engine pinging and backfires and stalls

ok guys, this one is kicking my ass and has me going nuts.
new engine installed, 5.7, thunderbolt 5 with knock sensor. 4 barrel weber. proffesionally rebuilt carb, new knock sensor, new mechanical fuel pump, new ignition sensor and coil, new plugs, wires, installed remote fuel tank to insure good clean gas. timing checked several times , base and advance system ok. the boat will idle and rev in newutral all day long but put a load on it and out of the gate you hear it pinging, if i do not pull back on the throttle, it will backfire and stall out but start right back up. and act the same way.

New engine or remanufactured engine?

Double check TDC on cylinder 1 and confirm timing mark.

Also it really sounds like your off on firing order...

Maybe issue with valve train, maybe lifer adjustment maybe stuc valve...

There is my thoughts...

A engine NOT under load can appear to run fine...
Usually its timing / firing order...
Are the ignition parts all Mercruiser?
Did you replace the cap and rotor?

engine is new, new. the timing is set up 10 btdc. firing order is correct..
yes, ignition parts are merc. cap/rotor , wires, coil, plugs. all new

even when it is detonating, it still runs smooth, no vibration, no stumbling, nothing to indicate an issue.

last trip out, i tried moving the distributor, and stil had detonation advancing and retarding timing


ill dig out my dead stop tool and check true tdc.

Don't laugh, but is the accelerator pump working and its squirt nice and strong?

Jeff

This happens when there is a compression leak between cylinders or a cracked head. Check the head gasket, Pull all the plugs and pump air in each cylinder, you will find the leak.

fastjeff said:

Don't laugh, but is the accelerator pump working and its squirt nice and strong?

Jeff

Click to expand...

The carb was rebuilt and then after the 1st occurrence I brought it back to him yo go through it again with this issue in mind. He found nothing wrong.

Docksidemarineservices said:

This happens when there is a compression leak between cylinders or a cracked head. Check the head gasket, Pull all the plugs and pump air in each cylinder, you will find the leak.

Click to expand...

If I pump air in the cylinders, the piston gets pushed down and the valves open so unless I back off Ann the rockers I don't see that working

Well you will bring the pistons to TDC of the compression stroke in each cylinder with a breaker bar and a 5/8 socket........ and hold it there while you test.

I guess you never did a leak down test before.

Docksidemarineservices said:

Well you will bring the pistons to TDC of the compression stroke in each cylinder with a breaker bar and a 5/8 socket........ and hold it there while you test.

I guess you never did a leak down test before.

Click to expand...

never had the need. thanks for the info