to just barely get it up on plane at 3000 rpm with only the occasional misfire. Over the course of another half hour I was able to work it up to 3500. I cruised there for about 2 hours and now the issue is completely gone. It runs WOT perfectly fine, idles perfect, holeshots perfect. Last season I had the EXACT same issue. I ran it hard for awhile and then it ran perfect all year. It's hard to troubleshoot since it runs just fine on the muffs. The erratic tachometer was pointing me towards an ignition issue but it kind feels like it was running out of fuel. But, now it's perfectly fine so good luck trying to troubleshoot the actual issue. The only thing that seems plausible at this point is sticky valves that free up after it runs for awhile. What do you guys think? Am I missing something really obvious?

 

[alldodge]

Carb is gunked up and it takes some running to clear it up. If your running Stabil year round I would stop, only use it during winterization. The misfire at high rpm is lean condition so carb is not supplying enough fuel until things free up and flow better

 

[Southtowns27]

Carb is gunked up and it takes some running to clear it up. If your running Stabil year round I would stop, only use it during winterization. The misfire at high rpm is lean condition so carb is not supplying enough fuel until things free up and flow better

I forgot to add that part. I rebuilt the carb last season when I was having this issue and it had no effect.

 

[alldodge]

Sounds like a fuel issue. Gas is going bad or its getting water in it.
Maybe where you buy gas at end of season. Does it sit outside where rain can get to fill port?

To late this season maybe, but try dumping filter contents into clear container to see if there is something except clean gas

Only other thing is a bad connection on the Dizzy module. Doesn't really sound like it because it would continue to act up off and on at times. The old module had 2 wires and used the body as ground. Merc changed to a 3 wire so it has a designated ground wire

 

[alldodge]

Maybe antisiphon valve on top the tank but usually if they clog they don't free up. Same goes with collapsing fuel line, tank pickup and fuel pump

 

[Searay205]

Gas tank vent, fuel hose delaminating inside and pinching off flow. I pretty sure you have coast guard approved fuel lines so they should be good but might want to check.

 

[kenny nunez]

How old are the spark plugs?