Hard starting when hot - vapor lock?

[Night Enforcer]

I have been through some of what your are experiencing with my own carbuerated cars and those of other rodders. MOst of thetime it is vapor lock. It could also be the carb or timing. I have found what Tom K is suggesting is rigt on as to the fuel pump and the location of the regulator. I would add that the return line will help your car run better CONSISTENTLY, so even if ithis is not the exact problem, I would encourage you to put one in with a high end fuel regulator with an accurate fuel pressure gage installed.

Answering a few questions would help give you some suggestions.

Have you checked your fuel pressure? If so what is it?

Do you have a fuel regulator. Is it adjusted to the proper fuel pressure for your carb. Too much fuel flowing to the carb at start up is just as bad as too little.

How old is your electric fuel pump and what brand is it? Is it mounted correctly? Some units need to be mounted so that the unit is horiz vs vertical. Also they need to be mounted on the chasis rail so that the incoming fuel line is at a certain distance from the fuel tank and cannot have a point in it that is lower than the inlet of the electric fuel pump. If installed incorrectly they will cavitate for awhile causing a hard start problem. That's why it's a good idea to know what your fuel pressure is at the carb or fuel log at strat up and while running.

Have you noticed that when the car is off and sitting for a while, and you first open the fuel cap, that your hear a slight sucking sound for a split second, like there is a vacumn in the tank? We have found a couple of SPF cars that had a problem with the gas tank venting properly. It caused a vacumn in the tank especially on hot days and caused problems for the fuel pump.

What size is the fuel line?

Have your floats in your carb been adjusted to the proper height?

What is your vacumn at idle?

Have you adjusted the idle circuit lately?

What type of fuel does the area of Michigan have that you live in when you experince this problem? Oxygenated or not? We have found that some oxy fuels in Arizona, boil at 140F. That means that they can boil in the fuel lines and/or float bowls on a med warm day. Different parts of AZ have different oxy fuels and other parts don't have oxy fuels.

What type of fuel log are you using?

Have you checked ALL of the the fuel filters? Besides an inline unit, sometimes there is one in the fuel logs and the inlet of the carb.

What is your initial timing?

I know it is a lot to consider, but each one could influence your situation.

I would say knowing that your fuel pressure is correct is the first thing you should check out.

[Dustin Mustangs]

Thanks for all the feedback.

The fuel pressure is at 6.5 pounds as recommended by the engine builder. The regulator (Aeromotive) has a gage attached and the readings are consistent.

The electric fuel pump is two years old and also Aeromotive properly spec’ed for my set up. I have talked to the Aeromotive application engineers about my specific needs. I believe it is mounted correctly in the vertical position and on the frame below the tank. I do have a 100 micron filter between the tank and the pump where half the diameter of the filter is (barrel shape) geometrically below the pump pick up. Pump cavitation (the noise we are hearing) is a concern and this may be an element. The cavitation happens after full warm up on warm days, at low engine speed and usually in a turn – not a hard turn. You can hear it as it affects the regulator which is mounted on the firewall. I do not believe that the cavitation and the “vapor lock” (if that is what it is) are related.

I have checked the tank venting and could not detect any problems. Just to be sure I added a vent to the cap deep in the labyrinth above the gasket so as not to leak fuel. I think this can be checked off.

Fuel line is the 3/8 inch braided stainless.

Floats and carb has been inspected and tuned by a pro within a year. He believes that the problem is vapor lock as well. He theorizes that the Superformance fiberglass body is helping to insulate the heat (He wants to sell me an aluminum body).

Can’t tell you off hand what the vacuum is at idle, I have pretty bad vacuum at low engine speed – overlap I’m sure. In a hard stop with the clutch in the engine will normally stall for lack of vacuum.

I have not touched the carb settings since the pro work. The problem has existed from the beginning, before and after a number of professionals have worked on different parts of the drive train. None have worked specifically on this problem.

I’m going to guess Michigan fuel is not Oxygenated. Don’t know for sure.

The fuel log is just an Aeroquip set up with “T”s, see attachment.

I check and service both filters regularly, 100 micron before the pump and 10 micron before the carb. The problems existed before the filter installations.

Total timing is 34 degrees BTC. Don’t know off hand what the initial timing is. It was set by a very experienced professional about a year ago.