offthefront

iTrader: (1)

Twin 454 mags ..one with 10 hours one with 30 or so .... at WOT or close one motor will backfire ... so far only twice ..and just once per occurnace .... I am guessing carb but dont really know which motor .. Everthing seems ok oil pressure temps ect .Suggestions on where to start ? Thanks ..

cuda

What type of carb do you have?

Tinkerer

I would say that you have a valve hanging up or going bad.
At WOT I dought the carb could be the cause.
If it was a lean condition it could cause the valves to get TOO hot and that could cause a backfire.

Tinkerer

Possibly you had valve float.

Crazyhorse

I'd be looking at the cap and rotor. It's quick and easy.

offthefront

iTrader: (1)

Joe .. I think they are Carters ..what ever was stock .... Please dont say valve ...

Edward R. Cozzi

Is it popping back through the carb or out the exhaust? A lean mixture will cause the carb to pop at sustained high RPM as the engine starves for fuel.
Usually if a cap or rotor is bad the idle will not be good, but not always. Are both engines pulling the same RPM?
How much time is on the valve springs?

offthefront

iTrader: (1)

Ed ..this is a 311 with silent Thunder ....and its only happened twice ..first time I wasnt even sure what it was ...maybe hit something or ? ..But today Def. a backfire ..even my wife knew what it was ...and I think its thru the carb but not sure ....The Starboard motor is Brand new.. (reman jasper) .. which was replaced by Jasper due to old motor (16 months) sticking Valves ...The Port motor is also Jasper (about 30 hours) ..I dont know which motor it is ...But seems port side ( but just a guess) .. going 60-65 at 4500 plus ...kinda hard to pinpoint .. and yes both motors seem ok as far as both at same rpm at speed ...thanks for the input ... I know I'll get blasted for this but I have put 18K in these motors since I got the boat in May ..could have done alot better I think for my $$ than this ..... Valve springs ...the 30 hour motor had heads redone this summer due to reversion ...the new motor is ...well new ...<10 hours ...

Edward R. Cozzi

After a good run I would pull a few spark plugs and see if they read lean. Are these Gen V engines with the mechanical fuel pump mounted on the raw water pump?
You may want to hook-up temporary fuel pressure gauges to each engine and see if one side has a bad or clogged fuel pick-up in the tank. Did you have the carbs gone through?
I wish you were here in SE Florida where I could help you HANDS ON!

Thunderstruck

If you can not verify that the motor is leaning out try the ignition system. Had the same problem. It was the coil. Watch the tachs while you are coming up to WOT rpm. You will see the tach bobble on the motor that pops. Change the coil on that motor (hell or both motors, coils are cheap). Also change/check plug wires, plugs, distributor cap and rotor.