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Carter Mechanical Fuel Pump M6903 Pressure Fluctuation #2444209
01/31/18 08:13 PM
01/31/18 08:13 PM
Joined: Mar 2013
Posts: 488
Spring Hill Fl
65Fury440 Offline OP
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65Fury440  Offline OP
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Spring Hill Fl
I just installed this fuel pump as an upgrade.
Worked fine for the first 30 miles, now it drops pressure for a few seconds intermittently.

The inlet fittings are tight from the tank
The tank is vented

Pushrod length is 3.120.

Wouldn't the pushrod lose pressure at higher RPM if too short?
No problems before other than limited delivery.

I took it apart to re orient fitting location, did not see anything amiss, went back together fine.

Pump drops pressure for a couple seconds and slowly comes back up at idle or slightly above

Any ideas?

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

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2444216
01/31/18 08:24 PM
01/31/18 08:24 PM
Joined: Jan 2003
Posts: 6,591
Canton, Ohio
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Sport440 Offline
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Canton, Ohio
Maybe its just your floats opening and closing showing the fluctuation. How are you reading the PSI?

You may have line or filter restrictions going on as well maybe enhancing the appearance of the psi fluctuations.

I ran that pump as well, but it was the higher 9 psi version and it couldn't keep up with 550 hp.

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2444294
01/31/18 10:35 PM
01/31/18 10:35 PM
Joined: Nov 2003
Posts: 34,744
Lincoln Nebraska
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RapidRobert Offline
Circle Track
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Lincoln Nebraska
Quote:
I just installed this fuel pump as an upgrade.
Worked fine for the first 30 miles, now it drops pressure for a few seconds intermittently.
I would toss the old pump back on for a test (& see if it acts up the same at the psi it provides). If no "drop" then it'd about have to be the pump itself, if it drops psi then obviously upstream (toward the tank) or downstream (to/in the carb) from the pump itself there's a problem. Holler what it ends up being. the old pump was fine as far as no pressure drop I am assuming so I am thinking the pump valves


live every 24 hour block of time like it's your last day on earth
Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2444364
01/31/18 11:55 PM
01/31/18 11:55 PM
Joined: Jan 2003
Posts: 3,526
Milford, OH
markz528 Offline
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markz528  Offline
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Milford, OH
I could never get those pumps to work. I gave up and went electric. When I put the original motor back in I put the stock one back on.


67 Coronet 500 9.610 @ 139.20 mph
67 Coronet 500 (street car) 13.300 @ 101.75 mph
69 GTX (clone) - build in progress......
Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: markz528] #2444430
02/01/18 02:03 AM
02/01/18 02:03 AM
Joined: Nov 2003
Posts: 34,744
Lincoln Nebraska
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RapidRobert Offline
Circle Track
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Lincoln Nebraska
Quote:
I could never get those pumps to work.
Agreed, it's called the Carter Curse.


live every 24 hour block of time like it's your last day on earth
Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2444437
02/01/18 02:14 AM
02/01/18 02:14 AM
Joined: Jan 2003
Posts: 1,233
Windsor, ON, Canada
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Diplomat360 Offline
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Windsor, ON, Canada
Hmm...interesting, very similar to what I saw with my M6902 pump. Is the M6903 a newer version of the M6902?

Oh, on my end, I never figured out what was causing this. I actually tested out two pumps (both were brand new), both showed the same behaviour. Besides, my bigger concern, and focus point, at that time was that I was seeing low PSI at all times...turned out I had plumbed the fuel filter incorrectly having put it BETWEEN the tank and the pump inlet. Once I moved the filter to AFTER the pump the PSI picked-up.

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2444442
02/01/18 02:18 AM
02/01/18 02:18 AM
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Posts: 4,435
Phoenix, AZ
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MoparBilly Offline
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Phoenix, AZ
Is the engine starving for fuel? Vapor locking? I have the same pump on my 440, 67 B-body with stock 5/16th line, feeds both the 750 Holley and the 160 shot of nitrous, and it's been 11.24-122. If the car doesn't fall on it's face, I wouldn't sweat the pressure gauge moving around a bit.

14324431_10154650818513938_2085996524283702274_o - Copy.jpg

"Livin' in a powder keg and givin' off sparks" 4 Street cars, 5 Race engines
Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: MoparBilly] #2444449
02/01/18 02:30 AM
02/01/18 02:30 AM
Joined: Jun 2003
Posts: 9,133
Charleston
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sixpackgut Offline
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Posts: 9,133
Charleston
I was having a weird issue on a new mechanical fuel pump and can't remember who said it to me but they said "do you really believe anyone cares about the quality of a mechanical pump they are selling these days?"

Last edited by sixpackgut; 02/01/18 02:33 AM.

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Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2444929
02/01/18 11:26 PM
02/01/18 11:26 PM
Joined: Jan 2003
Posts: 19,633
Niles , Ohio
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therocks Offline
Its Lost
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Posts: 19,633
Niles , Ohio
We had 3 carter all hipo new.2 BBs and 1 Sb.One never worked out of the box.1 was on the 413 for like 100 miles and the spring on the arm popped and it quit.Lucky we got the spring out of the pan.1 ran for like 1 hour on my 440.went back to my hipo Holey pumps.Had one I bought in the 70s that ran better than a new carter.Now we run all Holleys.rocky


Chrysler Firepower
Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2446324
02/04/18 05:06 PM
02/04/18 05:06 PM
Joined: Jun 2006
Posts: 4,878
CT
GTX MATT Offline
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GTX MATT  Offline
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Posts: 4,878
CT
I have that pump with a 3/8 line on my 440 in my '67. Pressure drops to 3 psi by the top of 1st gear. If you wack the throttle idling it'll drop from 7 to 6...


Now I need to pin those needles, got to feel that heat Hear my motor screamin while I'm tearin up the street
Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2461277
03/05/18 01:31 AM
03/05/18 01:31 AM
Joined: Mar 2013
Posts: 488
Spring Hill Fl
65Fury440 Offline OP
mopar
65Fury440  Offline OP
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Posts: 488
Spring Hill Fl
I had a chance to pull the pump off and apart.
The diaphram and spring look good.

I have a cowl mounted gauge, the weird part is the problem happens at low rpms, no real way to reproduce it, just happens when it wants.
The whole fuel system is new from the 3/8s sending unit up to the carb.

I am thinking of replacing it with the Edelbrock pump just for the simple fact it has 3.8 fittings.
Does anyone have good or bad experiences to report?

http://www.manciniracing.com/mechanical.html

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: markz528] #2461293
03/05/18 01:52 AM
03/05/18 01:52 AM
Joined: Jul 2006
Posts: 4,177
Puyallup, WA
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StealthWedge67 Offline
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Puyallup, WA
I’ve been using a 6903 pump for years with good success. My guage used to move a round a little (3-7), but never dropped to zero. Since going to 1/2” line, it now stays much more stable. Like Billy said; if the car isn’t running out of fuel, I wouldn’t worry about it too much.

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2461320
03/05/18 02:45 AM
03/05/18 02:45 AM
Joined: Mar 2013
Posts: 488
Spring Hill Fl
65Fury440 Offline OP
mopar
65Fury440  Offline OP
mopar

Joined: Mar 2013
Posts: 488
Spring Hill Fl
UGH, I searched the Edelbrock pump, they have pressure problems also,
There's not much out there for a BBM.

I'm worried it is going to completely quit.

What should I look for when looking at the valving in the pump?

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2461794
03/06/18 12:43 AM
03/06/18 12:43 AM
Joined: May 2017
Posts: 50
WI
B
Brewzer67 Offline
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WI
I just saw a used Clay Smith mechanical pump for sale on Facebook marketplace. I have been running one for 10 years with no issue. It has supported 600 hp so far and I'm hoping it will support the 775 I'm shooting for.

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2461830
03/06/18 01:39 AM
03/06/18 01:39 AM
Joined: Mar 2013
Posts: 488
Spring Hill Fl
65Fury440 Offline OP
mopar
65Fury440  Offline OP
mopar

Joined: Mar 2013
Posts: 488
Spring Hill Fl
Got a link?

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2462373
03/07/18 12:45 AM
03/07/18 12:45 AM
Joined: May 2017
Posts: 50
WI
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Brewzer67 Offline
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WI
Sorry, by time I found it again it was sold. I figured it would go pretty quick. Since they are so hard to get these days they move pretty quick if they are priced reasonably.

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2462714
03/07/18 06:41 PM
03/07/18 06:41 PM
Joined: Jun 2008
Posts: 2,828
usa
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lewtot184 Online content
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lewtot184  Online Content
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usa
this is typical of mech pumps. they're pulse pumps, not constant pressure/flow. fluctuations will be more noticeable the lower the rpm. vapor separator return lines will make the pulses more noticeable. rev the engine up a little and see if the pulses stabilize to a steady pressure.

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2463468
03/09/18 12:35 AM
03/09/18 12:35 AM
Joined: Mar 2013
Posts: 488
Spring Hill Fl
65Fury440 Offline OP
mopar
65Fury440  Offline OP
mopar

Joined: Mar 2013
Posts: 488
Spring Hill Fl
When it does drop out is when the RPMs are low, for about 5 seconds.
Even when I speed it up to maybe 1500, it stays low, then comes back up to about 6.5 psi, It's weird.

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2463606
03/09/18 10:45 AM
03/09/18 10:45 AM
Joined: Oct 2008
Posts: 399
Harleysville, PA USA
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Tommy D Offline
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Tommy D  Offline
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Harleysville, PA USA
I've run this pump at the rear, and a Carter mechanical up front for years in our race car and in my street car (10.80/11.07). I have a cowl-mounted gauge as well. My pressure fluctuates at low rpm, but is steady at 7PSI at 6800 RPM through the traps. I drive the same setup on the street and never had a failure. Probably been 20 years and 5000 miles. I do not typically drive it long distances. Maybe 50 miles at a time. I run a toggle switch under the dash and shut it off at lights and when cruising through town. Then it runs on the mechanical pump alone. I wouldn't sweat it.

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2463634
03/09/18 11:47 AM
03/09/18 11:47 AM
Joined: Feb 2010
Posts: 1,294
Phila. Pa.
Mattax Offline
pro stock
Mattax  Offline
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Posts: 1,294
Phila. Pa.
At low rpm, if the carb bowls need filling, the pressure may be a little low. I'd expect to see 3-4 psi, be somewhat concerned about pump or supply line if its 2 psi, and very concerned if its 0 - 1 psi.

Going down the track, I'd like to see 3 psi or higher.


maybe an explanation will help.

With diaghram type pump potentially flow more with increasing rpm.
In general more valves inside increase the flow possible.
Pressure is controlled by the spring. The pump arm pulls the diaphram up, but the spring pushes the diaphram down. When the fluid pressure is high enough, the spring can't push down. This cuts off the pressure from increasing.
So pressure on the gage is a combination of these.

With the smallblock, I had good luck with the Holley 110/Kieth Dorton series mechanical pumps. But of course mine was made 10-15 years ago, could be different now. However fuel pumps for small blocks have space limitations when running passenger side inlet water pumps. With a big block there should room to run a 6 valve pump.

If you really get stuck, contact Ryan Brown Racing. They deal with mechanical pumps as they have a lot of circle track customers. I had them test some small block pumps for me.

Seems to be some trouble uploading pics this morning.
Go here - about 3 post in row here with pump graphs and photos
https://www.tapatalk.com/groups/racingfu...1032.html#p9568

Below is a flow graph of the 110 mechanical pump from Holley's catalog. (It used to be on a webpage, now you have to find the catalog.)
Freeflow is pretty much meaningless, even though that's what they all advertise. There's always some resistance in the line and inlet valve.

f110GPHchrt.jpg
Last edited by Mattax; 03/09/18 11:56 AM.
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