return line too small???

grumpyvette · Oct 12, 2014

Rod S said:
I just installed a Mallory 140 fuel pump and a Mallory 4309 regulator. The instructions said to back the lock nut off and take the set screw out. If, upon priming, I had over three pounds of fuel pressure, my return line was too small. I just finished plumbing a 3/8 return line, the same size as my feed line. Upon priming, I had about 5 lbs. of fuel pressure. With the engine running I can't get it much below 6 lbs. I know, the obvious answer is that I have to plumb a 1/2 inch return line. I don't think there is any way around this. Is it best to get a roll of steel tubing and start bending, or is it best to get some of the teflon core stainless steel wrapped 1/2 in stuff and go with 8an fittings? Thanks in advance, Rod

GET A FUEL PRESSURE GAUGE AND MEASURE DON,T GUESS
how can you possibly set up your fuel system unless you know the pressure and flow rates required and what currently exist's

READ LINKS

http://garage.grumpysperformance.com/index.php?threads/setting-up-your-fuel-system.211/

http://garage.grumpysperformance.com/index.php?threads/fuel-pressure-regulators.635/

http://garage.grumpysperformance.com/index.php?threads/how-big-a-fuel-pump-do-you-need.1939/

http://garage.grumpysperformance.co...ss-fuel-pressure-regulators.12776/#post-65998
never GUESS ALWAYS TEST AND DEAL IN FACTS
many guys with fuel systems that use return style fuel pressure regulators with EITHER manual or electrical fuel pumps , find that the original fuel system either has no return feul line to the tank or if there is one its either too small or restrictive!
the first logical thing to do is test if the problem IS THE 1/4"-5/16"- 3/8" RETURN LINE SIZE BEING A RESTRICTION, causing the issue your having BEFORE you go investing time and effort in changing the current set up, hook a short section of lets say 12-16 feet of 1/2 line on the out side of the fuel pressure regulator, too semi, duplicate the larger and longe , less restrictive to flow,return line too the fuel tank,that youll have once its installed in the car, so its pouring the return fuel in a 5 gallon bucket ,as a quick and easy test, then test if you can reach the desired pressure levels, indicating the increase in cross sectional area on the return line actually cures that problem, rather than assuming and guessing., IF that cures the issue then proceed with swapping out the return line in the car knowing that will if properly installed, cure your problem, rather than go all thru the work only to spend the time and cash to potentially find it might not be or was not the issue.
if you find out that installing and allowing the larger and less restrictive ,temporary return line too drain in the bucket, has no measurably effect you will obviously have proven that is not the cause of the problem.if it cures your problem then you obviously go ahead with the install, and once its completed, if you no longer get the desired results youll know that a restrictive fitting or the routing is likely the cause.
a return line will be by definition low pressure, measure carefully, and you can have a local hydraulic supply custom fabricate teflon flex hose of the correct size type and with the correct fittings

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grumpyvette · Oct 12, 2014

while testing I,d point out a very commonly over looked component and factor is the internal passage size and cross sectional area of the fittings.while the NPT ,female thread size in your fuel pump and fuel pressure regulator will normally be either 3/8-NPT in most standard performance versions or 1/2-NPT in the larger higher out put versions,the barb length, diameter, and inside passage diameter will vary by manufacturer a great deal, easily varying by over 150% in cross sectional area

Grumpy · Sep 9, 2015

hey grumpy, Ive got to ask a question,I replaced the 350 sbc engine in my recently purchased 1970 corvette, that was basically trash,it had been sitting for years with a bad set of bearings.
I replaced the SBC 350 with a crate 454 BBC, the fuel pump is totally different in appearance that the sbc pump,
and has an additional outlet for a fuel return line,
I tested the cars existing fuel return line to the tank,and its blocked,
I tried blowing it clear with high pressure air but its not clearing,
can I just plug off that extra return line on the fuel pump and run it,
or do I need to find a big block pump that doesn,t have a return line port design?

Ideally youll install a clear replacement fuel return line and correctly hook it up, and verify it flows fuel back to the tank from the designed return port on the fuel pump.
WHEN TESTING<be sure the fuel pressure gauge you test with reads correctly by comparing it to a second test gauge, these old fuel pressure gauges are frequently defective

yes it sounds like the return lines to the fuel tanks blocked ,
BUT ID check with a different test gauge because the stock manual fuel pump usually maxes out at about 8psi, but if its not the gauge, Id try blowing high pressure air thru it after removing the tanks gas cap,

as a test as a start point, if its a clear return line back to the tank its probably a defective fuel pressure regulator
most carbs are best set up to run on a lower 4.5-5.5 psi fuel pressure meassured at the carb inlet port and obviously youll want to correctly set the carbs float level and verify the needle & seat valve
most manual fuel pumps have a return line port to maintain consistent fuel pressure, use of a return style fuel pressure regulator, and a large low restriction fuel return line is highly recommended

http://garage.grumpysperformance.com/index.php?threads/fuel-pump-push-rods.11093/

many guys in the days past CHEATED on the fuel system install, and did the modification below, while it will work its not recommended and its not as consistent. (it also allows far more heat to build up in the fuel pump,
as cool fuel is nor reaching the fuel pump nearly as consistently )
(something to think about if your dealing with vapor lock issues)

return line too small???

grumpyvette

Administrator

grumpyvette

Administrator

Grumpy

The Grumpy Grease Monkey mechanical engineer.