Vapor Lock - Carburetors

At the carburetor, heat from the intake manifold can vaporize the gasoline in the fuel bowl(s), thereby exacerbating vapor lock.  Fuel boiling in the carburetor is known as percolation.  See the following YouTube videos:

• Boiling Vapor Locking Gas In Carburetor Float Bowl
• Edelbrock Fuel Boil
• Quick Fuel Carburetor percolating 30-45 minutes after driving.

It is difficult enough to pump fuel with a hot fuel pump but a hot carburetor will take more time to fill with enough fuel to run.  Even if the carburetor has not completely emptied, the rich fuel mixture from the vented vaporized fuel could also cause a flooded engine condition.

One solution is to add insulation between the carburetor and intake manifold.  This can be done by adding thicker gasket or an insulating spacer and this usually doesn't affect the throttle linkage or other connections at the carburetor.  Adding a carb spacer adds volume to the intake manifold plenum, which tends to shift torque to a higher RPM and are best used with single plane intake manifolds.  Street-driven vehicles need to maximize low RPM torque and V8 engines are often equipped with dual plane intake manifolds.  Use a 4-hole with your dual plane intake to improve bottom-end torque. Insulating heat dissipator gaskets are easily available from aftermarket sources.  If you can't find a commercially-available spacer for your carburetor, you can also make one yourself.  Some people have even successfully used materials such as wood but wood is porous and can absorb gasoline.  I would use an phenolic material of suitable thickness to solve a hot carburetor issue.

Some engines (like the Chrysler slant six) run the fuel line from the fuel pump to the carburetor in front of the engine, which exposes the fuel line to the hot air blowing through the radiator.  Some vehicle owners have had success improving their hot temperature operation by rerouting the fuel line to reduce this exposure.  See Fuel Line Mod Improves Starting & Driveability.  A simple and cheap way to add a heat shield to the fuel is slip some split flex loom over the fuel line between the fuel pump and carburetor.

Vehicle manufacturers have worked to combat vaporization (and hydrocarbon emissions) in the fuel lines with additional vent lines to the gas tank and the charcoal canister.  Adding vent lines to cars that were never originally equipped with them adds unnecessary complexity to the fuel system and many have found this to be ineffective at controlling vapor lock.

Some people have added a return fuel line to the tank to prevent percolation of the fuel in the supply line to the carburetor.  This will work in the short term but serves to heat the fuel in the gas tank.  Besides causing unnecessary evaporative hydrocarbon emissions at the gas tank, this can result in a longer cool-down time if the bulk temperature of the fuel in the tank becomes too hot.  In addition to hot fuel recirculated back from the engine, the gas tank is also picking up thermal radiation from the hot pavement below.

I do NOT recommend disabling the intake manifold heat system (heat riser) system to keep the intake manifold cooler.  Street-driven engines need intake manifold heat to work properly and blocking the cross-over passages in the V8 will also disable the EGR system, which is necessary to minimize NOx emissions and engine knock and will affect the carburetor's tuning.  See Intake Manifold Heat.

More insulation is better and thicker spacer between the carburetor and intake manifold usually improves engine performance.  Excessive insulation can cause clearance issues between the air cleaner assembly and the hood and/or linkage binding issues dues to the new linkage geometry.  Check your hood clearance by putting some putty (or kids' play dough) on the air cleaner lid and closing the hood.

4bbl Carburetor Insulation

2bbl Carburetor Insulation