Carter Mechanical Fuel Pump M6903 Pressure Fluctuation

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

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.

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

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.

Agreed, it's called the Carter Curse.

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 Is the M6903 a newer version of the M6902?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.

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.

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?"

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

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...

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

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.

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?

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.

Got a link?

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.

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.

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.

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.

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.

maybe you have some restriction on the suction side. hopefully you don't have a fuel filter on the suction side..

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

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.

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...

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.

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.

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.

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.

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.

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