Carter Mechanical Fuel Pump M6903 Pressure Fluctuation
#2444209
01/31/18 08:13 PM
01/31/18 08:13 PM
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Joined: Mar 2013
Posts: 571 Spring Hill Fl
65Fury440
OP
mopar
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OP
mopar
Joined: Mar 2013
Posts: 571
Spring Hill Fl
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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
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Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation
[Re: 65Fury440]
#2444294
01/31/18 10:35 PM
01/31/18 10:35 PM
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Joined: Nov 2003
Posts: 36,040 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
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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
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Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation
[Re: 65Fury440]
#2444364
01/31/18 11:55 PM
01/31/18 11:55 PM
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Joined: Jan 2003
Posts: 4,267 Morrow, OH
markz528
master
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master
Joined: Jan 2003
Posts: 4,267
Morrow, OH
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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) 14.82 @ 94 mph 69 GTX (clone) - build in progress......
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Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation
[Re: markz528]
#2444430
02/01/18 02:03 AM
02/01/18 02:03 AM
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Joined: Nov 2003
Posts: 36,040 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
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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
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Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation
[Re: 65Fury440]
#2444437
02/01/18 02:14 AM
02/01/18 02:14 AM
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Joined: Jan 2003
Posts: 1,753 Windsor, ON, Canada
Diplomat360
top fuel
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top fuel
Joined: Jan 2003
Posts: 1,753
Windsor, ON, Canada
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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.
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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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Joined: Jan 2003
Posts: 4,448 Phoenix, AZ
MoparBilly
master
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master
Joined: Jan 2003
Posts: 4,448
Phoenix, AZ
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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.
"Livin' in a powder keg and givin' off sparks"
4 Street cars, 5 Race engines
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Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation
[Re: MoparBilly]
#2444449
02/01/18 02:30 AM
02/01/18 02:30 AM
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Joined: Jun 2003
Posts: 9,225 Charleston
sixpackgut
Drag Week Mod Champion
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Drag Week Mod Champion
Joined: Jun 2003
Posts: 9,225
Charleston
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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.
Gen 3 power 6.22@110, 9.85@135 Follow @g3hemiswap on instagram
performance only racing, CRT, ultimate converter, superior design concepts, ThumperCarbs
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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
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Joined: Jan 2003
Posts: 21,050 Niles , Ohio
therocks
oh wait.but hey.lets see.oh yeah.
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oh wait.but hey.lets see.oh yeah.
Joined: Jan 2003
Posts: 21,050
Niles , Ohio
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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
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Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation
[Re: 65Fury440]
#2446324
02/04/18 05:06 PM
02/04/18 05:06 PM
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Joined: Jun 2006
Posts: 5,155 CT
GTX MATT
master
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master
Joined: Jun 2006
Posts: 5,155
CT
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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
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Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation
[Re: 65Fury440]
#2461277
03/05/18 01:31 AM
03/05/18 01:31 AM
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Joined: Mar 2013
Posts: 571 Spring Hill Fl
65Fury440
OP
mopar
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OP
mopar
Joined: Mar 2013
Posts: 571
Spring Hill Fl
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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
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Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation
[Re: markz528]
#2461293
03/05/18 01:52 AM
03/05/18 01:52 AM
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Joined: Jul 2006
Posts: 4,319 Puyallup, WA
StealthWedge67
master
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master
Joined: Jul 2006
Posts: 4,319
Puyallup, WA
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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.
LemonWedge - Street heavy / Strip ready - 11.07 @ 120
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Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation
[Re: 65Fury440]
#2463634
03/09/18 11:47 AM
03/09/18 11:47 AM
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Joined: Feb 2010
Posts: 1,645 Phila. Pa.
Mattax
top fuel
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top fuel
Joined: Feb 2010
Posts: 1,645
Phila. Pa.
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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#p9568Below 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.
Last edited by Mattax; 03/09/18 11:56 AM.
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