Fuel Pumps

[John Hofstetter <hofs@domain.elided>]

>Date:
>From: Roger Scow <rscow@domain.elided>
>Subject: Electric vs. mechanical fuel pump - or both?
>
>While I was trying to figure out the poor starting characteristics of my
>'79 Traveler, I added an electric fuel pump.  I blocked off the mechanical
>pump on the block.  I have since come to the conclusion that the problem
>lay in the 1)stuck valve in #1, 2)a couple of bad plugs, and 3)lack of a
>carb insulating gasket (4 bbl Performer carb sitting on heated manifold -
>thanks FAQ guys!).
>
>Now that I'm putting it back together, is there any advantage to scrapping
>the electric pump, and putting the mechanical one back?  Or should I just
>keep the electric one?  Or, both, and run'em in series?
>
>Any takers?
>
>Merry Christmas
>
>Roger Scow
>'78 Traveler/392/TF727
>
>
>***
>It's funny, you'd think that all of those guys in MENSA, the genius club,
>could have figured out that, in Spanish, "mensa" means "stupid."
>
>------------------------------
>
>Date: Tue, 23 Dec 1997 12:11:26 -0500
>From: tsm1@domain.elided (Tom Mandera)
>Subject: Re: Electric vs. mechanical fuel pump - or both?
>
>>Now that I'm putting it back together, is there any advantage to scrapping
>>the electric pump, and putting the mechanical one back?  Or should I just
>>keep the electric one?  Or, both, and run'em in series?
>
>IMHO, run both.  Hook the electrical pump up to 1) ignition switch and then,
>in series, 2) a rocker/toggle/whatever switch.
>
>Why?
>
>Well, typically I'd leave the switch OFF, so no electrical pump.. just run
>the mechanical pump.
>
>Then, if the mechanical pump dies (and, when it does, you'll be 300 miles
>from nowhere, by yourself, without telling anyone where you're going, on an
>unmarked trail, in the desert with no water), you can just flip that toggle
>switch and you're back in business running the electric pump (which should
>still have a long life, since you haven't been using it.
>
>Then change out the mechanical pump at your leisure.
>
>Others have suggested using the electrical pump just before starting your
>Scout.. hit the "RUN" position, flip the switch, and let the electrical pump
>(typically mounted very close to the fuel tank) prime the lines and push the
>fuel up to the mechanical pump, then hit the starter.  Save your mech. pump
>some wear n' tear, particularly if you let your truck sit for a long time,
>and all the fuel drains out of the carb, and back down the line.. it can
>take a while for that mechanical pump to suck enough air to get some gas.
>
>Adding a backup electrical fuel pump is on my list of things "To-Do-Later"
>after I get my main build up done.  Probably do the electrical pump at the
>same time I get the $$$ and time for a 33 gal fuel tank.
>
>- -Tom Mandera, Helena MT
>http://www.tmcom.com/~`tsm1/scout
>'72 and '77 Scout IIs

Roger and Tom,
Here's what I think about that. On the Travelall I had, vapor lock was a
big problem. I helped it a lot by installing an electrical pump right at
the gas tank and then let the mechanical pump maintain the pressure into
the carb. They were wired in series. If, however, I was using an electrical
for backup, I'd plumb them in parallel, so that when the mechanical pump
failed, the electrical pump would bypass the mechanical with the busted
diaphragm. If my memory serves me correctly, if the diaphragm fails, and if
there were pressure in the chamber above the diaphragm, the gas would go
through the hole in the diaphragm and into the crankcase. Isn't that
correct? Plumbing in parallel with appropriate valving or check valves
would eliminate that possibility. Also in unimaginable situations where you
needed lots of gas, you could turn on the electric pump and have both
supplying gas to the carb. On my Holley EFI installation, I originally had
an electric high volume pump getting the gas out of the tank and then the
high pressure pump sending it on to the injectors. (Another series
installation) I've never done a parallel installation, but it makes sense
to me. What do you think?
John

John Hofstetter a.k.a. Ol'Saline, who's been a lot of places none of us is
allowed to go anymore and that's a real tragedy, and who to quote the
senator from Texas, "has more guns than he needs but not as many as he
wants"
79 Scout Terra with 8000 lb. Warn Winch, Detroit Locker in rear and limited
slip in front, 4" Trailmaster lift kit, Holley Electronic Fuel Injection,
Mallory Unilite Distributor                                  MSD ignition
"It's a legend", but it's too darn long and too darn wide sometimes to go
where I ask it to go.

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