Mechanical Fuel Pumps and “Vapor Return” Lines

Some common problems when running a mechanical fuel pump and a carburetor include vapor lock and poor hot restarts. The problems are magnified during the summer especially when stuck in traffic. Today's pump gas doesn't help. Before the late 1980's (when most cars still used a carb), pump gas, on average, had lower vapor pressure. This helped to reduce heat related problems with the fuel system. But today, higher vapor pressure improves cold starts without creating heat problems because the electric fuel pump in the gas tank keeps the fuel under pressure from the tank all the way to the injectors.

One way to reduce heat related problems in a carburated/mechanical pump fuel system is to use a “vapor return” (VR) line. The VR line circulates a small amount of fuel from the fuel pump ((or fuel filter) back to the gas tank. This lowers the fuel temperature and reduces the chance of the fuel turning to vapor (boiling).

RobbMc 550HP mechanical fuel pumps have always had an 1/8 NPT port for an optional, extra cost ($15) RobbMc VR fitting. This VR fitting has a very small inside diameter (about .040") to restrict the amount of fuel returning to the tank. Without the restriction, the pressure and volume delivered to the carb is too low.

The RobbMc VR fitting is barbed for a ¼" rubber hose because most OEM VR lines are ¼". Unfortunately not everyone wants to use a fitting for a ¼" hose. Some people would like to use a larger return line or a different style of return line (such as an AN line).

For these reasons, RobbMc 550HP pumps are now made with the .040" restriction built into the pump itself rather than in an extra cost fitting. This means that any 1/8 NPT fitting can be used for the VR. It also means that there is no $15 extra charge for those who want to use a VR line. A 1/8 NPT plug is provided for those who don't.

The RobbMc Test Vehicle

Below are a couple photos of Robb's 1965 351C powered Ford Falcon. Not only is it Robb's daily driver, it is also RobbMc's primary test vehicle. The Falcon uses a RobbMc starter, fuel tank sending unit, fuel filters, and PowerSurge. Before changing to fuel injection, the Falcon also used a RobbMc 550HP mechanical fuel pump for several years.