Could really use some help.. I have a 1987 27' Mach 1 Concorde. Powerplant is a 454 mag 330 horse coupled with an alpha outdrive. Having wot problems. Wants to fall on its face at wot. This will not allow me to reach max rpm. Have changed plugs, prop,rebuilt carb, changed secondary metering rods. Now it will reach 4400 rpms and 49 mph. Since changing all of this it is now backfiring through the carb at wot. I am going crazy trying to figure this out. Any help would be appreciated. Thanks

A compression test and check for water in fuel. Would be the first things I would check.

Already checked for water in the fuel twice this is not the problem. I agree a compression test is in order I am trying to find the time to get it done. thanks

Could have a bad cam lobe.

if the cam lobe and or lobes are wore will this show up during a compression test.

Backfire through the carb usually indicates a lean mixture, which I don't think is your problem. The second thing I'd look for would be burned valves, which will show up on a compession check. As far as losing lobe on a cam, I doubt it. Will usually happen before 21 years. 49 with that power plant on a 27 foot boat ain't bad. Btw, it's not a 454 Mag, it's just a regular 330 horse 454. That was the only year that Merc ever mated a BBC to an Alpha drive. It just has a sticker on the drive that says 454 Mag. I have one of those drives in my garage. Just a standard Alpha drive.

Best of luck!

I was thinking the exact same thing.

I had this problem with a truck one time and it was the cam that was bad. good luckm .02

Thanks for all of the input. Hopefully it will quit raining here sometime this week and I will be able to perform the compression test. Hopefully this will tell the tale. Much appreciated.

Sounds like burnt intake valve. My 289 '66 Mustang would do that.

If compression test show nothing, check for leaks or rot at intake manifold gasket.
That would make a cylinder run lean, but then that should show on the spark plug.