Developed a leak at the lower gasket on the pump. I took it apart to clean out the old gas. Its been sitting 1 year with only about 1 hour of use on the pump. When I removed the lower portion I found a black square hard sponge. Ive never seen this. Could this have caused a problem with the check valve, which caused the leak? HELP

i actually think the "sponge" is some sort of noise suppression. i've taken them out and now i hear the pump thumpin. leak is probably uneven mating surfaces. anymore i run all the mating surfaces over some wet or dry emory paper to get hem flat. i've had good success doing that and have picked up 1/2lb pressure doing it.

It is a pulse damper to assist the air dome diaphragm from collapsing too much on the pressure stroke. Not all pumps have it. When you say leak, what type of leak? Internal into the isolated chamber or external to the pump.

Gas was actually coming out of the lower gasket. I thought the check valve was clogged. Took it apart and it all gaskets looking good.

Any screws loose when disassembling? The bottom surface looks good other then the center bridge. The pocket with rubber should have been dry. What does the top and bottom surface of the middle body and top look like?

All surfaces look good. Should that middle area be machined? It looks as cast. I think gas spilled into that chamber when it was disassembled.

i think if you run the mating surfaces over some emory paper on a flat hard surface you'll notice that the surfaces aren't always true. they are "as cast" surfaces; don't trust them to be flat.

As stated it needs to be flat. If a high spot the middle section won't be a flush seal it would pivot slightly on the high spot. That might be your issue. You do need to check carefully for cracks at the fitting bosses. Over tightening the pipe thread fitting can crack the aluminum casting.

another thought; the "bowl" is on the suction side of the pump. the chamber with the black sponge should always be dry and the chamber under the two valves is wet but not under pressure. the middle valve section of the pump above the bowl is pressured. i wonder if your "leak" is coming from the pressure side and not the suction side? i would carefully examine this area. after all any fuel leaks there will pond around the bowl.

I was going to post this same question about the sponge.
Since not all pumps have it, is there a downside or advantage to leaving it out?

The flat smile shaped airdome diaphragm service a few purposes beside isolating the suction and pressure side of the pump. It is meant to give into the dry chamber when the plunger comes down generating the pressure stroke. This flexing reduces the max pressure at the beginning of the stroke, and toward the end of the stroke as pressure is reaching it's lowest point returns to it's original position which smooths out the pressure pulse and extends the movement of fuel out. A trick to raise pressure on a pump with low output pressure would be to double up that diaphragm. Or reduce it's inward flex with the hard sponge.

all carter 3 valve pumps i've messed with have the sponge. i take my pumps apart and re-work them and don't put the sponge back in with no adverse effects. i say keep it if you want, remove it if you don't; hasn't made any issues one way or the other for me.

Thanks guys! Im going to wet sand it flat and see how it works.

If you are dealing with original fp from the 60s and early 70s you won't see that foam. Later version and the 6903/4862 you will, but not always.

factory 2 valve pumps don't have the chamber so a sponge couldn't be installed.

Of course. I am talking about rebuild able pumps. 4024, the 2500 series, even very early 6903. Plus the marine pumps. I have disassembled and documented many of the Carter orig pumps. Cast number, part numbers, orientation, volume, spring pressures etc...

what are the spring pressures?

4024 orig hemi = Brown spring 35lbs/in 2.0625” L x .9375”OD; Compressed length = 1.625” = 15.3lbf preload

6903 = Blue spring 21lbs/in 2 9/32” L x 15/16”OD

4862 C9 = Brown spring 30lbs/in 2 1/32” L x 31/32”OD