Running Rich Mark III

[Beemer]

I have a Superformance MK III with a 351 stoker with 2 Edelbrock 500 CFM carburetors. It also has an electric fuel pump. It has a running rich problem causing the spark plugs to foul I am trying to solve. After speaking to a number of folks, one item brought up is when I open the gas tank filler, there is a lot of back pressure whereas my friend with the same car except he has a Roush 427R with a mechanical fuel pump has no back pressure when opening his gas tank filler. One person suggested I may have a clogged breather causing the pressure leading to the problem. Another person said it is just a carburetor issues and need to set the mixture. Another person said I need to put a pressure sensor in line to ensure the fuel pressure doesn't exceed the Edelbrock requirements.

Any thoughts?

[Blas]

Pressure or vacuum?

[WardL]

My SPF tank is vented to the atmosphere right under the gas cap. There is an overflow tube to direct excess fuel to the ground. I believe most SPFs were initially set up this way. If your tank is a sealed system, it is likely a custom setup. If your system is not intended to be closed system, then your overflow tube is definitely plugged and should be replaced or unplugged.

[Beemer]

Blass. It is a pressure when I open the gas cap.

[Beemer]

I just spoke with my installer, and he said it is an open system and shouldn't have the pressure when opening the gas cap. He said the vent tube installed by the factory would have to have a kink in it to cause that situation. He also said they had a similar problem in the Daytona coupes and had to drill a small (1/8") hole thru the filler that would allow the tank to vent. It wouldn't be a safety issue. Thoughts or experiences.

[Sawdust]

Beemer
If your vent hose is kinked it's an easy fix. These hoses are prone to kinks. Simply remove the right rear wheel, remove the inner fender and you'll see the vent tube going down the filler neck and into the tank. Go you your local parts store and buy about 3' of fuel line and replace it. Tie wrap it in place. SCOF has a tech article on it (if you are a member)

[JBCOBRA]

Maybe it is the gas cap? I cut a little larger groove in the metal under the rubber gasket to get more venting/breathing in the cap itself.
Just an idea.

[D-CEL]

It doesn't make sense to me that you have that much pressure building up in the fuel tank.
1) Is it a return or dead head type fuel system?
2) Do you have a fuel pressure gage plumbed in ? Id be curious to see what it reads when you first start the car VS. after it has had time to build up pressure.
3) Have you tried leaving the cap off (closing the quick fill?) and seeing if the condition persists?
4) What type of ignition do you run?
Id put my money on carb adjustment.

JASON

[Beemer]

We verified the vent wasn't kinked or clogged. My neighbor has an earlier Mark III and we switched the gas caps and the problem went away. An investigation of the caps showed his inside was different than the one on my car. I spoke with Olthoff people and they said it is a common problem with the later model gas caps. They are too tight. The newer caps have raised rings under the gasket causing the problem. They had me remove the gasket and cut 3 equal distant spokes about a 1/32" deep from the center to the edge of the inside edge of the cap across the rings under the gasket. I replaced the gasket and it worked and the pressure in the tank is gone. Thanks all who commented.