This is going to be a long one but I want to give all the details. So I just rebuilt the carb on my 78 f150, 351m non California, non catalytic. It is a Ford replacement carb, ink stamped D8PZ. It was pretty grungy on the inside of the bowl, granular textured bits. I cleaned everything up good and rebuilt with a new kit. I set the dry float level to 31/64, as prescribed in the kits instructions. I got the carb back on the motor and got it running, but I didn't want to idle. After bringing the idle mix screws out a couple of turns from 1.5, it now will idle well, in gear too. But it is seeping gas from the front passenger side corner of the air horn gasket. I figured the float level was too high so I removed air horn and the fuel level measured 33/64 from the machined surface of the bowl down the the fuel level. I'm not sure if this is correct or not because neither the 5 volumes of my ford service manual nor the two sheets with the kit give the wet level. I ran the engine with the top off to see if the level changed, it didn't but I noticed that fuel was geysering out of the seat about an inch or so above the carb, so I shut it off. I've never ran a carb with the top off so I don't know if this is a normal amount or if this is why I'm getting gas seeping past the gasket.

 

I've never run a carb with the top off. But gas in a bowl sloshes everywhere with starting/stopping/hills/curves/etc. No gas should be getting by the gasket.

Has all of the old gasket been removed, no residue left what so ever?

Some carbs used to have cast-in fine lines on the mating surfaces, to compress the gasket more. Did those lines (if there) get removed by over-energetic gasket scraping?

A big problem I had with the Ford/Autolite 4300 4 barrel carbs was distortion of the carb's main body over time due to the design of the casting. No amount of gasketing would fix the resulting lack of planarity, they became leakers.