Fuel starvation

[IGOTBIT]

I need help from you Cobra gurus

Here is the problem: After running about an hour of so, the car starts to run like sh**. It spits and coughs, and doesn't want to rev. The fuel filter looks empty. After I turn it off for about 15 minutes everything is fine.

Pertinant details: 351 W, mechanical fuel pump, in-line filter upstream of the pump. All this in a Johnspeed Cobra.

Looking at the system, I am not surprised. The filler cap seals tight. There is a breather hose from the top of the tank back into the filler neck, for burping during filling I suppose, but that still leaves it a closed system.

It seems to be worse (happen sooner) with a full tank. However, when it is acting bad, and I open the filler cap, it does not fix the problem. Even if I ride around and slosh the tak with the filler cap open. Also, the burp line does not seem to be working. I have to fill real slow.

Here is my plan: I will pull off the burp line and make sure it is free of obstructions. I will splice into it near the filler neck, and run into a check valve and out into open air. I am suspecting that once it is pulling a vacuum, it starves the pump, and it will not re-prime until the car is off for a while. I don't know.

So, my question is, do you think this will work? Where can I get a check valve that will handle gasoline vapors? Could it be something else? Heat related or something? Should I add an electric fuel pump at the tank with a switch? I am open to suggestions, but there is nothing worse than being stopped at a red light when the fan beside you says "lets hear it!" and you rev it and it sounds like sh**. And oh yeah, getting stranded on the side of the road ain't too cool either.

[IGOTBIT]

No takers huh? Well, I bought a PCV to use as a check valve, and plan to install it tomorrow. Unfortunately, I will have to wait until spring to know if it worked.

[lineslinger]

Your analysis sounds good, at least the vacuum scenario. Once the suction gets "inline" it is hard to eliminate. It can trap itself within the fuel line, a vacuum pocket if you will, that the fuel pump continues to fight, the pocket continues to expand and contract depending on the amount of draw against it, in certain situations you have to let it work itself out by equalizing the tank pressure, or you can equalize the tank and disconnect the fuel line somewhere before the carbs to let the pressure adjust itself on both sides of the line.
Yeah, your tank is developing negative pressure to the point where the fuel pump cannot override it.You need to equalize the tank pressure with the ambient air pressure, let the tank "breathe" and I bet your problems will disappear. Sounds like the pcv valve should work as long as you run it in such a manner as to open the system to ambient air pressure.
Hope this helps.

[sllib]

Call Summit or Jegs (or your local speed shop) and ask for a rollover valve. It will vent your tank but closes off if you decide to go inverted.
Bill Stradtner

[nevermind65]

To be honest, it sounds more like a vapor lock problem. Is the fuel line run close to the exhaust in any way, or is heat getting to the line somewhere near the engine?