I have a 1987 OMC 4.3 with cobra outdrive, Holley 4brl carb edelbrok manifold on a 24 ft. deckboat. I have been chasing this prolem for some time without success. I have read your forums and tried many of your sugestions. The problem is when I try to go to WOT it starts out ok but when I get to full throtle it starts popping, backfiring and finally dies. If I go to 3/4 throttle it goes great for 10-15 seconds then starts the popping and backfiring. If I pull back on the throttle it will idle down and run fine until I hit the gas again. This only happens when under a load. It idles great. It sounded like a fuel supply problem to the mechanic (carb specialist for 30 yrs) who completely rebuilt the Holley carb that is on it. I replaced the fuel pump and all screens and filters and cleaned water seperator. No change. I then put in an electric fuel pump. No change. I started looking for problems elsewhere and replaced rockers, lifters and a cam shaft. All these parts were visibly in need of replacement. I had already done the wires plugs and converted to a module replacing the points and replaced the springs for the centrifigal timing advance. No change. Still thinking fuel supply I ,for testing purposes only, elimated the fuel seperator, anti siphon valve and anything else between fuel and carb. Basically I had a pick up line going to a new fuel filter going to a new fuel pump going to the newly rebuilt carb. No change. I know there are more test I should run but I thought this might narrow it down a little. I did check the fuel pressure with a borrowed fuel pressure gauge. It read 5 PSI as close as I could tell. I have not checked the exhaust flaps that I was just reading about. Can anyone point me in the right direction? I am running out of time patience and money. Thank you.
