Ok, so I have been banging my head against the wall for 2 years now trying to figure this problem out, this is the year I will find and fix it! Im on as mission! I have an older 454 Mag 365 HP, all stock. After the boat sits for a week or longer, I go to start it, and it will start fine (on all cylinders) and then 30 seconds to 5 Minutes later, its like someone hits a switch, and it drops a few cylinders, and will only run if the throttle is bumped up a bit (will not idle). I let it run for 15 minutes or so, and then once again its like someone hits a switch, it snaps out of it, and starts running on all cylinders and it will then run perfect the rest of the weekend. It can sit overnight, and start and run fine the next day. It will only do this 75% of time, there are times I will go to use it, and it will not do it.

Things checked and replaced:
Plugs/Wires
Cap/Rotor
Coil
Pickup Coil (under cap)
Have checked all wiring
Choke not sticking
Carb recently rebuilt
Module Box

Im thinking it is an electrical problem somewhere?? My mechanic (who loves to spend other peoples money) is insistent that there is an internal problem somewhere, and insists the motor should be torn down.. I however disagree, because when the motor is running right, it is healthy sounding, no ticks, knocks etc. Has good oil pressure etc. And runs like a raped ape. ANY HELP from you guys is greatly appreciated!! Thank You!

Sounds like a idle circuit issue in your carb. Could be the carb itself or fuel delivery issue.

Wont be electrical, as its not like you are running a cop set-up that can drop out spark on a single cylinder. With old style cap rotors its all or nothing, unless plugs and wires are junk.

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Excuse me? Do I know you? My motor does not have 1500 hrs on it, it has somewhere around 500. Thanks for high jacking my thread.

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Dude seriously, are you high or something? I come on here to get help, and have some bozo who cant spell acting like he knows who I am, and trying to tell me what MY boat has.

Can a moderator please delete this non-sense. Thank You!

you may have a faulty needle&seat valve in the carb,not totally bad but weak and sometimes allowing excess fuel to the engine,also water or trash in the fuel can also cause your problem.

has the distributor been pulled, the vortec SB's HVS gear gives out. normally low power and then low power/vibration. was carb boiled out/ultrasonic cleaned, the gunk is really stubborn when it gets inside the small passages.

I had a similar problem that turned out to be a combination of bad coil to distributer wire, and plug wires cross-firing. Make sure the plastic plug wire separators are in place. before you tear down the engine, I would certainly run a compression or leakdown test.