Low fuel pressure after adding return line
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
regulating pressure to carb that is clearly already too low seems to be attacking the wrong problem ...His return is too large, thereby lowering his available supply pressure .a regulator set at 6psi will not help 2 psi available supply.. . reduce return flow, increase supply pressure .. then , if you want to, regulate it to carb
Drifting
Joined: Sep 2007
Posts: 1,572
Likes: 51
From: Wichita Kansas
My fuel lines are -6 braided with AN fittings. After read an old post from Lars I installed a return line. In install an AN -6 "T" at the rear feed (Holly style dual feed). From the open connection I used an AN -6 to -4 reduced. I treaded the -4 end for 10-32 and installed a brass screw. Then cut the screw flush with the flair fittings tip. Then drilled 040 hole through the screw. Install a -4 180* hose end and ran -4 hose to the stock 5/16" return line with a 5/16 / -4 AN male fitting to the tubing. Hooked up line and works great. Not only prevents vapor lock it stops the pump from dead heading.
Thread Starter
Burning Brakes
Joined: May 2014
Posts: 884
Likes: 128
From: Rhine Georgia
old post from Lars
Thanks to all who shared their ideas. And I can see all of them working out. My current FP seems to be working fine and all I need is a return. At this point I'm going to work on limiting my return line size and hopefully ending my vapor lock issues.
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
My fuel lines are -6 braided with AN fittings. After read an old post from Lars I installed a return line. In install an AN -6 "T" at the rear feed (Holly style dual feed). From the open connection I used an AN -6 to -4 reduced. I treaded the -4 end for 10-32 and installed a brass screw. Then cut the screw flush with the flair fittings tip. Then drilled 040 hole through the screw. Install a -4 180* hose end and ran -4 hose to the stock 5/16" return line with a 5/16 / -4 AN male fitting to the tubing. Hooked up line and works great. Not only prevents vapor lock it stops the pump from dead heading.
Drifting
Joined: Sep 2007
Posts: 1,572
Likes: 51
From: Wichita Kansas
I connected the -4 return line to the return line on the right frame rail that was used for the mechanical pump return. Used a 5/16 compression fitting to -4 AN male to connect the -4 AN return line. My electric pump and filter are located back at the tank. The return line goes directly back to the tank as the factory installed the hard line.
Thread Starter
Burning Brakes
Joined: May 2014
Posts: 884
Likes: 128
From: Rhine Georgia
Update:
Limited my return size and moved my pump to carb fuel line to a cooler location.
For some reason after moving line my fuel pressure gained 1 psi and now idles at 8 with return line connected. When I shut her down it drops to around 3 to 4
I went for a drive to warm the engine and let her idle of 35 minutes after I got home. No vapor lock so far.
Still wanna test some more before I trust the car but I'm happy so far.
Limited my return size and moved my pump to carb fuel line to a cooler location.
For some reason after moving line my fuel pressure gained 1 psi and now idles at 8 with return line connected. When I shut her down it drops to around 3 to 4
I went for a drive to warm the engine and let her idle of 35 minutes after I got home. No vapor lock so far.
Still wanna test some more before I trust the car but I'm happy so far.
Tech Contributor
Joined: Aug 1999
Posts: 14,375
Likes: 6,375
From: At my Bar drinking and wrenching in Lafayette Colorado
This is the setup I was trying to copy. Only mine is not elegant.
Lars
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Drifting
Joined: Sep 2007
Posts: 1,572
Likes: 51
From: Wichita Kansas
That setup has a .060" diameter hole drilled into the stainless reducer fitting coming out of the rear "T." The plugged and welded fitting is shown in one of the photos below the installation setup. This hole keep fuel pressure from dropping much more than about 1 psi.
Lars
Lars
Not high jacking just another example.
Last edited by Aggitated Monkey; Jun 10, 2016 at 11:16 AM.
Thread Starter
Burning Brakes
Joined: May 2014
Posts: 884
Likes: 128
From: Rhine Georgia
I'm interested in this too.
I just added a restriction to my return line so I have no idea what size opening I have. Only data I have is that I run 7-8 psi at idle and it drops to 3ish when I turn off engine and them slowly drops over time.
I just added a restriction to my return line so I have no idea what size opening I have. Only data I have is that I run 7-8 psi at idle and it drops to 3ish when I turn off engine and them slowly drops over time.
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
FWIW, I never run return lines on carb'd stuff with mechanical fuel pumps... Usually no problems with vapor lock assuming I have a GOOD pump on it. Between my dad and I, we have 7 cars with carbs/mechanical pumps, and I set them all up the same way.