I've found it also helps when replaceing the plunger to soak the leather cup in auto tranny fluid overnight to help swell it, then run a flat tip screwdriver around in between the inside of the cup and the spring to open it out a bit. This technique can also give an old hard plunger a new lease on life.

when taking off the top cover of a carb before you put it again in place you have to check somethings.

1st the gasket is good. it is the top part of the carb that needs to be filed. most of the carbs do this.

the top of the carb tends to ret round in face for about 1mm so what u need is to file the corners and check the gap by putting it over the body of the carb.

when the gap is at around half that of the thickness of the gasket. then you can have a good working carb. even problems with pump will eliminate them selves

Mine is the same. The pump doesn't work, but once the engine is up on temp, it's all good. The advantage I have is that my car has an auto, so there is much less 'off & on' the throttle & throttle opening is slower & more progressive which the engine can handle without difficulty. I did a major overhaul of the carb back when I first got the car, so the plunger was new.
Otherwise, the engine runs flawlessly, as one would quite naturally expect from a maintained Datsun.

I suspect that the bore for the accelerator pump becomes worn & the plunger doesn't seal too well any more, even when a new plunger [piston?] is used & the fuel simply bypasses the plungers 'washer' If you test the pump by pulling it up into an unworn part of the bore the washer seals & the pressure of the fuel is enough to expand the lip of the washer to maintain a seal over the full stroke.
Once returned to it's old position it's up to its old tricks again.

Well, that's my 'theory' & it matches up with the known facts, so it's entirely possible.

I replaced the old cork gasket with a new paper one, it's much thinner and wont swell up when it gets fuel on it. It needs to swell up to seal, so I took it off and made one out of good cork material I found at work. All the holes are correct and matches really good.
Didn't think about the venting.
I would think it would vent through the little rubber thingie around the top pump shaft, but maybe I've something wrong.
Thanks.
I'll go check it when it worms up outside....LOL

I haven't heard of this problem with the stock carb.

My guess is the carb top gasket is the wrong one, and it's not letting the passage breath correctly. The ball goes in first, but it needs venting to let the fuel go in no the down stroke.
Quote:Is the fuel level correct? If it is a bit low, it might cause this problem. The fuel level should be centered in the bowl glass.

OK, I've done everything I can think of, now I have to ask.
I have a stock carb on my A12 and I can't get the accelerator pump to work.
The things I've replaced;
New pump, new check valve needle, new top gasket.
Things I've checked; check ball, pump walls for scratches or pits, check ball seat for pits and corrosion.

If I take the pump arm off and pump it a few times by hand, it will work and the car runs great. It will continue to work till I let it set for awhile. I try it again later and no pump shot. I pull the air cleaner top and pump the linkage by hand, I can see there is no fuel coming out the sprayer.
If I pull the top of the carb off and pump the accel slowly I can sometimes see the fuel moving back and forth in the bowl, then after a few times it strats working.
If I hold down the needle valve so the fuel can't go through to the sprayer, the pump wont go down, that tells me the chack ball is sealing in it's seat.
I've checked and cleaned verything I can think of and it still wont hold a charge.
Is this a typiacl issue with the stock carb?
I'm at my wits end.
P.S. I've fixed all the vac leaks. It runs good and pulss fine, after ya get it moving. I even tried pushing the gas peddle down slowly and lifting it quick to help suck some fuel up into the pump chamber before I try to start it. When it does work, I car put the peddle to the floor and it revs clean, no bog or hesitation, if the pump isn't working, it wont rev without a big hesitation and sometimes dies.
So?, any suggestions?
Oh yea, good morning to all.