Really low fuel pressure

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Motorboy
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Location: Lansdale, PA

Really low fuel pressure

Post #1 by Motorboy » Thu Jul 19, 2018 5:27 pm

Hello all,

My project is coming along slowly, i have an offy c-series mated to a 2150 w 1.08 venturi which i bought from National Carburetor who said they would jet it for my engine(We'll see when i get to proper tuning). I'm running the stock mechanical pump that is 38 yrs old and has Who knows how many miles on it. It starts and idles fine with just a hair over 4psi. BUT if i blip the throttle the psi goes to 0-0.5 and maybe gets up to 1psi and cant recover. Obviously i need a new pump. I want to stay mechanical (I'm trying to keep this truck as analog as possible). Summit racing has 2 Carter pumps 1 with 25 gph and another with 35gph. Should i go for the high flow pump, or should i stay with a new stock one? My only experience with carbureted engines is with mid 70's Honda motorcycles (I build several cafe racers) so I'm not a total mechanical noob, but I'm definitely in new territory with this project. Any advice would be appreciated.

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THE FRENCHTOWN FLYER
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Re: Really low fuel pressure

Post #2 by THE FRENCHTOWN FLYER » Thu Jul 19, 2018 8:52 pm

Gasoline weighs about 6.3 lbs per gallon.
A well tuned engine has a brake specific fuel consumption at WOT of about .500 lb/hp-hr.
A pump delivering 25 gph would be delivering 157.5 lb/hr (25 x 6.3).
At .50 BSFC that pump will support 315 hp (157.5/ .50). The 35 gph should support 440 hp.

The 25 gph will be enough.

I also don't think the 1.08 2150 will require re-jetting. Maybe idle circuit tweaking, and part throttle air bleeds - depending on whether or not you are using EGR.
Last edited by THE FRENCHTOWN FLYER on Fri Jul 20, 2018 10:17 pm, edited 1 time in total.
FORD 300 INLINE SIX - THE BEST KEPT SECRET IN DRAG RACING

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Motorboy
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Re: Really low fuel pressure

Post #3 by Motorboy » Fri Jul 20, 2018 12:37 pm

Thanks FTF! Your sage advice is much appreciated.

94f150rules
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Re: Really low fuel pressure

Post #4 by 94f150rules » Fri Jul 20, 2018 11:12 pm

Also if you can check your fuel lines. I had a similar problem with a Firebird back when I was in high school. I would have close to 5psi on start up I'd take it for a ride and it would bounce between 2 and 3, then randomly it would drop to 1 or 0. Then it would take forever to restart and finally it would run again like there was nothing wrong. Then it would start all over again.I installed 4 new mechanical fuel pumps, even a heavy duty Edelbrock. Nothing cured the problen. It turned out I had a couple pin holes in the line coming from the sending unit to the fuel pump. The funny part is I had no visible leaks, an older mechanic told me to start checking the fuel lines. Not being any good at bending fuel lines at the time I replaced the piece with rubber and the car ran fine.

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THE FRENCHTOWN FLYER
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Re: Really low fuel pressure

Post #5 by THE FRENCHTOWN FLYER » Sat Jul 21, 2018 9:04 am

94f150rules wrote: It turned out I had a couple pin holes in the line coming from the sending unit to the fuel pump. The funny part is I had no visible leaks, an older mechanic told me to start checking the fuel lines.

Yup. My dad had a '61 Ford inline six that started consuming oil. We couldn't find the leak. While I was fiddling under the hood I noticed a film of oil on the backside of my hand after I closed the hood. I stuck my hand back under ther hood and followed the origin of the oil with my hand. It turns out the oil filter had a pinhole leak in it that was spraying a very fine mist of oil up into the air and on to the ground.
FORD 300 INLINE SIX - THE BEST KEPT SECRET IN DRAG RACING

SteveP
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Re: Really low fuel pressure

Post #6 by SteveP » Sat Jul 21, 2018 6:44 pm

I have an Offy C with a 2150 from National Carb with 1.21 venturis. It came with 50 jets. I bought a set of each 48, 49, 51, and 52 jets. It ran terrible with each of these and runs great with the 50s.
I'm running a stock fuel pump (new from AZ when I installed the engine) with new 5/16 lines on mine and I don't have any fuel delivery issues.

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