92 Excel 351W quits under load

Hello New here to the forum! Looking for some advice or possible guidance. The problem I am expierencing is engine runs fine under no load (in neutral) at various RPM's. Once under load anything over 2400-2500 RPMS the engine will run fine with no hesitation, backfire, or issues for 10-15 seconds and then engine cuts out but if I back out of the throttle it will idle fine (most of the time) wait at idle speed or lower RPMS then retry again to have same thing happen. If I run at higher RPM it will run for a shorter period of time.

I installed a new engine long block 351W in August of 2019. The engine has approx 20 hrs. End of last season the original Pro-Tec ignition failed and replaced with DUI marine 7000 Distributor and new wires, and plugs. After spring un-winterizing running on a hose and bucket at home everything engine ran fine, went to launch at lake and wouldn't start. Found broken rotor parts in distributor under cap. I removed broken pieces, and replaced the rotor didn't seem like anything else was damaged, boat started to not run under load.

To rule out any fuel issues (I always run 91 non-ethanal and used marine fuel stabilizer in 2 tanks before winterization) I have replaced fuel filter, checked fuel shutoff, checked anti siphon valve at fuel tank, drained and refueled tank with fresh gas and additve, checked inlet screen at carburetor (which was a little plugged, and carburetor was "rebuilt" @ engine installation. Ive checked the mechanical fuel pump (produces 6psi and holds for over hour), replaced spark plugs and re-gapped new plugs to .55, checked the timing with timing light (28 deg @3000RPM).When I removed the plugs they were not fouled and were a tan brown color.

Removed the Holley 4160 Primary and Secondary Float bowls and checked the jets in primary section, checked the float needle valve and everything seemed good. Adjusted the idle air mixture screws (turned in all the way and backed out 1.5 turns)
It seems like its starving for fuel. When I removed the plugs they were not fouled and were a tan brown color.

Not sure if I have a carburetor issue or a issue with mechanical fuel pump. Can a fuel pump show within spec pressure but not pump required fuel? I bench tested the fuel pump and checked for pressure as it was off the engine I replaced. It doesn't show any fuel in clear line going to spark arrestor on top of carburetor. Thanks for any input!!

Brian0445 wrote: I've checked the mechanical fuel pump (produces 6psi and holds for over hour),

Please elaborate on what "holds for over hour" means. A fuel pump that is failing may be able to keep up at idle speed, but might struggle at higher RPM while under load. I'm assuming that your "long block" did not come with a fuel pump, correct? Is the mechanical fuel pump a Carter? You may want to do some additional fuel pump evaluation.

JQ

Thanks for the responses! I forgot to mention when I pumped out the fuel of the fuel tank I also removed the tank pickup tube and inlet screen was clean. I tested the fuel pump with a pressure/vac gauge. I installed it on the outlet port of the pump and cranked the engine and gauge read 6-6,5 psi. I left the gauge installed for over a hour and it held the 6-6.5 psi.

Update.... Ive since replaced the mechanical lift pump with the original Carter pump. Water tested and same issues. I tried running out of different fuel tank to eliminate any fuel issues in the system. Water tested again with same issues. Called DUI distributors explained my issues said it may be the coil or cap due to rotor issue. They are going to send out a replacement. Hopefully that will fix the issue. Felt like a fuel issue but I'm at whits end! Has anyone else experienced a failed cap/coil to run under load then quit, and rebound if you throttle back to idle?

Surely wouldn’t hurt to triple check your firing order. How do the plugs look?

The transmission doesn't seem to lock up causing the engine to quit. The engine stumbles and if you back out of the throttle the engine will idle down and continue to run. I spoke with DUI they said they will be sending a new distributor cap and coil. They said it may be the problem.