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Mechanical Fuel Pump 101

khryslerkid

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I was having a "higher than I wanted" fuel pressure using a Carter M6903 mechanical pump on my 440 with two 600 Edelbrocks. It was running at 8 psi and the last oil change I'm getting a strong fuel smell. The oil wasn't over full or thinned out but the fuel smell is telling me fuel is getting past the rings somehow. The engine wasn't running rich but there was 8 lbs showing on the guage each time I shut it down and it was probably bleeding off overnight. This pump is supposed to be rated @ 7 psi.

So doing some research I find the Holley 12-440-11 is the same pump just chrome plated and it's rated 6.5 - 8 psi. (Not for me)

Then I looked at the Edelbrock 1722 (same pump again) and it's rated @ 6 psi. Did a little more research on this pump and the ratings are about 50/50 on quality and function. I also found out Edelbrock had Carter make this pump with a lighter spring just for the fact Eddies don't like more than 6 psi. (I'm not liking the ratings)

So I'm thinking if I could find a rebuild kit for the Edelbrock 1722 it would have a lighter spring and less pressure. No such luck finding a rebuild kit for any of these newer Carter fuel pumps. They are made to be disassembled but you're not going to find a kit. Makes no sence except for the fact they are just a throw away pump just like the stock ones.

Speaking of stock pumps, there are several out there that have a 3/8" inlet (another feature I need because of having that size line from the tank) but they are rated @ 8 psi also.

You might be thinking, why don't you just get a regulator? Going to a regulator setup would require additional lines, the regulator and mount it on the firewall or inner fender. I'm not really that fond of that look on this car plus the added cost would be well over $100.

A little more research and I stumbled onto this outfit in Massachusetts that rebuilds fuel pumps.

Then and Now Automotive
447 Washington Street
Weymouth Ma. 02188
Ph# 781-335-8860
www.then-now.com


I emailed them and talked to Mike. Told him my situation. Part of his email reply...

The diaphragm assembly from the factory is not changeable. What can be done, is if you send me the pump, I would disassemble it and make an entirely new diaphragm assembly with a lower pressure spring on it. After that, it can be changed.

So I boxed it up, sent it to them and a week later...

I fixed up your pump this morning with a weaker spring. Just so you know, the diaphragm assembly I took out wasn't going to last very long. I'm really not surprised either. The spring that is in there now, should give you the lower pressure you're looking for. They put a killer spring in there from the factory. Anyway, I am sending the old one back with the pump so you can see and feel how strong the spring was. Look closely at the holes where the screws go thru the diaphragm material. You'll notice that the holes are elongated which is a no-no. That tells me that when whoever at the factory was assembling the pump, they neglected to pre-load the diaphragm. By not doing that, the diaphragm tends to stretch and fail rather quickly. Those oval screw holes were a dead giveaway. Pre-loading the diaphragm is extremely important especially in a Carter pump.

Here's the diaphragm he removed
20200523_125459.jpg

20200523_125448.jpg


The spring reminds me of a inner valve spring, spring! That got me thinking about checking the condition of the pushrod.
20200513_121433.jpg


This started out as 3.220 In just under 1500 miles it's worn almost 1/8".
DO NOT USE THE HOWARD'S CAMS 92152 PUSHROD ON THESE HIGH PERFORMANCE FUEL PUMPS! In fact, I would not use one with any fuel pump. Just using the "Search" feature on here, there are complaints of failures with this pushrod all the way back to 2015.

So I pulled one from the original 361 engine that has over 140k on it and it still measures 3.220
20200513_135857.jpg


Inspection of the cam lobe for the pushrod looked good. (Cam lobe is harder than the Howards push rod!)

I like to use a remote starter button to bump the engine around to find the shortest stroke of the pushrod. Just install the rod and while holding it against the cam lobe bump the engine until it's in as far as it goes. It makes it so much easier to install the pump not having any pressure on the fuel pump leaver.
20200523_130041.jpg


A little sealer on the gasket and stick the bolts though just a few threads before installing. Most gaskets will hold the bolts for you.
20200523_133005.jpg


All buttoned up.
20200523_143549.jpg


Before...


After...


Check out Then and Now Automotive's website. They have a lot going on and not just antique vehicles. Thanks Mike!
 
I didn't know that !! Glad you didn't just put on a fuel regulator and go. Solved all your present and future problems in one step. I bet it runs a little cleaner now. Good for you!!
 
Great post, Mr. Kid. Always well thought out and articulate. I have a cousin that is well into his 80s that was an engineer for Airtex, a company that manufactured pumps for Chrysler and Ford. He rebuilt the pump that I have on my 65, always thought it was a Carter, no it was from some company that I’d never heard of. After his retirement he built custom pumps for people all over world. Just goes to show you that with aftermarket parts, you never know exactly what you’ll get.
 
I was having a "higher than I wanted" fuel pressure using a Carter M6903 mechanical pump on my 440 with two 600 Edelbrocks. It was running at 8 psi and the last oil change I'm getting a strong fuel smell. The oil wasn't over full or thinned out but the fuel smell is telling me fuel is getting past the rings somehow. The engine wasn't running rich but there was 8 lbs showing on the guage each time I shut it down and it was probably bleeding off overnight. This pump is supposed to be rated @ 7 psi.

So doing some research I find the Holley 12-440-11 is the same pump just chrome plated and it's rated 6.5 - 8 psi. (Not for me)

Then I looked at the Edelbrock 1722 (same pump again) and it's rated @ 6 psi. Did a little more research on this pump and the ratings are about 50/50 on quality and function. I also found out Edelbrock had Carter make this pump with a lighter spring just for the fact Eddies don't like more than 6 psi. (I'm not liking the ratings)

So I'm thinking if I could find a rebuild kit for the Edelbrock 1722 it would have a lighter spring and less pressure. No such luck finding a rebuild kit for any of these newer Carter fuel pumps. They are made to be disassembled but you're not going to find a kit. Makes no sence except for the fact they are just a throw away pump just like the stock ones.

Speaking of stock pumps, there are several out there that have a 3/8" inlet (another feature I need because of having that size line from the tank) but they are rated @ 8 psi also.

You might be thinking, why don't you just get a regulator? Going to a regulator setup would require additional lines, the regulator and mount it on the firewall or inner fender. I'm not really that fond of that look on this car plus the added cost would be well over $100.

A little more research and I stumbled onto this outfit in Massachusetts that rebuilds fuel pumps.

Then and Now Automotive
447 Washington Street
Weymouth Ma. 02188
Ph# 781-335-8860
www.then-now.com


I emailed them and talked to Mike. Told him my situation. Part of his email reply...

The diaphragm assembly from the factory is not changeable. What can be done, is if you send me the pump, I would disassemble it and make an entirely new diaphragm assembly with a lower pressure spring on it. After that, it can be changed.

So I boxed it up, sent it to them and a week later...

I fixed up your pump this morning with a weaker spring. Just so you know, the diaphragm assembly I took out wasn't going to last very long. I'm really not surprised either. The spring that is in there now, should give you the lower pressure you're looking for. They put a killer spring in there from the factory. Anyway, I am sending the old one back with the pump so you can see and feel how strong the spring was. Look closely at the holes where the screws go thru the diaphragm material. You'll notice that the holes are elongated which is a no-no. That tells me that when whoever at the factory was assembling the pump, they neglected to pre-load the diaphragm. By not doing that, the diaphragm tends to stretch and fail rather quickly. Those oval screw holes were a dead giveaway. Pre-loading the diaphragm is extremely important especially in a Carter pump.

Here's the diaphragm he removed
View attachment 954409
View attachment 954410

The spring reminds me of a inner valve spring, spring! That got me thinking about checking the condition of the pushrod.
View attachment 954417

This started out as 3.220 In just under 1500 miles it's worn almost 1/8".
DO NOT USE THE HOWARD'S CAMS 92152 PUSHROD ON THESE HIGH PERFORMANCE FUEL PUMPS! In fact, I would not use one with any fuel pump. Just using the "Search" feature on here, there are complaints of failures with this pushrod all the way back to 2015.

So I pulled one from the original 361 engine that has over 140k on it and it still measures 3.220
View attachment 954424

Inspection of the cam lobe for the pushrod looked good. (Cam lobe is harder than the Howards push rod!)

I like to use a remote starter button to bump the engine around to find the shortest stroke of the pushrod. Just install the rod and while holding it against the cam lobe bump the engine until it's in as far as it goes. It makes it so much easier to install the pump not having any pressure on the fuel pump leaver.
View attachment 954432

A little sealer on the gasket and stick the bolts though just a few threads before installing. Most gaskets will hold the bolts for you.
View attachment 954435

All buttoned up.
View attachment 954438

Before...


After...


Check out Then and Now Automotive's website. They have a lot going on and not just antique vehicles. Thanks Mike!

What kind of power is your motor putting out? Reason I ask is that I am have a detonation issue on a newly built stroker motor that is 595hp and 605 ft lbs. I am using the same 6903 mechanical pump. The engine builder is blaming the pump for not putting out enough volume once the secondaries open. I don't know if I am buying into that.
 
Good stuff! I second not using the Howard's push rod. Here is the one that failed on me.

IMG_0589 (Medium).JPG
 
What kind of power is your motor putting out? Reason I ask is that I am have a detonation issue on a newly built stroker motor that is 595hp and 605 ft lbs. I am using the same 6903 mechanical pump. The engine builder is blaming the pump for not putting out enough volume once the secondaries open. I don't know if I am buying into that.

When I hear the term detonation I'm thinking of too much timing, heat, too low of a octane fuel.

Not enough fuel when at wot would create a lean condition.

I'm only making a estimated 400 hp. I never had a problem with a lean condition at wide open throttle. :)
 
Last edited:
Good stuff! I second not using the Howard's push rod. Here is the one that failed on me.

View attachment 954533

I saw your post in another thread about your pushrod and it had me second guessing using this one myself. I was going to alert you of this thread just to confirm your findings. They should pull this item, I might just contact them :)
 
When I hear the term detonation I'm thinking of too much timing, heat, too low of a octane fuel.

Not enough fuel when at wot would create a lean condition.

I'm only making a estimated 400 hp. I never had a problem with a lean condition at wide open throttle. :)

Exactly my thoughts as well. That is why I think their is something else besides fuel feed issues going on with the motor he put together for me.
 
I saw your post in another thread about your pushrod and it had me second guessing using this one myself. I was going to alert you of this thread just to confirm your findings. They should pull this item, I might just contact them :)

Had about 1500 miles on it with a standard mechanical pump in a 383 HP.
 
I learn something new every day. I was a GM man for years and would start with a dart block and for pumps, not wanting electric, I used police interceptors. Good for a little more flow. I'd use 750 quick fuel carbs. When done I'd take it to a friends that was a distributor for quick fuel and had a dyno. He'd set the pinion and change out everything in it till it ran as good as it could for what it was, the best you could ask for. They were always 383 strokers with 500 to 550 hp running 3/8 fuel lines. Pump gas only and Forged everything. Thanks again for the lesson on Mopar.!!
 
I fixed up your pump this morning with a weaker spring. Just so you know, the diaphragm assembly I took out wasn't going to last very long. I'm really not surprised either. The spring that is in there now, should give you the lower pressure you're looking for. They put a killer spring in there from the factory. Anyway, I am sending the old one back with the pump so you can see and feel how strong the spring was.

First of all great post and thank you. I have been deliberating on whether to get the Edelbrock mechanical fuel pump for my 800 avs2 on my stroker build but have heard of quality problems. I was thinking, could it be a possibility these pump pushrod wear problems could be associated with the very high spring pressure in these pumps in general?? I will most likely be getting a new Carter pump for my 440 and send it in to these guys at Then and Now Automotive. Spoke to them today and they seem terrific to deal with. Thanks again.
 
I was thinking, could it be a possibility these pump pushrod wear problems could be associated with the very high spring pressure in these pumps in general??

I was thinking the same until I saw T2R9's post...
Had about 1500 miles on it with a standard mechanical pump in a 383 HP.

Yeah, Mike is great to deal with. We talked the other day and I'm second guessing if 5 1/2 psi will be enough on the dual 600s at WOT. He said if they end up starving he'll send me the next stiffer spring. (Shipping is a little slow right now so there's that time line to consider)

He's making motor mounts also. I haven't asked about those yet.
 
Yeah, Mike is great to deal with. We talked the other day and I'm second guessing if 5 1/2 psi will be enough on the dual 600s at WOT.

My gathering is psi and flow are completely different things.
 
Excellent post, my friend!
When it came fuel pump time, I did as many have done before and fetched the M6903
Carter from Mancini:
https://www.manciniracing.com/cabheen5psis.html

Quoting from their description:
"The product specialists at Mancini Racing worked directly with the Federal-Mogul/Carter Fuel
Systems brand team in bringing back these highly sought after mechanical fuel pump..."

So - Mancini "commissioned" these pumps to start being produced again?

Also from their description:
"5.5 to 6.5 PSI"

Question for KK: Did you get your Carter from Mancini? Do they not have an "exclusive" on the
one being produced now and if not, do others have different internals?
Or perhaps, is Mancini's description (or the manufacturing process) have that much "wiggle room"
in the rated output pressure?

Oh, as an aside....my own 440 has managed to wipe both the "good" Comp Cams AND the Howard's
pump pushrods in short order.
I'm inclined to believe the Carter may have something to do with that.
What else could it be?
 
Excellent post, my friend!
When it came fuel pump time, I did as many have done before and fetched the M6903
Carter from Mancini:
https://www.manciniracing.com/cabheen5psis.html

Quoting from their description:
"The product specialists at Mancini Racing worked directly with the Federal-Mogul/Carter Fuel
Systems brand team in bringing back these highly sought after mechanical fuel pump..."

So - Mancini "commissioned" these pumps to start being produced again?

Also from their description:
"5.5 to 6.5 PSI"

Question for KK: Did you get your Carter from Mancini? Do they not have an "exclusive" on the
one being produced now and if not, do others have different internals?
Or perhaps, is Mancini's description (or the manufacturing process) have that much "wiggle room"
in the rated output pressure?

Oh, as an aside....my own 440 has managed to wipe both the "good" Comp Cams AND the Howard's
pump pushrods in short order.
I'm inclined to believe the Carter may have something to do with that.
What else could it be?
I know Mancini says exclusively for them, but that is BS. I've seen the 6903 for sale elsewhere. You can find it at JEGS for example.
 
My gathering is psi and flow are completely different things.

You are correct. I don't know if the lighter spring that changes the pressure will effect the 120 gph that these are rated or is it the valves inside that dictate the flow?

Excellent post, my friend!
When it came fuel pump time, I did as many have done before and fetched the M6903
Carter from Mancini:
https://www.manciniracing.com/cabheen5psis.html

Quoting from their description:
"The product specialists at Mancini Racing worked directly with the Federal-Mogul/Carter Fuel
Systems brand team in bringing back these highly sought after mechanical fuel pump..."

So - Mancini "commissioned" these pumps to start being produced again?

Also from their description:
"5.5 to 6.5 PSI"

Question for KK: Did you get your Carter from Mancini? Do they not have an "exclusive" on the
one being produced now and if not, do others have different internals?
Or perhaps, is Mancini's description (or the manufacturing process) have that much "wiggle room"
in the rated output pressure?

Oh, as an aside....my own 440 has managed to wipe both the "good" Comp Cams AND the Howard's
pump pushrods in short order.
I'm inclined to believe the Carter may have something to do with that.
What else could it be?

I bought mine from Summit.

https://www.summitracing.com/parts/crt-m6903

They rate them as having 7 psi then futher down in their add they state 5.5 to 7.5 (trying to cta!) I know mine was a steady 8 psi. I guess it's all about what springs they decide to install. The Edelbrock was said to have been a special request from Carter for the 6 psi rating because of the Eddy carb specs. You wonder if Mancini did the same or they have the wrong info listed? "Wiggle room" Only a good gauge will tell and not everyone has a guage.

As I stated earlier about the heavy spring causing pushrod failure, member T2R9 said he had a Howards fail using a stock fuel pump. (Post #9)

I did a search on here on the Howards and found five or six post on them wearing prematurely clear back to 2015. This is inferior material or not having the correct heat treatment. I'm sticking with a factory one even if it's used and still measures 3.220
 
You are correct. I don't know if the lighter spring that changes the pressure will effect the 120 gph that these are rated or is it the valves inside that dictate the flow?



I bought mine from Summit.

https://www.summitracing.com/parts/crt-m6903

They rate them as having 7 psi then futher down in their add they state 5.5 to 7.5 (trying to cta!) I know mine was a steady 8 psi. I guess it's all about what springs they decide to install. The Edelbrock was said to have been a special request from Carter for the 6 psi rating because of the Eddy carb specs. You wonder if Mancini did the same or they have the wrong info listed? "Wiggle room" Only a good gauge will tell and not everyone has a guage.

As I stated earlier about the heavy spring causing pushrod failure, member T2R9 said he had a Howards fail using a stock fuel pump. (Post #9)

I did a search on here on the Howards and found five or six post on them wearing prematurely clear back to 2015. This is inferior material or not having the correct heat treatment. I'm sticking with a factory one even if it's used and still measures 3.220
Wish I had a stock one to fall back on....
I have a gauge on my fuel line too. I'll check it out right now, in fact....
of course, who's to say how accurate any of these little gauges are, anyways?

EDIT: Just went out and fired the GTX up. At idle, the fuel gauge on the rail
says 6psi. Sorry, I know that doesn't help anyone.
Could be my pushrod is already shortened, rendering a lower psi? :)
 
Last edited:
Well I ordered a new Carter M6903 off ebay cheap and will send it off immediately to these guys. Sucks that they don't assemble them correctly to begin with. If I have any problems down the line they can rebuild it for me.
 
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