Hi guys!
I have always had an apparent fuel delivery problem in my '68 HEMI Charger. On hard acceleration from slow roll or stop, it begins to pop and loose power around 80mph.. It pulls really hard if one is just cruising around in 4th gear and steps on the throttle. Its also really strong up to 80 mph when accelerating hard. After experiencing the problem, I installed a high output fuel pump, but it still didn't cure the issue.
The 426 HEMI has a moderate build and requires 50% race gas.
Someone told me that I should put a fuel flow restrictor in the return line to the tank, that is designed to prevent vapor lock, that the carbs would get more fuel and wouldn't run low on sustained acceleration. He said I should drill a certain sized hole into a bolt, and insert in into the 3/8" line to reduce the amount of gas that's returning to the tank. I'm thinking of putting a vice grip on the return line, to see if it solves the problem, then I'll make a restrictor for the return line if the vice grip solves the problem.
The fuel gauge just quit working, so I'll check the filter on the end of the pick-up line when I'm working on the sending unit. Maybe it's plugged, or maybe there is an air leak in the fuel line somewhere???

Is it a mech pump or electric... what carbs..
what size needle/seat.. have you done a flow
check(gals per minute).. do you still have the
stock sock on the pick up... do the filters have
enough flow... thats a start

Check the fuel pump pushrod to see if it is worn. If it is, it won't give the pump a full stroke.

It's a new high output mechanical fuel pump. It didn't seem to help with the supply problem. I'll check the sock this week.

Factory carbs and jetting, new mechanical pump.

From reg to carb you want 6-7 psi running, having a gauge set up so you can see it when the problems start will tell you. Is your fuel pressure dropping off ?

I struggled with a high output mechanical pump on my 440. I consider myself pretty competent and I gave up. Went electric.

Did a bunch of testing and tried a couple of pumps. All were Carter.

To fix the problems you need to know first what they are I use to race two non original street Hemi cars in NHRA stock back in the early to mid 1970s. Both had 5/16 lines and pickups, both needed the Carter 3 valve race Hemi H.P. NASCAR type pumps to run with enough fuel supply
Good luck, let us know what you find and do to fix that problem

Get rid of the sock filter in the tank.

Remove and measure the length of the fuel pump pushrod to make sure it isn't worn.

You should be able to eliminate the return line.

Wasn't the special OEM fuel filter used on the original cars suppose to be a vapor seperator? Not for liquid returns is what I remember

the aftermarket vapor seperaters have too large of a return orifice. i solder the hole up and re-drill to .046". not all aftermarket "high performance" mechanical pumps are equal.

Mechanical heat sinking pumps =

X3, see what you have AT the carbs at the offending RPM/load with an accurate gauge on the cowl or plumbed inside (just for the test of course! or with an isolator for safety)

I never heard of a return line to the tank, vapor can like Cab said, could be broke and just circling back to pump. Hemi cars came with 3/8 line. That stock pump "should" pump enough fuel for the carbs, cover the lines from heat to stop vapor lock. My two cents.

The sock in the tank will separate water until
its totally under water.. but its not 100%..
the return just has a orifice.. pretty sure back
then it was .060
EDIT
Yes they did have a return to the tank... any of
the so called performance engines had a return..
any that had a 3/8 supply had a return

U
Return line or vent line? How big a performance engine did it have to be? My a-body m-code 440 only has one 3/8 line, maybe because it started as a 383?

If t was factory and labeled HP it had 3/8 supply
and a 1/4 return and a 1/4 vent... a 3 tube bundle
clamped together in the 70s with the charcoal canister
up front.. otherwise it had the vent in the frame rail
on older years but still had the 3/8 and 1/4 going forward
if it went into the fame rail it was likely a 3/16 vent

Some of the Mopars had the 3 tube fuel filters which many called a vapor seperator. The third line did go back to the fuel tank but it was not for fuel flow like a fuel pressure regulator can use it was a vapor return line as it had a small orifice somewhere around .050 to let fuel vapors escape out of the fuel line/filter and route it back to the tank. A small amount of fuel could also go back to the tank along with the vapor but the orifice was small enough that only a very small amount of fuel could get through the orifice which was not enough to cause a fuel pressure loss but letting the fuel vapors escape helped the car run better. Ron

I had a pretty new carter crap the bed. So have many others. I dont think the quality is there with their mechanical pumps. If all else checks out, try a holley pump.

Have a 72 RR that I restored about 5 years ago and put a 440 + 6 in the car and the engine was mildly build but it couldn't even spin the tires.Have a 3/8 fuel line and the vapor line to the front. After checking everything possible after all the searching I stumbled on to the problem about a month ago. I didn't have a vented gas cap on the car and had created a closed system. I drilled a small hole in the center of the cap and the car run's like a house on fire! P.S. I couldn't find a vented cap they are hard to find thus the hole in the cap!

process of elimination. try electric fuel pump first. when i restored my stock hemi. at track i couldn't get out of third gear. installed a elec. fuel pump NO more problem.

I repaired the faulty fuel sending unit (float had developed a leak), and also checked the fuel flow at the carbs.. I got 1 quart per minute of running on the starter, just like it says in the shop manual.
I wasn't able to measure the pressure during the flow test.
Now, I'm wondering if there are screens in the carb inlets on the factory HEMI carbs.? If so, maybe one of them is partially blocked. The car pulls really hard in a fourth gear punch. It only gives me trouble when I run it hard up through the gears. Starts missing around 80 or 85 mph., with 4:11 gearing.

Myself I completely agree with this. I wont even waste my time with a mech fuel pump on my hotrod or muscle car for a number of reasons that make good sense.
But many do still prefer the mechanical pump with the pushrod on bigblocks and Hemi's and many get them to work good for themselfs. And if thats what you want its ok but I feel you will have more problems running a pump gas combo using the mech pump then you would with an electric pump. Ron

I had a venting issue on my Dart. Small vapor/vent line was clogged at the tank. Either pump won't pump from a tank that's not vented.

What 383man said. I run an electric fuel pump, when I put the repro vapor separator on, I turned the pump on to check for leaks, I could hear fuel flowing like crazy, turned out there was no orifice in the separator and most of the fuel was flowing back to the tank. If that is not it, next best bet is to check the pump pushrod for excessive wear, lots of soft ones got out several years ago, I have a couple in the toolbox that were obviously dished.

(1) put a 3 gallon can of gas on the pass floorboard & run a neoprene line up to the mech pump inlet (cap the hose you take off of course). (2) retry it. (3) clamp the return line shut with a pair of vice grips with something to protect the hose from the jaw serrations (keeping the 3 gallon can operational) (4) retry it. holler back. PS. I would clamp the return line first as you & others' mentioned & see what happens as that would be the easiest (& a practical) first move