Engine Missing Causes.
 

I have a 79 Formula with a Merc 260. Went through the whole boat this past winter. The motor ran well last summer so I left that alone. It has 600 hrs on it and is in very good health. It has an edelbrock carb on it. I took it out for the first time today since the resto. I noticed going through the harbor, the idle missed a tiny bit. I just assumed from sitting. I warmed up then hit the gas. It ran well. Did 54mph on GPS @5000rpm with full fuel and a passenger. we then settled into a 3000-3500 rpm cruise. The boat developed a bad miss. Struggled to get on plane. It was also missing when idling in gear. Out of gear it seemed ok. I have brand new plugs in it. Also I swapped out the cap and no change. It ran great all last year. Any ideas?

Btw the points have been converted to petronix I think. There is no points.

How about the plug wires? Sure they aren't arcing out? A good test is in the dark, get a spray bottle filled with water. Spray the wires with mist of water, and look for sparks coming from the wires.

Any popping from the carb and or exhaust?

New plugs, wires, cap and rotor are a great idea to do all at once. After you have done that then I'd start trouble shooting bigger things.

Changed the cap an rotor. The plugs are also brand new. The wires are relatively new. It was also surging a bit idling back to the marina. I have heard that its a fuel delivery issue.

I had a miss in the mid range went thru my whole electrical. Thought for sure it was some kind of electrical causing it, bad plug, cap, rotor, wire, Ect. Nope it was the dam carb. Bought new carb(wanted a little bigger anyways) and was fine then two years later it did the same and was carb again, rebuilt and was fine. Ethanol gas sucks .

I am leaning toward the carb. I screwed up and topped it off with regular. I was even at a 100% gas pump. I hit the wrong button on the pump.

Regular 87 octane wont cause whats going on with your merc 260. If its fuel related, you probably have a gummed up carb or filter.

There are a couple threads on here about those carbs. Something about epoxying something in the bottom of them. The guy that I rent a shop from did this last year and cleared up all of his loading up and missing.

I went thru this a few times with Q-Jets. The ethanol anymore ruins rubber parts in Carbs...just know this and run either pure gasoline or learn how to go thru your carbs periodically. I am in the process of securing a new pair of Quick Fuel Double Pumpers which will help alot.

That $hit plays hell on everything rubber thats not designed for it. My 79 Formula is getting all new hoses in the fuel system because of it.