Just getting ready to fire up my Hemi for the first time. I have the Carter street strip m4862 question is will it require a regulator?

8 to 9.5 PSI. Yes It will need a regulator.

it is possible to regulate that pump with the return line orifice size. both the 375hp 440's and street hemi's used a higher pressure pump but the return orifice in the vapor separator helped regulate the pressure. I played with this a little bit "bench" testing pumps.

Sure any addition in the flow path could cause a pressure drop, but Chrylser wanted the higher pressure with the Carter carbs starting somewhere in the early 60s. Mopar and Carter documentation state that. In addition, the test procedure in the service manual is to measure the pressure at carb inlet, not pump outlet.
That is after the vapor separator. The Needle valves and floats on the AFB and early AVS are different then what Edelbrock uses. The float could handle the added pressure/force. It was calibrated that way.

The orifice is designed to flow vapor, not liquid. Mostly about preventing vapor lock after a stop. Open it too much and start moving liquid defeats the purpose of having sufficient volume flow at high speed.

If the pump is putting out too much pressure for your combo, you could take it apart and put in a softer spring. It can be removed from the pullup rod.

really? and how does a mechanical fuel injection regulate itself? i'm not with you on this.

I run one on my 2-4 edelbrock hemi without one and even thou the pressure is high I haven't had any problems.

I would try it first, if it makes the carbs. flood then fix it with a regulator

Well that pump is not going to do the job as is. Car fired right up and ran great but started loading up. My fuel gauge was fluctuating badly so bad I had no idea what the pressure was 15 psi summit gauge. Put a old liquid filled and it was reading 18 psi so the needle/seats surely will not like that. Pump is new and has been in the box for 15 years. That's crazy pressure for a pump rated at max. pressure 8 to 9.5. So now I have to decide whats best. Seeing its all plumbed I might just go with a step down to the M6903.

What don't you follow or agree with? You want to regulate pressure with the vapor separator, fine. He may not even have one installed, but either way that is a lot of trial and error to set it, and then have all sort of variables like temp, and altitude effect the set point?

My only point is Carter can handle the higher pressure. Look at the float, needle and seat orifice used as compared to edelbrock and you can calculate a basic force on the float. Same relationship as Master Cylinder size other then the counter force of the float(buoyancy).

The pump is a positive displacement diaphragm pump. With sufficient head to flow the velocity and mass needed. Reduction of volume to reduce pressure because the pump is built wrong with the spring is not what you want. The chamber volume is constant on these pump.6903/4862

I am not sure how these latest pumps are pushing 18 plus psi. Would love to take one like that apart and measure the spring and see how they assembled the diaphragm and plates. I have had no issues with the older ones or the rebuild kits to fix them.

I recently fitted one on my 440 six pack and its been good so far.

I have run that pump on 3 different combos. All 440s, one w/ a Carter 750 carb, an 830 Holley DP, and a six pack setup. None had a regulator and had no issues w/ them. Always ran great.

I have a Carter 6903 that I rebuilt with a kit from Mancini. It put out 15+ psi and would flood the engine running an Edelbrock carb.
I had to add a regulator, I'd love to have the correct spring, don't see how it can be changed without changing the diaphragm also.

I am baffled by what this pump is doing. At first I thought it was float levels even though it was the same Pro Systems 950HP that it was dynoed with. On the dyno it was run with the dyno's set up which was electric. It fires up and sounds right but after a short time it is loading up. I lowered the floats no change and then being alone it was hard to get out of the car before it would die. So I jumped the starter relay and was able to observe the 15 psi gauge pulsating very bad. I put a older 30 psi liquid filled and that's were I saw it at 18 and was not fluctuating like the other gauge. Pump is new but has been sitting on the shelf for 15 years. There must be something odd with this pump. It is just a street car 528 single carb hemi. I can plumb a regulator in but this pump is still about double its max 9.5 psi rating. For my intended use I think the one step lower pump would be my best choice.

It can be cut off with a Dremel if you can't unwind it. Then and now kits use a lighter spring, they also have a different method for holding spring and top seal on. It allows the spring to be removed and replaced easily. When I have rebuild various original hemi, and early model pumps including the 6903/4862 I have documented the part and casting numbers of the various assemblies. Also measured spring length and force. I have not had a mancini or holley version of the pump, so I am not sure what they could have done to get the pressure doubled from 7-8 to over 15psi. It would take a very hefty spring, or they have internally modified the diaphragm and housing area to significantly reduce available surface area of the assembly to force pressure up that high when flow stops. Force = Pressure x Area and the spring is the Force. When flow stop the diaphragm just hang in the pulled up position as the lever floats, until sufficient pressure has bleed off through the check valve or check valve bleeds, or the float opens the needle again.

The only other issue could be the damper diagram. It is the one between the lower and middle housing that separates the input and output chambers. It act like a pulse damper. It is thin and soft, and when the spring pushes down on the fuel in the chamber it also depress that gasket. Increasing chamber volume and keeping pressure low (some force of spring used to depress the pulse diaphragm). When the lever pulls the pump diaphragm back up the pulse diaphragm returns to original position and continues to push fuel out of the pump while the main diaphragm is starting to pull fuel in to the pump. That dampens the pressure pusle.

In an old pump when that gets hard, it no longer deflects and the pump pressure actually rises.

Old army trick for jeeps was to double up that damper gasket if the pump pressure was too low.

^^^^^Good info, thanks!