-I posted this on the fourm for 1967-72 F100, I just found this one and figured I should put it here too.

Hey guys, I have a 1969 F100 that I have had since my freshmen year of high school. I have done lots of work and improvements in the past 4 years, including switching from the original 3-on-the-tree transmission to a 4 speed t-18, and rebuilding a junk yard 300ci I-6 and swapping it with the original 360ci V-8.(I am rebuilding the 360 which had just over 100,000mi on it and I plan to swap it back) Anyway, I dropped the new engine in last June and it has been a dream ever since, until about a month ago when it started flooding like crazy. Fuel runs all down the carburetor and down intake manifold. When I remove the carb, there is fuel pooling inside the manifold. I first checked the needle valve on the carb because I figured it had failed, I went a head and rebuilt the carb. The problem was back within 15 miles. Next I checked the pressure that the fuel pump was delivering and found it to be correct. I put a second carb that I had on to see if it would make any difference, and it got me from Denver to Fort Collins (60 mies), but I was began getting trouble again inside the city, and now I’m back to where I started. Any ideas, I’m stumped.

 

the only thing I can think of is that the fuel metering system may be stuck so that the needle valve cant close all the way, or the float is leaking and full of gas. but rebuilding the first carb would have fixed that.Try swapping in a carb that you know to be good and see if that works