hard starting when hot!

[Night Enforcer]

Sounds like good ol' vapor lock. I have a great deal of experience with this issue both with Cobras and other carbed rods that I own and my compatriots rods.

A couple of questions are in order. Are you using a mechanical or electric fuel pump? Return fuel line or no return line? What part of the country do you live in? Oxygenated fuel or not? Elevation where you live.

We have done extensive testing and found that certain oxygenated fuel mixes in Central and Southern Arizona will boil as low as 140 degrees F. Even some “Race Gas” is oxygenated and boils at lower than expected temps and can cause a bit of vapor lock after long hard runs and then letting the car sit and the heat to percolate. The low boiling point means that the fuel in your fuel log and float bowls may be boiling after the car sits for a while after a run, and the heat off of the manifold and engine in general, goes to work on the fuel. In addition the fuel in you fuel line running under your car to the mechanical fuel pump or to the carb can heat up enough to boil the fuel. In Tucson in the summer the pavement can reach 160F easy.

Keep in mind as you read the rest of my post that a mechanical fuel pump is basically sucking the fuel from the tank. This lowers the pressure in the fuel line thereby lowering the boiling point even further. (Boyles Law) An electric fuel pump is for the most part pushing the fuel and thereby pressurizing the fuel in the line and raising the boiling point of the fuel. (Boyles Law)

We have found several things and combinations that work to help alleviate vapor lock. The best is number (1). I have placed these in order of being most successful. (1) Electric fuel pump and return line to the tank. (2) Electric fuel pump. (3) Mechanical fuel pump and return line to fuel tank. (4) Holley aluminum reflector /diffuser plate and phenolic or wood spacer between the carburetor and the manifold. (5) Wrap fuel lines in reflective material and place reflective material over mechanical fuel pump.

Depending on where you live and the fuel you are using, the deflector and the carb spacer is enough to solve 80% of the problem. Let me know if you need any additional info.