Boil, Boil, Toil, and Trouble
I've just put together a 406ci (0.030-over 400) small-block Chevy with a supercharger. It dyno'd at 460 hp and 524 lb-ft of torque. The problem is that the fuel in the carb boils when the outside temperature is above 85 degrees. It usually happens when I decelerate after pushing it kind of hard. I've made it better by installing a heat shield under the carb, replacing the mechanical fuel pump with an electric fuel pump, moving the fuel lines away from the block, messing with the timing (it's never knocked), and mixing in some aviation fuel with premium gas. What's next?
There should be no less than 3/8-inch clearance (and preferably 1 inch clearance) between
Try the following:
(1) Check the clearance between the underside of the air cleaner lid and the carburetor's vent tubes (see illustration A). Run the car without the air cleaner; if the problem disappears, you've found the culprit.
(2) Install at least a 1/2-inch phenolic (plastic) spacer under the carb to prevent heat transfer from the engine to the carb (these plastic spacers are much more effective than a simple aluminum plate).
(3) Recheck the fuel line routing. You said you routed the fuel lines away from the engine, but what about the exhaust system? Sometimes exhaust heat transfers through the frame to the lines even if there's no direct contact between fuel lines and exhaust pipes.
(4) Pushing fuel is better than pulling-is the electric fuel pump installed back near the fuel tank at or below the tank's fuel level?
A standard Holley fuel pressure regulator can be used as a bypass valve by plumbing it as
(5) Install a 5/16- or 3/8-inch fuel return line. Recirculating the fuel in a constant loop back to the tank ensures a fresh supply of cool fuel, even under low-demand (deceleration, cruise, and/or idle) conditions. Manage the return rate by installing a check-valve in the system's return side. You can use a standard Holley fuel pressure regulator for this purpose (PN 12-803, 3/8-NPT ports, 0.220-inch restriction; or PN 12-704, 1/2-NPT ports, 0.4375-inch restriction; see illustration B for correct hookup). BG Products offers a single-stage diaphragm-valve bypass (PN 171021).
(6) Metallic fuel lines and steel/braided "racing" hose retain more heat than plain polyester-braid hose or Nomex-braid hose. However, cheap parts-store rubber fuel-line hose doesn't hold up and is not sufficiently fire resistant. Aeroquip's FC300 AQP braided polyester hose and StarLite Nomex hose are now NHRA-legal for plumbing the entire fuel system. Other similar hoses may also be acceptable; contact the National Hot Rod Association for the latest rules.
(7) If your car has been upgraded with a performance dual-exhaust system, check its routing where it passes the fuel tank. Often, routing is tight and the exhaust temperature heats up the fuel inside the tank.
1695 Indian Wood Cir.
Maumee, OH 43537-0700
BG Fuel Systems
1450 McDonald Rd.
Dahlonega, GA 30533-2424
Holley Performance Products
P.O. Box 10360
Bowling Green, KY 42102-7360
National Hot Rod Association
2035 Financial Way
Glendora, CA 91740-0950
I have some questions regarding solid-lifter cams. First, why do manufacturers give solid cam specs at zero clearance when the cam is not run at zero clearance? This is really misleading when buying a solid cam and looking for a certain lift and duration.
Second, wouldn't the duration figure given at 0.050-inch lift be different if you figure in the lash clearance of the cam? Just how much does the lash clearance change the duration anyway?