I have a Holley 800 double pumper marine carb with electric choke. It is misfiring at pedal down full load. It is on 454 marine engine. This new carb work great for the first 8 hours of use, but now it is started to back firer or hesitate on fast throttle take off, but if you bring the throttle up slow it does not back fire or hesitate . It works just fine on wide open cruising throttle or idle. It is just on fast throttle take off that I am having the problem. Also fuel is at 6 PSI when running, it squirting fuel form the nozzles when throttling and choke is working proper. Any help?

check and make sure that all the rubber plugs are still on the vacuum/signal ports. Good idea to tie-wrap them on

You could have an issue with your ignition or its advance..

Sounds like your power valve is blown, threads into the metering block. It will blow when there is a backfire.

sounds lean on the primary side,

I had the same issue on my ski boat. It ended up being a simple air passage being blocked. I used high pressure air to blow them out and sure enough, one was blocked. It took about a month to figure that one out.

Darrell.

Sounds like a lean condition in the idle circuit. Back off the idle screws half a turn. I don't know where you live, but as the air cools, the circuits have to be richened. The best way to check the complete RPM range is with an air/fuel ratio meter.

hay guys if it works okay on easy accells then the carb should be okay the enrichment curcuit is the accellerator pump right when he hits it hard and a buch of air goes in it needs fuel thats when the pump should help sounds like a lack of pump shot to me my three cents worth inflatoin you know

I was thinking the same, he said it was squirting and will run wot.

Trash in the carb or a weak ignition component, maybe a mechanical advance issue.

Back to basics.