Merc 4.3 backfire @ 3500

[slogfish]

I ave a new 4.3 in a '93 18' bowrider. The engine starts great, has good low end power. But when I reach 3500 rpm, it starts to backfire and lose power. The whole engine is new, with new mercarb 2bbl, thunderbolt 4, new plugs, wires, cap, everything. Gas is fresh and 93 octane. This is frustrating, I can get on plane but only reach about 30 mph. What should the WOT and MPH be on this engine?

 

[cooter2506]

Re: Merc 4.3 backfire @ 3500

WOT should be 4400-4800. But I think it is starving for fuel. Try removing your gas cap to se if it goes away. If it does then you tank vent line is plugged. With everything being new probably a problem on the tank end. Do you have a new seperator filter on it. Dump the one that is on it out in a clear container and look for water. If it has water in it change it and run it.Might have water in the tank, vent tube clogged, pick up screen clogged, antisiphon valve stuck partially closed. Or even the new Mercarb isnt st up right.