I'm in the process of replacing my 83 Bird's mechanical fuel pump (I believe the diaphragm is torn/worn out because it spits gas out of it's side vents). I found a new Bosch pump that will fit my Chevy 350 for a good price and I just noticed that it has a vapor return. I don't believe my car's current pump has it (I have a vented gas cap so I don't think it'd need it anyway). My question is, would there be a problem with just leaving the new pump's vapor return outlet open?

I got a Rochester Quadrajet in her also if that's relevant.

 

It's not a vapour return, it's the bypass return line for the internal regulator. It must be connected. The stock pump had it too. If your pump doesn't have one currently, it's been replaced at some point.

 

Oops wrong word. lol Yeah I bought my car for cheap from a friend of mine that knew very little about the car. I got it because of it's good condition. I've looked it over and it was modified quite a bit over the years. It originally had a 151 4 cylinder engine with TBI, c200 tranny, and other various things. Now it's got a 350 V8, Turbo 350 tranny, fiberglass hood, alloy rims etc. I'm not sure why it's current pump doesn't have a bypass return.

Forgive me but I'm not sure I understand how the bypass return and the internal regulator work etc. I guess I'll have to do some reading about the process. If anybody could explain to me it's purpose and why it's necessary, that would be very helpful.

 

When the desired pressure is reached, the regulator valve inside the pump will open up and send excess fuel back to the tank through the return line. If you plug the return line, there's nowhere for excess pressure to be vented, and all the fuel will be sent up to the carb, flooding it.

The regulator in a non-return pump will just circulate excess fuel within the pump body itself.

 

Thank you. Now I'm beginning to understand it.

 

Oh, I was gonna say that I originally had the idea that the bypass return was to pump air back to a sealed tank to keep it from making a vacuum that would prevent the fuel from flowing. That's what led me to think that it would be alright to leave it open, I thought it would just push air out of it.

I don't know where I got that Idea from. I probly confused it with something else.