Hitachi 328 carb accelerator pump

[tractormal]

I am rebuilding my original hitachi carb on my '72 Datsun PL521. The accelerator pump bore in the carb has two different ID's The pump fits nicely in the top bore but will not go down into the lower part of the bore. I am thinking this is by design. But then the pump only moves in the top part of the bore, and only about 5/16" travel. Does this sound right? The replacement accelerator pump is diffrerent than the one that was in the carb. It has a leather plunger vs. rubber and it is not spring-loaded. And it's a different length. No doubt it is wrong. But even the "original" did not move very much in the bore. Any thoughts? Thanks

[LenRobertson]

It seems everyone is too busy preparing for Canby to answer your post. It has been awhile since I've been into the accelerator pump on a Hitachi. I'll look at one of my old carbs when I get a chance, but it may be the weekend before I do. Maybe someone else will answer your question by then. It seems like the pump stroke should be more than that, but I have no idea.

 

Len

[LenRobertson]

Finally had a chance to look at a carb. I had hoped someone else would jump on this question, but they are all at Canby, so you'll have to settle for what I know! Yes the accel pump bore is two diameters - larger above, then a short tapered section narrowing down to a smaller dia. I believe this is so the pump rubber (or leather) will easily insert into the larger bore, then the taper reduces it as it slips through into the smaller bore. I think it will take a bit of force and twisting to get it down into the smaller dia. It needs to be a good tight fit in the lower bore in order to work properly. I don't know if a little gas dripped down the bore would lubricate the pump to help get it in or not. WD40 might be fine too, but you don't want anything that might attack the rubber or leather. I didn't take measurements of the pump rubber diameter, but it seemed to be about the dia of the upper bore. If your pump rubber/leather is so big it is hard to fit into the upper bore, it may be the wrong one. The only springs on my pump are one that coils behind the pump arm pivot shaft, and hooks hooks over the pump arm, and a thin coil spring down the pump bore, under the pump plunger. It is approx the dia of the smaller pump bore with a bent section across the lower end to hold down on the 1/8" dia ball that lives in the very bottom of the pump bore. This ball can be stuck in place if gas has dried in the carb. Spray a bit of carb cleaner down the bore and poke the ball with a small screwdriver to free it. Be careful not to blow it out and lose the ball if you use compressed air on the carb.

 

The pump stroke I measured was about 3/8", surprisingly little. There are adjustment holes in the arm for minor changes to the stroke.

 

Let me know if this rambling explanation isn't clear or makes things more confusing. Wouldn't be the first time! I can measure anything on my old carb if you need it.

 

Len