Engine Misfire Saga - Update

[Silverback51]

First off, glad to hear it's running well again.

One word of advice. Only change one thing at a time. Now if it happens again, you do not that knowledge to fall back on to check first.

This is true of upgrades also. Change one item, get it running right, and then move on to the next.

[Terry Stapley]

After many trys at fixing the engine miss I put a different carburetor on it to see if that would make any difference and the engine ran fine with the new carb after some fine tuning. The Carb is a Quick Fuel Technologies 850 DP. So I rebuilt the Holley 750 DP that was on the engine using a Holley rebuild kit and put the 750 back on the engine and after some fine tuning it ran okay but was still a little rich at idle. But as I revved to 2000 - 2500 rpm it started missing again and black smoke was coming out of both sidepipes. Original engine missing problem was back.
Frustrated I turned it off and left it overnight and when I lifted the hood in the morning there was fuel in the intake manfold on both sides of the carb. About a tablespoon full in each of the indentations below the carb on both sides, so fuel is leaking from somewhere. The floats are new and the float levels were adjusted a bit low if anything!! Every moving part in the carb is new!!
I put the 850 carb back on and it needs some fine tuning but otherwise the engine runs fine.
What suggestions would you offer about the 750?? Is it toast??

[Bobcat]

Terry , you didn`t say how old the Holley is/was , but one thing I have found on older Holleys is that the butterfly shaft seals leak as they get old . If you check , you should find the fuel puddles/leaks very close to the ends of those shafts . When the engine is running , the vacuum doesn`t allow the fuel to run out .... only when the engine is shut off over a period of time . I can tell you from experience , that unless you work on these carbs regularly , it is a real bear to replace those seals . Don`t see where anyone else suggested you check your metering blocks and carb body to see if they are warped, but that is a possibility ... also check the gasket under the Power valve . If it is a fiber gasket type , typically , they are a two piece and can separate . The garbage that is called gas these days has some pretty nasty chemicals in it and they attack the gasket . I went to a nylon one from and problem solved . When I took the PV out of the metering block , half the gasket stayed on the metering block and the other half was on the valve . That is an internal leak . On my 427S/O Galaxie both of those items were a problem ... and I had the same fuel puddles on my manifold until I replaced those seals .... and a rich condition until the PV gasket was changed.
Sorry for the long post .

Good luck