drdrdaddeo

Newbie to the Third Gen forum. I'm ground up restoring a 1985 base 5.0 L firebird (ECM/4 BLL Carb) to an emission's free street machine (going with Edelbrock Performer manifold and 1706 carb). Original GM mechanical fuel pump has two lines into it from the tank, one for fuel in and the other for vapor out to the tank (? providing tank pressure). The gen three firebird fuel tanks have four fuel lines out, two as above to/from the fuel pump, and two more to/from the charcoal fuel cannister (tank vapors sucked into the cannister and returned to the tank to control emmissions and maintain tank pressure all under ECM control). Since I'm trashing the entire ECM system, I've removed the cannister entirely and plugged both those vapor lines. I also plan to use a vented tank gas cap to relieve tank pressure but I don't know which mechanical fuel pump I should use.....i.e. a standard one line in pump (plugging off the old return vapor line), or a newer, two line mechanical pump, leaving the vapor return to the tank (and risking excess tank pressure build-up), assuming that tank pressure should be relieved by venting through the gas cap. Any suggestions out there?

Apeiron

The second pump line is a solid fuel return line, not a vapour line. Return style fuel systems maintain more constant fuel temperatures.

drdrdaddeo

My sincere thanks to Apeiron......that is incredibly informative (and not at all well-explained in either Haynes or Chilton manuals)! It also fully explains why I got a mouthful of liguid gasoline when I sucked on what I thought was the vapor return line (I thought my pump was bad). Most importantly, it also suggests that the return line should not in any way increase pressure in the fuel tank, as it is simply returning whatever gas has been pumped out of the tank but not used in the carb (actually probably leaving a slight negative tank pressure during engine operation, easily normalized by a vented gas cap. I'll guess I'll stick with a new dual line mechanical fuel pump unless anyone knows of other advantages to the fancy new single line pumps (with the return line capped off of course).