I'm a little curious why no one has mentioned this yet (and perhaps I'm the one missing the point?) but what about installing a pressure regulator?

I use a Carter M6902 mechanical pump here, and even though it's advertised as 6-7.5 PSI sometimes it would peak over that...now it is set to max 6 PSI.

chrysler built their emissions into the carbs and ignition. the 6bbl carbs have a somewhat unique idle circuit. the secondary carbs have a mixture screw and probably so for a lean idle burn. the center carb, which i think is the real culprit, has the idle jet tube in the main well contrary to conventional holley performance carbs. that idle jet is .023" and the idle air bleed is about .062". the relationship between the two is heavily biased towards a lean burn. the idle jet should be at least .031" for that .062" air bleed. in my opinion there's no way the center carb, which is the brain of the three, can supply enough low speed fuel. so this means we have to over compensate the end carb idle fuel to make up for it. normally this can be made to work, done it many times, but i don't think it's right and i've never got a good clean burn doing it. if i ever do this again i'm going to open up the center carb idle jet and maybe use a screw in air bleed for the center carb. another option is simply change the center carb metering block. promax has a metering block but i'm pretty sure you can't use a vacuum advance distributor with it and i use a vacuum advance. another option is an aftermarket universal metering block with the idle jet in the conventional location. back in the day some folks would put piece of lead shot in the center carb idle air bleed and drill that out to .046". this would put the air bleed and jet in a better relationship but i think there still may be a volume problem i bought a universal metering block from quick fuel (IIRC) that will do what i need and if IIRC correctly has a .032" idle jet that can be easily opened. you'll still have to get into the secondary carb mixture screws for adjustment whether you stick with what you have or change parts. looking ahead will only make the start up and final tune easier. you may have some main metering issues with the 440 metering plates in the end carbs but that shouldn't prevent a good tune for driving.

two reasons i give up on this was finding a regulator that i thought was good with a pulse pump (not saying there aren't any, i just didn't know which one to choose), and cost to do it all. i figured it might cost about $300 to do what i wanted and using a return line to regulate the pump was only a few bucks. if someone feels comfortable with a regulator than go for it. to me it's just something else to go screwy.

if the return line is smaller than the feed, [such as 1/4" return, 3/8" feed] does the restrictor need to be larger or smaller than .060 ?

i think it depends on the pump. Chrysler chose .060" for their high pressure pumps. seems to work, but a lower pressure pump will need less or none at all, and the modern high pressure electric is a different game. return line size such as 1/4" doesn't mean much for these mech pumps; I've done smaller. i think it really depends on how much you want to take the pressure down. from what i've done .060" didn't take something like a 9lb carter down enough for me. going too large will screw up high rpm fuel pressure; been there done that. maybe a juggling act?

Very well put. I agree completely. I figured this out some time ago trying to tune these six pack, but I could not put it into a clear paragraph like this. I’m not an engineer and some of this is at my mental limit but I’m working on a solution to this with a billet metering block that is a little more stealthy than what is out there currently on the market. I’ve used the Promax block but as you mentioned, it does not have a vacuum advance feature. Mine will. My block won’t have the numbers on it for the restoration guys, but it will solve the problems you mentioned.

i started messing with 6pak stuff in 1970. i was 22yrs old when i really got into this. i didn't have an A12 car; simply couldn't afford one. i bought new parts over the counter and my first foray into this was actually a dp6b. there were so many problems with them that guys actually took them off their A12 cars and put 4bbs on. there was no real info out there and if you could find something you were always at least 6 months behind because you had to wait for the info to come out in print. i believe it was in the middle of 1970 that car craft magazine did an article on 6paks. it was pretty comprehensive for the day. i use to save all that stuff but like a dummy gave it away. anyhow, it was the car craft article that showed us "shade tree" mechanics how to work the center car IAB. it explained the difference built into -1 carbs, deficiencies of the '69 carbs. i can't say i understood it all in 1970 but there's stuff i've never forgot. at that time i tried corvette carbs, 6pak mechanical, and factory carbs. by 1974 i had quit on it all, like most us in that era, and didn't pick it back up again until 1985. from 1985 until 2014 i was a die hard 6pak guy but have moved on since then. power isn't a problem with a 6pak; manufacturing errors (specifically edelbrock!) and emmissions are. get through those two and a 6pak is the best dual-purpose street performer.

i have to give credit to a member here, BSB67, for giving me a starting point on chasing the return orifice thing. i had a 6pak on my '65 coronet and always thought the power was a little lack luster. now this engine was never built to be a "real" performer but a good dual purpose street engine, i suspected some fuel delivery issues and BSB67 suggested tying a fuel pressure gauge to the windshield wiper and go hammer it! i did and what an eye opener. i was only getting 1.5-2lbs pressure at WOT with a 6903 pump. next came trying pump mods and juggling return orifice sizes but still keeping the pressure below 7lbs for street use. this helped a bunch. i never achieved 4+lbs at WOT but i did pick up about 1.5lbs and that was noticeable. fast forward to 2022, about 4 weeks ago, and i'm back into this again on a stock 440HP engine. this engine is in my '69 r/t and i decided to take the 272/.455 cam out and put the stock magnum cam in (cam swap was in june 2022 and i was chasing power brake issues). i put a new 6903 pump in it at the same time and gradually began to have some noise issues. thought the pump was the culprit, took it apart, did some fixing and re-installed (pump wasn't the noise issue ). car really ran crappy going down the road. did a fuel pressure check (which is what i should've done the first time) and now i'm cruising at 8lbs pressure and i knew that wouldn't work. you can build more pressure into a pump but i haven't figured out how to take pressure out of a pump. anyhow i took the .110" needle and seats out of the carb and installed .100", but i believe the real relief was opening the return orifice up to .053" vs the .031" i had been running. car drove beautifully. when the weather warms i'll do another pressure check but i believe i'm in that 6-6.5lb range now. the return does two things it's supposed to do, bleed vapors and control pressure. i have the original HP pump from my R/T and believe or not this thing still works! it'll do 8-9lbs at the pump but at the carb pressure will be noticeably reduced with .060" return orifice. a long rode of trial and error but i always learn something along the way.

"you can build more pressure into a pump but i haven't figured out how to take pressure out of a pump." It is easy, you increase the area under the diaphragm. This allows a lower pressure to resist the force of the diaphragm spring, while increasing volume of fuel pushed during each stroke.

It is a system, everything from the pump, and lines to the carb seat diameter and "float size" matter. That is what most folks don't realize. Orig carters used a larger float in the AFB. Edelbrock used a smaller float which means it can handle less force to maintain the same float level. That is why edelbrock has a lower fuel pressure limit.

I have posted this picture before. The 6903/4862 use all the same parts except diaphragm spring. It has a small outlet chamber of about 26cc. The scalloped chamber is the hemi fuel pump. It holds 32.5cc and has a greater surface area. Other pumps like marine pumps used this larger hemi chamber.

Pressure is a measurement of the restriction to flow. It is a balancing act. Higher pressure at inlet can also mean less flow out of pump because the pressure resists the full stroke of the diaphragm and it short strokes and hangs until the lever comes back up and pulls it up again. Of course, too low a pressure can also mean your pump is not sized right and insufficient volume to keep level in bowl high enough to start restricting fuel flow.

So you can take your 6903 and machine out some scallops in the bowl to mimic the hemi pump, increase area and volume under diaphragm, to reduce the maximum static pressure out of the pump while gaining more volume per stroke.

Maximum pressure in the system is when the spring is compressed at top of stroke because flow is at zero. The further the stroke moves off top dead center the more the spring unwinds reducing the force on the diaphragm which lowers the static pressure when diaphragm/flow stops (because needles closed).

So while you repurposed the vapor separator to manage fuel pressure it was never designed for that and you reduced the flow rate of the pump. If it is over sized for you combo, no lose in performance. But it does lower pressure and reduce available flow rate. You can do it just as easily with a throttle needle valve add as a return line. But as you lower pressure you lower flow "rate" too even in a constant displacement pump. Because the speed the diaphragm moves it not mechanically connected to the lever during the output stroke. It is spring force against system restriction, with force decreasing as the stroke increases moving towards the bottom.

with the above in mind, if one is not using a return canister filter with the orifice in place, and is using a regulator at the carb or on the fender, does it matter where the return orifice is located in the return line ?

i think as long as the return is on the pressure side of the pump it's ok.

trying to get a pic up of the fuel log on my dual quads were i took the return off the back of the log. it's probably better to get the return close to the carb to help prevent vapor lock. i'll keep trying.

ok, this is what i did when not using a vapor separator. inside those back fiitings is a .050" return orifice.

thanks Lew.