I'm running a 360 with a purple cam ported heads RPM intake thermoquad carb. It has a electric and mechanical fuel pump. I'm still running the 5/16 fuel line. When I floor it it pulls great till 3000 and won't rev much higher. Another time I was crusing about 2500 floored it and it pulled great to 5500 that's it. Now stock 5/16 from the sending unit to the electric fuel pump with fittings to the mechanical pump with fittings to the pressure regulator with fittings to fuel filter to the carb. Also have a fuel filter before the electric fuel pump and both are new. So it's a small fuel line and a lot of fittings Does this sound like a fuel restriction problem? Oh the carb was rebuilt by Rusty Rat Rod and he did a great job No bog and love the sound of those secondaries Thants. Jim
3/8 vs 5/16 fuel lines
- Thread starter sorewrist1
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thesiren74
Well-Known Member
I would just run a mechanical with 3/8" line back to front that's the way I've been doing it for years and run 12s, still with the stock 5/16" sending unit.
What ported heads and which purple shaft cam? What valve springs are you running? Which electric fuel pump, what mechanical fuel pump and regulator, and what's the fuel pressure? How many filters are in the system? Without being able to see what the fuel pressure is actually doing under demand, it's difficult to say what the culprit is,
I highly doubt it has anything to do with the fuel line diameter.
Ditto on the line size. Especially with two fuel pumps running at once.
MOPAROFFICIAL
Oogliboogli
This.
Thanks for the quick reply the purple shaft is hyd oval track with .495ish lift mech pump is Edelbrock filters both new inline just bowl work on the heads Kragen cheapest electric pump. I'm scared to bring a gas line under pressure inside to monitor
67Dart273
Well-Known Member
What, realistically, do you think the HP is? My "first guess" is with the others....doubt the line size is the issue. What kind of pre-filter? You have to be REALLY careful with suction side filters, and this is where I'd start.
What "exactly" are the pumps?
What "exactly" are the pumps?
72Dart6pack
Harder Better Faster Stronger.
Unless you can monitor fuel pressure. You are just guessing. Bigger isn't always better. Save your money putting in bigger lines.
my68barracuda
Well-Known Member
Keep the gauge under the hood, use an I phone or a go pro to monitor during testing. This ain’t brain surgery.
I added a fuel pressure guage right before my carb, after my filter... screwed directly into the metal edelbrock single feed line... I think it cost like 10-15 bucks for the guage? Now I know my fuel pressure and its an easy way to monitor it... and my problem ended up not even being fuel related lol.... I also rarely ever check it 
I know it doesn't sound like much of an increase to go from a 5/16-dia. line to a 3/8-dia. But it's a 44% increase in cross-sectional area.
So, yeah, it is kind of a big deal.
So, yeah, it is kind of a big deal.
If the carb was running out of fuel it would most likely start popping and sputtering from a lean condition .
I had a 73 340 4sp Dart Sport with W2 and a solid roller that would run like a champ until I bolted slicks on and ran it down the track . It would sputter and pop on thetop end .. guys were telling me I needed 1/2 fuel line ..lol
I said if the roller cammed 440-6 in my Charger can run with 3/8 this small block certainly can .
After months of chasing the issue I found the line coming out of the tank was kinked while I was swapping gears one day .
I had a 73 340 4sp Dart Sport with W2 and a solid roller that would run like a champ until I bolted slicks on and ran it down the track . It would sputter and pop on thetop end .. guys were telling me I needed 1/2 fuel line ..lol
I said if the roller cammed 440-6 in my Charger can run with 3/8 this small block certainly can .
After months of chasing the issue I found the line coming out of the tank was kinked while I was swapping gears one day .
Throttle cable ? Is the carburetor opening all the way ? 9 times out of 10 that's the problem
K.O. SWINGER
Meeting in the alley since 1976
I agree with others I do not think the fuel line is the cause of your problems. But that being said I would still change out to a 3/8 fuel line I believe that's about perfect for 4 to 500 horsepower
Even 1970 x 440 B body cars, with a vent canister and a 1/4" return line, only have a 5/16 fuel line.
I’d check the fuel filler cap to make sure it’s venting and/or ensure any tank vents are not plugged. Bypass the mechanical pump and see what happens, and vice-versa. But if you start throwing parts at it I’d start with a fuel pressure gauge to know what pressures you have and you might be able to identify the issue. Buy a fuel pressure tester, or a gauge and a few feet of fuel hose, route it up between the hood and cowl or fender, wrap a cloth around the gauge, use some tape/zip ties to keep it positioned so you or a passenger can read it and go out and drive. It’s a temporary connection for testing. You gotta improvise a setup to see what’s what.
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Exactly, you beat me to it.
Lots of good ideas and you guys helped me a lot in the past. I did block the tank vents but changed it like the older ones from the fill tube down to the chassis. I put the electric fuel pump in for easier starting. Manual choke too Put in a fuel line pressure gage and it reads 4 to 6 at idle. I'll move it so I can see it under load. I'll check secondaries when I get home 600 miles left. If it makes that much power on those little primaries I'll be happy when the secondaries come in .It should be making about 400 up Thanks all and Merry Christmas
I gotta ask why you are running both a mechanical and an electric pump and not one or the other!? We run electric boost pumps on some of our aircraft engines, to insure fuel flow in high flow / nose high attitudes in case the gravity feed mechanical pump loses head pressure, but they need to be correctly check valved and/or be a proper flow through design to work properly. Many electric pump designs don't allow fuel to be sucked through them (if they aren't running) by the mechanical pump.
moparmat2000
Well-Known Member
A buddy of mine gave me a new 25' coil of metal 3/8" fuel line. I will use it on both cars just because I have it, and both cars will need lines put under them at some point. My 408 might actually need it, but sonny's warmed over 318 wont. But that's ok. It will be in there if he ever does need it down the road.
It vapor locks sometimes so I just turn the key and fill the bowl
Okay, so is this electric pump running all the time or just to start the car? If only to start the car your mechanical may be starving for fuel it can't suck through the electric pump.
That said, no reason for the fuel bowl to be going empty. I can leave either of my cars for a month +, get in turn the key, tap the pedal and go.
That said, no reason for the fuel bowl to be going empty. I can leave either of my cars for a month +, get in turn the key, tap the pedal and go.
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