I have 2 trucks, both with 300-6's. Single barrel carbs that run so rich I'm getting 'fumed' out. Run rough. Tried rebuilding, no change. I can hear/see the raw gas going down the throat. Adjusting the idle mix screw has no effect. Any ideas or fixes.

 

The problem is in the float,the needle & seat and/or the float setting. These carbs are very prone to misadjusted floats & sticking needles. Being that you just recently rebuilt it, you can remove the top without tearing the gasket for an on truck adjustment.If you are not scared & are safe about it,you can start the engine and see how high the gas is getting in the bowl,provided the top doesn't house the needele,seat & float.If the gas does stop rising eventually,but too high,just an adjustment is necessary to bring the level down.If the gas continues to fill over the float,the needle & seat is probably defective & you will want to put a new float if it looks marginal.You didn't mention if you had the Holley or Carter carb,but the Holley was more prone to having this trouble.
Phillip

 

I have 2 trucks, both with 300-6's. Single barrel Holleys that run so rich I'm getting 'fumed' out. Run rough. Tried rebuilding, no change. I can hear/see the raw gas going down the throat. Adjusting the idle mix screw has no effect. Per a recent suggestion, I have checked the float levels and the floats and fuel level appear OK. Any other ideas. One truck has an ERG ('86), the other doesn't ('72).