I run a Carter 6903 mechanical pump, with a carter electric out back to help feed it during runs. I have a bypass line with a check-valve that runs around the electric so the mechanical can draw without the electric on. All 1/2” hardline. This system feeds a Holley 4150.

I’ve been seeing my pressure gauge bouncing around more & more recently, and last weekend I noticed it not only was bouncing but pressure was lower than its normal 6 psi, at around 3 without the electric pusher on. This was coupled by the car being more inconsistent than normal, and actually falling off completely at the top of 2nd gear on the last run of the weekend. It would seem that the mechanical pump is going away.

Ultimately, the 4862 is the better pump and Carter recommends using it if you run a series setup like mine. But before I dump some cash on a 4862, is it worth it to pull this current 6903 pump apart and take a shot at rebuilding it? If you know these pumps a little, please fill me in on what can be done.

Thanks-

Try replacing (or removing) the check valve first?

Joe

I'd suspect either the mechanical pump's diaphragm is going bad or there's something - debris, flap inside hose - in the system.

Had an Edelbrock mechanical a couple years back that announced its bad diaphragm by spewing fuel on the headers.

granted we are the complaint dept & we get the problems cuz that is what we are here for but I hear constantly about Carter mech pump issues (the Carter curse) they call it.

probably asking too much from the pump. they will only create so much vacuum to pull fuel. 1/2" line will diminish that signal plus the check valve (how much vacuum does it take to open the valve least wise pull something thru it?), and the mech pump won't pull thru a vane pump. the 6903 only has a 1/4" i.d. feed orifice in the bowl. the pump can be reworked and helped; I have done it. the high pressure does pull some fuel but you'd have to figure a way to regulate the pressure. I've had some of those carter electrics get stuck and sometimes they're a little short on pressure.

I chased a problem like that and after buying 3 M4862 pumps and a second Holley 220GPH electrice I finally woke up and checked the pump rod off the cam. Sure enough it was almost a half inch short due to wear. Replaced that and everything works great. Anybody want to buy a slightly used M4862? LOL

I’ll measure the fuel pump rod when I pull the pump. I find that it should measure 3.242. I’ll send you a PM on the M4862 pump, Jon.

“Back in the day”(95-97), I was running 10’s with the OE 5/16” line, pick up, a carter 120gph mech pump, and a wix in line filter.

Nowadays, I have little faith in the overall quality of mechanical pumps, since they are now such a tiny portion of the fuel pump market.

I’d research for a suitably reliable electric pump that would satisfy your needs, and simplify the fuel system...... and likely improve its capacity at the same time.

I would agree with you for a dedicated race car, Dwayne. But my car still sees the street regularly and I don’t want to be tied to a noisy electric pump when driving on the street all the time. The way it is now seemed to be working great until last weekend.

I pulled the trigger on a new M4862 pump and regulator. I’m now thinking of bending up my own carburetor feed setup with 3/8” line in order to place the regulator in a sensible spot. I was really surprised that summit doesn’t have much in the way of a ready-done hardline feedline setup.

It’s certainly a higher effort and expense to change it over....... it I’m thinking I’d like to do something like this:

https://www.holley.com/products/fue...bureted_electric_fuel_pumps/parts/12-600

Then, if I wanted to change over to EFI at some point...... it would just be a matter of swapping the regulator.

Well in the aviation world, mechanical pumps are STILL THE STANDARD, with electric pumps as a backup. All they are is a lever, diaphragm, check valve and some springs, the electric pumps have a lot more to fail and are more susceptible to corrosion due to the crap they put in as additives in gas now days, alcohol is even worse. I would just get a diaphragm kit for a mechanical pump as a spare part and run it. But I am old school and in my world a failure = potential loss of life.

******* UPDATE *********

I replaced the 6903 pump with a 4862 and added a regulator along with -6 braided line & AN fittings from the mechanical pump forward. Verified that the check valve was clear and added a small inline filter right in front of it. While the pump was off I inspected the pump pushrod. It looked perfect with both ends looking completely normal and it measured correctly.

All back together: Issue still looks the same at the gauge. I removed the brass filter from my summit carb feed line setup; no change. Strangely, I can’t get the pressure up over 6 psi no matter what I do with the regulator. I tried to attach a video to help, but apparently the file type my iPad creates is not uploadable.

Could this be just the guage? I had an issue on my last pass that really felt like fuel starvation. My sense is that the guage is correct, but it does bounce around a lot. ????

Do you have the "sock" filter at the end of the sending unit? That all plugged up?

6 psi might be all there is coming out of the pump, but I would put a vacuum gauge just before the mechanical pump and check for restriction. It could be the sock filter in the tank, or any type of kink in the line back to the tank.

Bypass the regulator, see what kind of presssure the pump will develop without it.

Part of my lack of love for current mechanical pump offerings is I feel the q/c is spotty.

I know of a SB racer a few years ago who tried 3 of the same part number Carter pump on his motor...... all three had a different pressure......... and not like 1lb different either.
It was like, 4,6,9...... something like that.
Car was having fuel delivery issues, which is what prompted the testing.
After seeing that....... it got an electric pump. Delivery problems solved.

Well, now I’m getting frustrated. Tonight I pulled the sending unit from the tank; clean as a whistle. Re-installed. Traced every inch of the fuel line (1/2” hardline). Found one area right in front of the tank when the E-brake cable had been rubbing on it. It didnt look worn through, and I saw no sign of fuel leakage, but figured a small hole, especially up above the fuel level which this was, and behind the elec. pump (no pressure) might suck air and not leak fuel (???... anything’s possible). So I JB welded it then wrapped in tape, hoping I found something. Started the car... still the gauge flitters like a humming bird and does not seem to change when I adjust the regulator.

I didn’t get the chance yet to bypass the regulator as Dwayne has suggested, but it’s notable that it was doing this before I added the regulator. I’m beginning to think the gauge is simply bad and maybe I never had a fuel related issue to begin with??? Ugghh. I’ll see if one of my buddies has an extra guage I can borrow to see what it says.

Any further help still appreciated.

I just replace my Magnafuel brand low pressure fuel gauge on my race car Magnafuel 4 port regulator, it had drop to 3.5 lbs. with no evident issues on fuel delivery. The two little screws that held the outer cover on the gauge had vibrated out allowing it to slide down a little over the main gauge and it might have touch the needle at one time or another. The new same part number Magnafuel gauge with the cover tight showed 5.5 lbs. that I had set that regulator up with 5 yrs ago
My message is anything mechanical can go bad or be built defective, almost like men and women, huh

Look to reduce as many bends in the fuel line as possible, especially 90 bends .

What if you bypass the mech pump and the regulator?

What pressure does the elec pump show?
Nice and steady on the gauge?

I also liked the suggestion of running a hose from the mech pump into a can and bypassing the entire inlet side of the system.

Do you have a filter before the pump?

I gave up on running a mechanical pump last year. I mounted a gauge on my cowl and once the engine was heat soaked my 7 PSI fuel pressure dropped to 5 or less, and the gauge nose dived at the top over every gear. The top of first gear had it down to 3 PSI.