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Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2463663
03/09/18 12:30 PM
03/09/18 12:30 PM
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lewtot184 Offline
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Originally Posted By 65Fury440
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.
maybe you have some restriction on the suction side. hopefully you don't have a fuel filter on the suction side..

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2464244
03/10/18 12:27 PM
03/10/18 12:27 PM
Joined: Feb 2010
Posts: 1,645
Phila. Pa.
Mattax Offline
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If it only drops a little, say to 3-4 psi, that's probably just the inlet to the bowl opening.

PS. More pressure is not better. It just needs enough pressure to move the fuel into the bowl so the level is maintained. More moderate pressures reduce the turbulance and foaming of the fuel entering the bowl.
Flood & Drain Fuel Bowls scroll down for a video
Fuel Foaming Talk

Last edited by Mattax; 03/10/18 12:34 PM.
Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2464414
03/10/18 06:57 PM
03/10/18 06:57 PM
Joined: Mar 2013
Posts: 571
Spring Hill Fl
65Fury440 Offline OP
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65Fury440  Offline OP
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Spring Hill Fl
Thanks for all the useful info.
I drove about 30 miles and made a few stops today, didn't act up at all.

The last time it dropped pressure, I went into the auto parts store for about 5 minutes.

I came back out, PSI was still 6. I started the engine, it ran about 20 seconds while I put on my seat belt, and dropped slowly to 0 PSI.

I didn't even back out, as I thought the car was going to stall.

I revved it up to a fast idle, around 1500 rpms, then after another 10 seconds,it came back up pretty fast.

I bought it from Rock Auto, and they quit carrying them in the last month.

I can send it back for credit, or pop it back apart and see i there might be a valve problem. The cost for a rebuild kit from Mancini is more than I paid for the pump.

I just can't leave it this way. I just put the gauge on when I installed the pump, so I don't know how the last Muscle car pump acted, but no fuel pressure just shouldn't happen.

I ordered a Comp Cams 4626 fuel pump rod to pop in while I have it apart.

Thanks again, I'm baffled.

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2464603
03/11/18 04:36 AM
03/11/18 04:36 AM
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Cotati, CA
Dave Hall Offline
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I've seen my gauge mounted on my 2-port reg. go pretty low like that at idle as well with a mech. pump. How long is the line to your gauge? Maybe when the bowls are filling the fuel could back down the gauge line? Maybe try taking the line from the hood and watch a gauge sitting on the floor? If the car isn't running out of fuel I have to believe that the pump is fine. They are a pretty much work or don't work kind of thing. My .02...

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2464655
03/11/18 11:53 AM
03/11/18 11:53 AM
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Phila. Pa.
Mattax Offline
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Some poossibilies:
Pinhole hole leak in fuel line to the pump?
Partially clogged sock filter?
Vapor lock, possibly on the pressure side.
- Having the filter outlet higher then inlet should help it burp out.
- One fix Chrysler used for this was "anti-flood" bypass around the fuel pump. Just a pear of tees with an .040 restriction in both. If you search, I've posted pictures of this kit a few years ago.

I wouldn't expect this to be a problem in Fla. but never know.
Up here, its easy to get vapor lock when the weather suddenly turns hot and we're still on winter fuel. Sometimes rerouting or insulating the fuel line can resolve it. Since putting headers back on, that's the one problem the reared its head - except in my case its on the suction side - which is hard to clear out once it starts.

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: Dave Hall] #2464658
03/11/18 11:55 AM
03/11/18 11:55 AM
Joined: Mar 2013
Posts: 571
Spring Hill Fl
65Fury440 Offline OP
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Spring Hill Fl
Originally Posted By Dave Hall
I've seen my gauge mounted on my 2-port reg. go pretty low like that at idle as well with a mech. pump. How long is the line to your gauge? Maybe when the bowls are filling the fuel could back down the gauge line? Maybe try taking the line from the hood and watch a gauge sitting on the floor? If the car isn't running out of fuel I have to believe that the pump is fine. They are a pretty much work or don't work kind of thing. My .02...


Yea I dont know if maybe the gauge is sticking high on pressure?

I have maybe 3 feet of line. It holds pressure for several hours.
I guess if the float bowls lost pressure back into the lines and the gauge stuck it would show like this.

Sure seems like the gauge responds quickly to changes though while it's being driven.
When the throttle opens quickly, the gauge drops right away.

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: Mattax] #2464668
03/11/18 12:11 PM
03/11/18 12:11 PM
Joined: Mar 2013
Posts: 571
Spring Hill Fl
65Fury440 Offline OP
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65Fury440  Offline OP
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Posts: 571
Spring Hill Fl
Originally Posted By Mattax
Some poossibilies:
Pinhole hole leak in fuel line to the pump?
Partially clogged sock filter?
Vapor lock, possibly on the pressure side.
- Having the filter outlet higher then inlet should help it burp out.
- One fix Chrysler used for this was "anti-flood" bypass around the fuel pump. Just a pear of tees with an .040 restriction in both. If you search, I've posted pictures of this kit a few years ago.

I wouldn't expect this to be a problem in Fla. but never know.
Up here, its easy to get vapor lock when the weather suddenly turns hot and we're still on winter fuel. Sometimes rerouting or insulating the fuel line can resolve it. Since putting headers back on, that's the one problem the reared its head - except in my case its on the suction side - which is hard to clear out once it starts.


Wow, lot's of things to think about...

So when t happens, is after sitting a short time. That may be a valid point.

The whole fuel system is new, from the sending unit to the carb.

I rechecked all the clamps on the rubber lines, no sign of any moisture.

I insulated the steel line with a heat deflector where it comes out of the stub frame, it's maybe an inch and a half to the closest point of the header.

I'll check out your post on the anti flood.

I am burning non ethanol 90 octane gas. Don't know how that equates with the problem.

Thanks for the help.

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2464673
03/11/18 12:21 PM
03/11/18 12:21 PM
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NJ central
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Scamp408 Offline
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I had similiar issues on my 383 I used the carter street pump strip pump and the stock pump all new and tried multiples. Back in the 90s I never had a problem it wasn’t until around 03 04 when I had issues. I changed the lines filter pu tube and rerouted always from hot sources no fixes found. I did more research and and poked around with my heat gun and found out the pump was getting hot on the side of the block 190 200 etc. normal motor temperatures .Herre’s the problem E10 or E15 boils at very low temps 150 160 180 so no matter what you do the pump will boil the fuel and vaporize inside. It’s the fuel we use and real fuel boils over 200.

Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2464676
03/11/18 12:32 PM
03/11/18 12:32 PM
Joined: Feb 2010
Posts: 1,645
Phila. Pa.
Mattax Offline
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Here's another place I posted it. Quicker for me to find.
https://www.tapatalk.com/groups/racingfu...ures-t1459.html

Reid Vapor Pressure is a good indicator of how sensitive a fuel may be to vapor lock from heat.

Years ago I must have had a pinhole leak in the original hard line near over the axle. That was my conclusion anyway - couldn't prove it, but had tried a lot of things, and thats when I got into the fuel pumps. Unfortunately with that I don't recall the details of what I'd see at idle when I installed a gage. The problem that I was focused on was the pressure going to zero about 2/3 of the way down the track (1/4 mile). It was always close to running out of fuel in the bowls through the lights. That was with a crappy little Holley chrome pump, and then with the Carter "Competition" pump. The Competition pump didn't solve the problem, but it did cause flooding at times because of the high cut-off pressure (7 - 7.5 psi). (This is just a street car, high 13s)

Overall, your problem sounds different. I offer that story as an example of how sensitive the suction side can be to small blockages or leaks. The location was over the hump and had a little rust on the outside - nothing terrible - clean Arizona car. I can't recall now if I ever noticed a damp spot toward the end and that's what spurred me to replace it with a new 3/8 stainless line.

Last edited by Mattax; 03/11/18 12:37 PM.
Re: Carter Mechanical Fuel Pump M6903 Pressure Fluctuation [Re: 65Fury440] #2464680
03/11/18 12:40 PM
03/11/18 12:40 PM
Joined: Feb 2010
Posts: 1,645
Phila. Pa.
Mattax Offline
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Looks like the holes were much smaller than .040" .. smaller than an #80 bit, but another guy there used an .040 jet on a vapor return line he made himself

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