Low fuel pressure after adding return line
06-04-2016, 05:08 PM
#1
Burning Brakes
Thread Starter
Low fuel pressure after adding return line
I have another thread asking for fuel pump with return recommendations. That thread is no longer very relevant.
I added a return line between my current mechanical pump and carb. This line connected back to the old factory hard line which was capped off. No leaks.
My fuel pressure without return was a solid 6-7 at idle. It dropped to below 2 with return line connected. Pressure gauge is located before return line.
Am I doing something really wrong or is my fuel pump too weak to support a return.
Fuel pump came with crate engine so I have no info on it. Return line is 1/4 rubber.
I added a return line between my current mechanical pump and carb. This line connected back to the old factory hard line which was capped off. No leaks.
My fuel pressure without return was a solid 6-7 at idle. It dropped to below 2 with return line connected. Pressure gauge is located before return line.
Am I doing something really wrong or is my fuel pump too weak to support a return.
Fuel pump came with crate engine so I have no info on it. Return line is 1/4 rubber.
06-04-2016, 06:11 PM
#2
Drifting
your return is nearly the size of your supply line .your return is too big.. place something in the line to restrict flow.. You only need a small amount of continuous return to keep it from vapor lock
06-04-2016, 06:59 PM
#4
Safety Car
Member Since: May 2004
Location: los altos hills california
Posts: 3,609
Received 1,126 Likes
on
730 Posts
The return line is for vapor, not full flow back to the tank, so it is no surprise that your pressure has dropped as much as it has. You should have a single supply line from your pump and a return line from an AC GF432 filter for the system to work as it was designed.
06-04-2016, 07:21 PM
#6
Safety Car
Member Since: May 2004
Location: los altos hills california
Posts: 3,609
Received 1,126 Likes
on
730 Posts
I actually have a setup like Lars with an electric pump running at about 12#'s. With the stock return line fully open I get about 6# at the carb. So what you are seeing is predictable. For your mechanical pump, easiest way is as Fishslayer suggests, a highly restricted return line. A quick read of the Lars post by me bears that out.
06-04-2016, 10:19 PM
#10
Team Owner
The fuel return line should have a small orifice in it to limit flow and maintain pressure in the carb line. Something with about 1/16" orifice should do the job.
06-05-2016, 07:43 AM
#12
Burning Brakes
Thread Starter
I'm going to add a in line ball valve to my return. That way I can control the amount of restriction and hopefully my fuel pressure too.
Fitech is looking better and better.
Fitech is looking better and better.
06-05-2016, 09:23 AM
#13
Safety Car
Where in the set up is a regulator ?
06-05-2016, 09:38 AM
#15
Le Mans Master
Member Since: Jul 2006
Location: charlotte north carolina
Posts: 8,117
Likes: 0
Received 100 Likes
on
92 Posts
Use a restriction of 050. That will allow fuel to circulate thru the pump and fuel line before it becomes hot, and yet not reduce fuel pressure. I soldered a copper plug in a brass fitting and then drilled a 050 hole into the plug. Another member just posted about tapping the pressed-in plug bore hole on the front of his qjet and using that as fuel return point- a very elegant solution that allows use of a stock pump to carb fuel line.
Last edited by jnb5101; 06-05-2016 at 09:41 AM.
06-05-2016, 10:41 AM
#16
Safety Car
Why not use a regulator ? It would make life easy and allow exact setting of fuel pressure
06-05-2016, 10:53 AM
#17
Melting Slicks
Member Since: Mar 2006
Location: Piedmont Va
Posts: 3,456
Received 100 Likes
on
85 Posts
St. Jude Donor '11-'12-'13,'19-'20
As I believe I posted in the previous thread, but feel the need to point again, that GM engineered the solution 46 years ago. ....just saying. 
http://www.zip-corvette.com/70-81-fuel-pump.html
http://www.zip-corvette.com/70-81-fuel-pump.html
06-05-2016, 11:24 AM
#18
Team Owner
A regulator sets line pressure and it is an expensive...and misapplied...alternative to a simple orifice. And a ball valve will need to be nearly completely closed to do the same job.
The simplest approach is almost always the BEST approach. And, in this case, the appropriate solution is a flow-limiting orifice in the fuel return path.
The simplest approach is almost always the BEST approach. And, in this case, the appropriate solution is a flow-limiting orifice in the fuel return path.
06-05-2016, 01:00 PM
#19
The ORIGINAL and bestest
Member Since: Oct 2000
Location: Toronto Ontario
Posts: 10,009
Received 234 Likes
on
143 Posts
Toronto Events Coordinator
As I believe I posted in the previous thread, but feel the need to point again, that GM engineered the solution 46 years ago. ....just saying.
http://www.zip-corvette.com/70-81-fuel-pump.html
http://www.zip-corvette.com/70-81-fuel-pump.html
Why not use the pump that GM engineered for the car?
06-05-2016, 01:18 PM
#20
Safety Car
A regulator sets line pressure and it is an expensive...and misapplied...alternative to a simple orifice. And a ball valve will need to be nearly completely closed to do the same job.
The simplest approach is almost always the BEST approach. And, in this case, the appropriate solution is a flow-limiting orifice in the fuel return path.
The simplest approach is almost always the BEST approach. And, in this case, the appropriate solution is a flow-limiting orifice in the fuel return path.
I will concede what is going on here is one way of doing it. But the best way is based on one's perspective and both ways are the best ways depending on how someone looks at it.
To me a regulator provides options otherwise not available to this orifice set up and allows for future upgrades if that ever takes place. It is plug and play, easy and 100% effective and only requires the turn of a screw to meet your needs.
GL OP .. I hope you get er done to your satisfaction ..