Soon, I will be finished rebuilding my 250.
For initial break-in, i am taking my machine shop guy's advice and only running one carb (i have an offy 3x1) so i dont run the risk of washing down the cyl walls and preventing ring-seat. I had planned on running an electric fuel pump for my setup, so i could prefill the carbs before turning the key,to give me better start-up. Right now I have my tank out to make new fuel lines. My question is, can i run my electric pump with one carb? Or is it too much pressure? I wanna going to use the Carter P4070 pump, (72 GPH and 5.75 psi). Or is there a better pump? I need some help. Thanks!

That electric pump will work.
I believe you can install a 292 pump that has a bigger outlet/feed tube.
Anyone can chime in on this.

What is nice about the mechanical pump is,,,at idle the pump does not make that much pressure as compared to high RPM,therefor,possible better mileage,won't overcome the needle @ seat to flood out the carbs, & for the most part ,,,,more reliable.

Anyone else care to comment?

MBHD

Is there a fuel pressure regulator in the system, or built into the pump? How much pressure can the carburetors take before the needles will not seat and seal? is there a fuel pressure gauge? The fuel pump should not care about how many carburetors it is feeding. However, starting for the first time with only one carburetor (the others removed and blanked off) reduces the chance that the engine will be running excessively rich or lean, as there only two screws that need to be adjusted instead of six.

First off; Your Carb. (one or more) don't require 'pre flling' as; they should stay full, running or not.

Hank's correct, either pump is okay. You'll only need the electric for long-high RPM usage.

If your floats are set right your not going to "wash off" the cylinder walls and only 2 of the 6 idle mixture screws (center) are used anyway. The end ones are closed because 3 Carb. 'set-ups' idle with only one.

Good luck.

I might have missed it, but does that mean i can leave that pump in for break-in? Its not too high of a pressure for the carb?
Thanks for the feedback

PS
There is no pressure regulator
Going electric will make it easier to start? When run in combo with a MSD box?

Actually, now that I read closer, I think the overall concensus, is I should stay with the mechanical fuel pump. Sounds OK to me.

Also, if I have the mixture screws set identically, would that cause me to run too rich?

If your going to run a elec. pump it is always a safety to use a pressure regulator & a small mount inline pressure gauge.This way You know just
what the pump is really pushing out.
And I do agree with John on the air mixture screws Close off the end carbs.Same can go for the Idle screw you can back that off as well & leave the linkage Not hooked up to them and run the center carb.
You can also do like Hyot said and leave the other
to carbs off.

I know many of you have seen my 3x4 tunnel ram
set-up. A test i did one time Just for (GP)Short
tirm ride up and down the street.
Is i ran each carb one at a time No linkage
hooked to the others and it ran Off anyone of the
carbs.Mind you my carbs sit right over each runner
Just normal take offs (just like a Old lady lol)
With having the plenum built as a balance tube
between the runners the fuel still got to each
end of the motor.Not a way I would want to really
drive the car But I just wanted to see if it would
run.And to see if I ever got stuck between a rock
an a Hard place.If sometime was to happen to anyone of the carbs.(you all know what i mean)
So I only made my 2cent comment to prove a point
you won't need to make intake swaps to break your
motor in.You can Use anyone of the above Ideas
and save your self some headacks.

I am unfamiliar with the Offy 3X1 intake so forgive me if this is an obviously stupid question. What Carbs are you running (singles yes, but which ones)? I see you say you will have the Offy 3X1 and a Carter Electric pump, but (unless I missed it) I don't see where you state what carbs you are using. I don't know for sure, but it is my understanding the BXOV Strombergs tolerate less fuel pressure than the Rochesters, I don't know about the Carters or the Zeniths.

If you want to know if that's too much pressure, I would expect that's need to know info. But maybe I'm goofy.


IMHO an electric pump is fine, as long as you run an oil pressure safety switch with it. A pressure gauge is definitely handy and a pressure regulator, if necessary. But that's me.

I am running the Rochester Monojet MV I believe, circa 1968.

I would use the '292 pump/line' sugested. It's the fastest & cheapest way.

Unless you've made 'major' modfications (engine)it will be plenty.

The electric's usually are needed only to "suplement" the mechanical one(s)
on trucks/boats/motorhomes with multiple
tanks/where the tank is a long distance
from the engine etc.

Happy trails.

OK, I'll give a little more information.

I really appreciate all the help you guys give.

250 Chevy
Bored .030"
New dished pistions, bearings ect.
Lots of port work done, polished, larger int. valves and stainless exhaust valves
Comp 252H Cam
Langdon's headers, Offy 3x1 intake with MV Rochesters.
I will most likely be adding a MSD box to the existing HEI I already have. I would like this to start as quickly as possible when turning the key. Essentially what I wanna know, is if an electric pump would help me reach that goal, or if I should stick with mechanical?

As far as using the 292 pump with a 3/8" outlet...
The carbs I'm using have 5/16" inlets, should I simply install adapters to use 3/8" line? Or in my case should I stick with 5/16" all around.
**AND WHAT YEAR 292 PUMP**?

Thanks AGAIN!!

250Nova,
one thing to consider,,,, the bolt in lump port that Larry sells.
These will greatly increase your torque & HP & better all around driving.

You should run the 292 pump,not sure on the year,,,,anyone????

Run a 3/8 line from the pump, to a fuel log ,then run your (3) 5/16 fuel lines to your carbs.
Remind us on your goal about deciding on an electric fuel pump or mechanical????
What compression ratio are you running?

MBHD

My goal was to have a motor that started very quickly cold or hot. And that had great throttle response.

As far as compression, my machine shop said all said and done, I should be really close to 9:1
It's .030" over, dished pistions, but the head and block deck were both shaved slightly.

**ALSO JUST CHECKED CARTER FUEL PUMPS. COMPARED MY 72' 250 AND A 69' 292, THE NUMBER COMES UP THE SAME.**
Which 292 pump has the 3/8" outlet??

you,ll have to cut the head & block alot to bring c/r up a full point. a cleanup surface wont do it. you should have used 307 flat top pistons

I would say take off about .060 on the head.
Get close to zero deck,if you can.

You might be close to 8.5 to 1.

MBHD

Most stock 250s are 8.5-1 even the 194 had a 8.5-1 compression.It wasn't until 73 on up did the compression drop to 8.25/8.0-1 compression so depending on the year of your 250 yours could have been factory 8.5-1

A few thoughts:

At a pump pressure of 5.75 psi AT THE TANK, the line pressure drop should be sufficient to drop the pressure seen at the carbs to less than 5 psi. Any major US brand carburetor in proper working condition will stand this pressure. The pump doesn't care if you are feeding 1 carb or 30, provided the total volume required is less than the 72 gph pump rating.

For fuel lines, since you have 3/8 to the carbs, and 5/16 into the carbs; why not simply take a page from Pontiac - the 1959-93 Pontiac used a 3/8 feed into a four-way (available from the Pontiac vendors) with 5/16 lines going to the 3 carbs.

Remember that by law (and a good one - MOO) permanent wiring on an electric fuel pump must come from an oil pressure switch (safety feature). You can still install a manual override push button switch to prime the carburetors.

As far as pressure regulators are concerned: - the testing we have done indicates the inexpensive dial type regulators will regulate to the dialed pressure ONLY AT RATED FLOW! At minimum flow (idle) the pressure will be equal to or very close to the output pressure of the pump. We do not sell or recommend the use of this type regulator (except for paper weights).

If in doubt of the pressure, a fuel pressure gauge may be installed at the line junction to the carburetors.

There is a relationship of carburetor fuel valve orifice size to the amount of line pressure which the float circuit can withstand without leaking. Unless this is a race vehicle, the original fuel valve seat specification will be adequate. You will not need to install a "high performance" fuel valve in the carburetors. In fact, adding the larger seat in the carburetor often creates flooding issues.

Sounds like a fun vehicle!

Jon.

Carbking, I appreciate your info.

Can you go into more detail about this priming switch? As this is something I'd like to be able to do. So that the engine fires as quickly as possible.

If you want it to have the engine to start as soon as possible,,, spray starting fluid down the carb until the mechanical pump feeds the carb some fuel.

With starting fluid,it will fire right up quick.

MBHD

The permanent wiring for an electric fuel pump should be fired by an oil pressure switch. That way, in some form of an emergency, if the engine dies, the electric pump ceases to function.

For starting purposes, one can easily wire a push-button override (like an older starter switch) to energize the pump without cranking the engine. Run the pump for 7~15 seconds (whatever is necessary) to fill the carburetors. Release the push-button, and the pump stops. Start the engine, which brings up the oil pressure and the pump restarts. Mount the push-button in the dash or under the dash or???

Jon.