That carb should be a square bore.

My question is how are you adjusting your carb?

What is your timing set at, at idle speed and say 3000 rpm? Do you have a vacuum advance distributor? It sounds like it could also be a timing issue if you are hooked to manifold vacuum instead of ported vacuum because that will cause similar issues.

Take some pictures.

Thanks for the help in ordering!

I am not quite that good at tuning, but I will take picks and try to tune properly.. WIsh one of you guys was close enough to give me some pointers in person..

Do you have a tacho on the truck ?
If you do , warm the motor up and set your idle around 600 rpm .
Make sure your choke butterfly is fully open .
With engine running turn the idle mixture screws one at a time in or out until you get the smoothest idle .
Mixture screws should be around 1 1/2 turns out or there abouts .
Adjust your idle speed back to around 600 rpm and repeat until it's running right .
This is assuming your timing is correct also .

Packrat56,

I have a 1955 F100 with a 390/C6 combo with an Edelbrock carb. I believe you have been given alot of good information and would like to help further, but have a few questions. What specific carburetor are you running (e.g., Edelbrock 1406, 1407)? The carb number will be stamped on the front of the carb (passenger side). Next, what jets (primary and secondary) and rods are you running? The jets and rods have numbers stamped on them as well. Further, what step up spring are you using (yellow 4" hg, orange 5" hg, Pink 7" hg, or Silver 8" hg)? Finally, what is your float setting (e.g., 7/16”)?

Once I get the information above, we can move to the nextstep in making sure your carb is running at it's best.

Lance

This is a very good thread about a problem I think a lot of people have especially this time of year. I am having the same issue only with a Holley carb. All the people I have talked to has told me it's "percolation" and the heat from the engine is boiling the fuel thus, expanding it out of the fuel bowls, into the intake manifold, and in turn making the engine run extremely rich, if not flooding it out altogether. Ethanol is extremely sensitive to heat, but it seems non ethanol gas is volatile too. In some style of Motorcraft carbs, they used a "hot idle compansator". This was a simple bi-metallic type of valve that opened up and created an intended small vacuum leak during high under hood temperatures. This raised the idle, to help combat the drag of early air condtioners compressors, but also had a side benefit to help control an overly rich idle mixture due to percolation. They were built into the carburators, and the early hot idle compensators were plumbed into the PVC to carb spacer hose. In addition to all the great suggestions mentioned by others earlier, Some things I am thinking about doing, but have not tried yet are:
1. Going to an electric fuel pump. Fuel should be cooler since it won't be heated up by running through the block mounted mechanical pump.
2. Installing a fuel regulator and a fuel return line. This way the fuel can be pressurized to help raise the boiling point of the fuel. The return line from the fuel filter or regulator back to the tank will help keep the fuel moving, not allowing it to be stopped by the float bowls of the carburetor. The fuel tank almost acts a like a radiator, it's taking a small, but warm, return trickle of fuel, and cooling it down by mixing it in with the fuel being stored in your fuel tank.
I have tried many different carb spacers, even a carb heat shield, they did help, just a little. As mentioned before there are many variables. But anything you can do to keep your fuel cooler, should help.