Fuelproblem

Good day
I have a 83 with the 305 HO engine 5 speed manual, and a mechanical fuel pump on engine. I had to replace the fuel tank and found it has a electric fuel pump in it. Replaced the electric pump and tank the Rochester carb overflowed with fuel when I tried to start it. I disconnected the electric fuel pump dried the fuel up and the car started fine. Question ? Why is there a electric fuel pump in the tank, and should it be there and if so how can I make it work properly. Thank you Steven K.

zed-028

electric pump was there to prevent vapor lock. the fuel hard lines run past the exhaust manifold and make a vapor bubble in hot conditions. the fuel cant get past the bubble and you kill your battery trying to get it to start.

by having an electric pump pushing from the fuel tank, you can (99.0274%) of the time overcome vapor lock... but not always.

i had the same problem as you did, the fuel pump in the tank was pushing too much pressure and the return off the mech pump wasnt gutsy enough to lower the pressure enough. the excess pressure blows the needle out of the seat of the carb when you stomp it, causing the far more literal flooding than the usual carby flood.

i put an adjustable regulator and limited the pressure just before the carb to between 3-5 psi

Fuelproblem

Thank you for the info, I had no idea about the vapor bubble in hot conditions, I had a feeling it was a psi problem, I check the Spec on the pump it says it puts out 13 PSI I will install the adjustable regulator and limited the pressure to the carb. Hopefully the volume of fuel will not change.


Thank you
Steven K.

naf

iTrader: (7)

the proper pump for that application should provide about 2-3 psi

look for a pump for a 87 LG4. same application but the helper pump was a standard item in the 87. some early L69's had the helper pump installed by the dealer

five7kid

iTrader: (14)

All 3rd gen f-body carb'd V8's had a return type mechanical fuel pump. I'm a bit surprised to hear anyone has had a problem when used with the factory in-tank electric pump.

The mechanical pump regulates pressure via a check valve in the return line. When pressure gets high enough, the check valve cracks and returns fuel to the tank. With the in-tank electric pusher pump, the return check valve is always open so there is a constant flow through the fuel supply line (keeps it cooler, and being under pressure reduces the chance for vapor lock).

If the in-tank pusher electric pump is overwhelming the carb needle/seat, you either have a needle/seat problem, a blocked return line, or an in-tank electric pump with too much volume.

FWIW, the Carter M6626 pump is a good choice for a mechanical replacement pump. Not much more $ (if any) than a "stock" replacement.

naf

iTrader: (7)

I ran an in-line external pump through a non-functioning carter mech pump with no issues for years and 1000's of miles, but I can imagine a scenario where the pump performance curve was not a match to the fuel system head curve.

Suspect a TBI/CFI pump was installed

the 6626 pump at Rock Auto is like less than a good six pack.

Fuelproblem

I was wondering if the Mechanical had is own pressure regulator, I put a new mechanical fuel pump from NAPA in when I got the car a year ago, maybe the check valve was defective or it was damaged, it did have a bit of rust go through the new one before I found the rust in the tank and changed it and the electric pump. The car sat for ten years in a garage before I got hold of it. (Barn -find).


Thank for the feed-back

Steve K

zed-028

agreed - however i couldnt find a low enough pressure electric pump fuel pump for the tank in Australia. Most people ive talked to would just ditch the in tank pump however with 45C+ days throughout summer - vapor lock is a real bitch in this country.

my carb only ever got the fuel diarrhoea when i fully stomped it - but not EVERY time i stomped it. Got some info from ruggles about the ideal fuel pressure for different applications on the quadrajet. These range from about 2-3 up to about 7

my regulator bounces between 3 and 6 pound

Five7 - i actually bought one of those carter mech pumps but havent installed it as yet, i wonder if the updated pump would regulate my pressure any better. TBH it doesnt bother me that much since the problem doesnt occur anymore