| Fuel Pressure for Quadrajet [message #369594] |
Wed, 04 May 2022 13:24  |
Melbo
Messages: 144
Registered: August 2018
Location: Albuquerque NM
Karma: 0
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I have gotten to the point of driving the motorhome. I had the quadrajet rebuilt and I use an electric fuel pump. When I went through the fuel supply system and replaced all the hoses I also added a connection for fuel return to the tank. Before I drive extensively I would like to install a pressure regulator that allows for any excess fuel pressure to bleed off so as not to flood the carb and or allow any raw fuel into the engine. I have read about engine failure from excessive electric fuel pump pressure destroying carbuerated engines. Now for the questions. What pressure should I set the regulator at so as to not over power the carb? Is there a good way to set this up to help avoid the dreaded summertime vapor lock. Thank you for any and all suggestions. I really appreciate the help and advice.
The Melbo
Albuquerque NM Bus Conversion 1978 MCI 1973 GMC
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| Re: Fuel Pressure for Quadrajet [message #369596 is a reply to message #369594] |
Wed, 04 May 2022 19:42  |
bwevers
Messages: 597
Registered: October 2010
Location: San Jose
Karma: 5
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My Carter 4070 fuel pump puts out 4 to 8 PSI if that's any help.
I have a Quadrajet.
Bill Wevers GMC49ers, GMC Western States
1975 Glenbrook - Manny Powerdrive, OneTon
455 F Block, G heads
San Jose
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| Re: Fuel Pressure for Quadrajet [message #369597 is a reply to message #369596] |
Wed, 04 May 2022 20:13 |
Melbo
Messages: 144
Registered: August 2018
Location: Albuquerque NM
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I was told that I should upgrade to a 4070 and I will probably do that while I am working on this. If I set the pressure regulator at 6psi anything over that will just bleed back into the tank and I should be in a good pressure range for the carb to function properly and keep a fuel supply even in the heat. I won't really know until it is field tested. I always read Matt's responses because he provides goodd information and his 5.5 to 6.5 seems high to me but compared to him I am a neophyte so I will go with his numbers.
Thanks for the guidance.
The Melbo
Albuquerque NM Bus Conversion 1978 MCI 1973 GMC
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| Re: Fuel Pressure for Quadrajet [message #369600 is a reply to message #369594] |
Thu, 05 May 2022 08:20 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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The Doug Roe Rochester Carb book states 4-6psi. It says they will “run” at 2 PSI. It says never to exceed 6 PSI at idle as increased pressure increases fuel level in bowl working against the float system leading to idle instability.
John Lebetski
Woodstock, IL
77 Eleganza II
[Updated on: Thu, 05 May 2022 08:20] Report message to a moderator |
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| Re: Fuel Pressure for Quadrajet [message #369602 is a reply to message #369594] |
Thu, 05 May 2022 23:09 |
Melbo
Messages: 144
Registered: August 2018
Location: Albuquerque NM
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John that is very interesting to know that it will run with pressure as low as 2psi. I am wondering if it would lean out a bit at highway speed with that low a pressure. I am glad I posted this question because my brain had me looking at a target pressure of 4 psi which is what I ran my webbers at in a sports car in another lifetime. The car had a vapor lock problem and I had a pump that ran about 8psi and everything over 4psi recirculated. Kept it running well even in the heat with the AC on. On a side note I used to live in Illinois and Iowa and I have noticed there are a lot GMC people there. I make a pilgrimage to that part of the country the last weekend in July every year in our bus conversion. It would be fun to catch up with some GMC people when we are in that part of the country.
The Melbo
Albuquerque NM Bus Conversion 1978 MCI 1973 GMC
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| Re: Fuel Pressure for Quadrajet [message #369604 is a reply to message #369594] |
Fri, 06 May 2022 08:54 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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You will note I put RUN in quotes for the 2 PSI. My guess it would go flat at WOT after a few seconds when use exceeded supply. Also I would think that being at the higher end of the range not to exceed the 6.5 Matt mentioned could be a good thing on a GMC with our higher fuel rate than a passenger car.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Fuel Pressure for Quadrajet [message #369811 is a reply to message #369604] |
Sat, 28 May 2022 19:01 |
Melbo
Messages: 144
Registered: August 2018
Location: Albuquerque NM
Karma: 0
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Well it being the holiday weekend and having no major plans I got the pressure guage installed and checked the fuel pressure to the carb. I am very disappointed. The pressure is at the maximum 5 lbs. of pressure. The pressure regulator and fuel return line are really unnecessary at that pressure. In fact when I leave the return line valve open the pressure never even shows up. When I start and run the engine it goes even lower (down to around the 2psi that was suggested as the lowest pressure the engine will operate at) so I'm guessing that I need to upgrade my fuel pump. Any and all recommendations for fuel pumps would be appreciated. I am thinking that I need a 14 psi or greater pump to get the pressure up to where the regulator will keep the pressure up where I would want it and still let fuel return to the tank. Once again the more help the better because the last time I did this (on a comletely different vehicle) I had a constant 4-5 psi and a nice return flow to the tank but I don't remember what fuel pump pressure or pump type I used. I remeber it was an electric pump. Too many years ago.
TIA
The Melbo
Albuquerque NM Bus Conversion 1978 MCI 1973 GMC
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| Re: Fuel Pressure for Quadrajet [message #369815 is a reply to message #369811] |
Sun, 29 May 2022 08:07 |
bdub
Messages: 1578
Registered: February 2004
Location: Central Texas
Karma: 5
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So, ... it wasn't broke but you fixed it anyway?
BTDT Ouch!
My lil cheapy Mr. Gasket boost pump, powered by the Aux Tank Selector was all I ever needed for hot, long, uphill grades. Really cheap, easy fix.
bdub
'76 Palm Beach/Central Texas
www.bdub.net
www.gmcmhphotos.com
www.gmcmotorhomemarketplace.com
www.gmcmhregistry.com
www.facebook.com/groups/classicgmcmotorhomes
www.facebook.com/groups/gmcmm
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