5.7 backfiring thru carb

[QBhoy]

I was meaning water ingress to engine through risers or exhaust.

 

[98bryant214]

Took the boat out yesterday, it has a mind of its own, now it's running fabulous, even under load, but after 3400 RPMs, the backfire starts up again.
Talked to a certified mechanic, he thinks the fuel pump is dying a slow death, and can't keep up with the boat under load, and suggested that I drain the tank to rule out bad gas....does this make sense?
Compression test tomorrow. I have checked all of the plugs, they look normal, as in no rust etc.
It runs super quiet, no ticking or clicking, no vibrations, maybe it's just possessed

 

[98bryant214]

One other thing, this adapter for the carb, is it supposed to be flat on top, or contoured?

 

[alldodge]

I would put a pressure gauge between the fuel filter and carb. If the pressure stays at least at 3 psi above 3400 then its not the pump. If it drops off then its the pump, tank, filter, other restriction or antisiphone valve.

Don't drain the tank, get a piece of fuel line and a 5 gallon gas can. Stick the fuel line in the tank and take it for a run. If it does OK, then its the tank or antisiphone valve. If it still acts up then its the pump, filter, carb or restriction.

I would install the fuel pressure gauge and use the 5 gallon can

 

[98bryant214]

Compression test checked good. Put a new fuel pump on today, drained the tank, fresh gas....problem still exists.
It's strange how a few days ago it wouldn't go past 2200 rpms without backfire, now it will go up to 3400-3500, and it runs so smooth, but after 3500 it starts popping thru the carb.
If I creep up to 3500 it will be smooth for a minute, pop a couple of times then either back to smooth or continuous backfire.

ICM module?
Ignition Sensor?

 

[alldodge]

Compression test checked good.

What were the readings?

new fuel pump on today, drained the tank, fresh gas.

Not to be harsh but unless your running test and not telling us the results, your just throwing money at it and its doubtful the trouble will be found except by accident

 

[dingdongs]

Double check that the plug leads are on the correct plugs.