In another post about intake gaskets somone mentioned they had gaskets with an open exhaust crossover which caused the fuel to boil.How do you know if the fuel is boiling?Does it spill out of the carb and what are the symtoms?I'm wondering if I have the same problem because my car runs different from a stop after it gets hot.

 

Related from the Holley site;


QUESTION After I shut my car off and come back out the next morning there is gas on the throttle shaft and puddled on the intake. What causes this?
ANSWER This is usually caused by percolation. This is when the engine is shut off and the engine temperature rises it causes the fuel to boil in the bowl and leak out of the boosters. There are a couple of things you can do to cure this one is make sure the fuel level is not too high. You can also lower the fuel level about 1/8" below the sight plug hole and this will cure it sometimes. The heat from the engine will rise into the carburetor sometimes and will cause the fuel to boil. Installing a phenolic heat spacer between the carburetor and the intake or a heat shield can cure this. These parts will prevent heat from getting to the carburetor and boiling the fuel.

 

Will changing intake gaskets to the blocked exhaust crossover improve anything at all? My car gets driven only in the warm weather months so starting is not a problem.I already have an insulator between the carb and manifold and wondering if blocking off those ports would be beneficial.

 

Do you have an aluminum intake or is it cast iron? Aluminum intake engines can block the crossover with a thin piece of stainless steel sheet metal. I would not block both sides on an iron intake. It will take too long to warm up. Aluminum manifolds heat up fast enough that the longer warm up is not hard to deal with.

I made my block off out of a stainless steel mixing bowl bottom. It was flat and about the right size so I just used my snips to make it. You want it just wide enough to fit between the 2 intake manifold bolts.

The wife still wonders where her mixing bowl went. "I HAVE NO IDEA"

-Mark.

 

Next time you drive it pull the air cleaner off as soon as you turn off the car. Watch the carb and see if you see vapors rising out of it. This usually indicates a sign of fuel boiling in the fuel bowls on a Holley. The best cure is to run a return line back to the tank. (i.e setup a return style of fuel system). Depending on how bad it is you can also try lowering your floats a little. Some guys also have luck with a heat shield between the carb and the intake as well as phenolic spacers if you have the clearance. Good luck.
P.S make sure your timing is right also.

 

After the car has sat overnight, pull the dipstick and smell it. If it smells gassy, it's a good chance fuel is boiling over into the intake, into the chambers through an open valve, and seeping past the rings

 

1) Weld the heat riser valve open or gut it.
2) Use a carb/intake insulator. This one is 0.320".



3) Use intake gaskets with crossover block plates.

 

I had a serious fuel percolation problem with my SBC L82 and Holley carb. I have the clear sight plugs in the fuel bowls, and you could see the fuel boiling. like water boiling to boil an egg.

I installed the standard alum plate heat shield between the carb and intake manifold. I also installed a 1/2" thick phenolic spacer there. I also got some alum-back fiberglass insulation from Longacre Racing and cut a piece to shield the front fuel bowl from the hot air hitting it from the radiator fan, covered the front, sides & bottom of the fuel bowl. When I put the air cleaner on, it holds the insulation in place.

I also covered the fuel lines to the carb with that orange silicone-coated fiberglass-braid insulation tubing (also Longacre) to protect it from engine compartment heat, and also covered the factory steel fuel line as it comes up by the frame with the same orange insulation to protect it from header heat.

I also made an alum shield to protect my fender-well-mounted Fram fuel filter and blue Holley fuel pressure regulator from engine compartment & header heat, and lined it with the alum-backed fiberglass insulation. fabbed from sheet alum "flashing" from the hardware store and pop-rivets.

of course, the heat cross-over passages in the intake manifold are also blocked.

alltogether, I suspect this is way overkill, but I wanted to kill the problem for sure (with extreme prejudice), so I covered all the bases. I don't have a fuel percolation problem anymore.

 

I solved my problem with a 3/8" carb gasket. I do not know who makes it, but I got it at a local speed shop.