flatout

right on thanks, sound like i should start with the fuel pump since i already changed the carb and filter.

Falcon

Definitely check the alpha ignition "interrupt" switches. They work by cable load and if the cable is pinched somewhere or if it isn't adjusted correct it will activate. It's only supposed to active momentarily coming out of gear.

flatout

where might those be located? i have two red switched on the front of both power plants which i assume is the kill switch if there is a fire. Is that the nutral safety switch which won't let you start it if its in gear?

GOODT

the shift interupter works when shifting and if a bad cable will stall when shifting in 25 yrs never seen a problem from it at 3000 rpm go to basics work from there if you want a flow chart of what to check pm me or call me and Ill point you in the direction of where to check

Falcon

I have seen the problem of sporadic interrupts from the switch, but not continuous and at a very specific rpm. Because it works on cable load, if something is pushing or moving the outer cable it may move the interrupt. Was just a thought because it's easy to check.
Look for where the shift cables connect on a common bracket, usually mounted on the engine. You may see 2 switches on some engines. The interrupt is mounted with a v-spring off of the cable mount from the drive. The neutral switch is mounted so that it operates on a cam on the lever. On later model ecu controlled engines, not sure about yours, the neutral switch may also be tied into a neutral rev limit in the ecu. The way to check either is to temporarily disconnect them, one at a time, but it will probably make it difficult to pull the drive out of gear with the interrupt disconnected.

Happy Hunting

mats

Check fuel pressure at WOT. That reveals any problem in the fuel delivery system which is the the most likey cause for backfires.

BadDog

Fuel pressure would only indicate available fuel to the carb, not what is getting through the carb. It would not take but 10 minutes to swap carbs left to right.

SB

iTrader: (1)

This is good advice. A fuel psi gauge will not show if a needle and seat is restricting fuel flow.

For someone reason , this past month, I need more than two hands to count all the people with needle and seat issues.

Just throwing possibilities at you - near impossible for us to diagnose without being there to check this and that.

brian41

iTrader: (1)

I have had a few boats with similar problems turn out to be the pickup in the merc distributor easy to swap and check if miss goes to other engine before buying new 1

NightHawk

I agree with brian 41.

It just seems there are other more likely things it could be such as a distributer/ignition problem or even a valve train problem. Wouldn't fuel starvation simply slow the engine down?