Does anyone have any idea why a 454 mag would be backfiring and missing at 4800 to 5200 rpm? It seems to run fine for a couple hours on the water, then it just starts backfiring and won't run at high rpm. It's a 28' sleekcraft with a 454 that has been bumped to a 468. It ran great all last summer and then I made the mistake of not putting any fuel stabilizer in it over the winter. I'm in Phoenix, so we don't have a big humidity problem. I normally run it a few times during the winter but was not able to this year. I recently ran all of the old fuel out and have refilled and replaced water/fuel seperator, cleaned carb, checked fuel pressure, replaced spark plugs, replaced cap and rotor. Runs fine until put under load. Please help.

Wow! Sounds like you already did the usual fixes.

If everything's okay at first and then it acts-up later, could it be heat related? Is a coil breaking down? The 454 Magnum came with a Quadra-jet carburetor. Did you check the in-line fuel filter at the carb inlet?

Years ago I had the same problem in a Corvette I had. Turned out to be faulty Champion spark plugs. Put back the AC's and everything was fine.

Usually backfiring through the carb is a lean mixture or the spark plug wires are cross-firing. How good are your plug wires? Remember to keep #5 & #7 separated.

Hate to be the messenger, but mine did the same thing last year. Ended up being a blown headgasket.

Hopefully its something simpler. Fuel supply problem?

It has a Holley 750 carb with no inline filters and I made sure the fuel lines aren't anywhere near the exhaust to cause vapor lock. All wires look fine and seperated. Still lost.

I would think it would do it all the time if it was a blown head gasket.

Weak valve springs? Did you change them when you rebuilt the engine?

He says it runs fine, then acts up. This would rule out valve springs, head gaskets etc. Sounds to me like you have a bad coil. They act up when they get hot and run like crap.

Coils are cheap, swap it out and see what happens.

Mine only acted up noticeably when under full or near full throttle and above 4600 or so. It got worse (more frequent) over time. It blew it between the center two jugs. No water in oil.

I forgot about the hot coil thing. But in my experience bad coils that fail when hot won't work at any RPM

backfiring and missing at 4800 to 5200 rpm?
Weak valve springs will do this, that is why I asked.

Yes, it has merlin heads.