1974 mercury 500 , half throttle/under load problem

[Rsbaker08@gmail.com]

At about half throttle under load motor misses terrible then takes a few seconds but then clears up &takes off like a brand new motor. Runs great at idle and full throttle. Rebuilt fuel pump twice had floats off twice cleaned readjusted per secs new gaskets tore both carbs off soaked and cleaned blew light compressed air through all orifices new gaskets also. Replaced fuel line& primer bulb, tried opening vent all the way on tank and closed. Also bought another distrubutor and replaced even though don't think it's a spark issue. All 4 plugs seam to be burning the same. Also new plugs. On a 14 duo tri hull runs 27mph with two adults and 12 gallons of gas. I'm stumped?

 

[mlbinseattle]

Have you checked the timing? Sounds like it's either a fuel delivery issue like the carb bowls are being depleted of fuel temporarily, or either the timing is off. Also, from what I understand, the carbs need to run a little on the rich side. When you rebuilt the fuel pump, did you check the fuel filter? The vent on the tank needs to be open, too.