VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc Let…s talk fuel pumps.

I was at a buddy’s house the other day and he had just gotten his new dual action fuel pump delivered. We took it out of the box to look at it and as I remember it, you could push the arm of the pump in by hand and hear the diaphragm pumping air and they had quite a bit of movemet. This pump arm moves about an 1/16" after the slack is taken up, the lift on the cam has to be more than that. Even when I leaned it against the edge of the work bench and pressed with my body behind it, it still only moves a small amount.
When I got home I took the dual action that I got form Chevy Duty last year off the shelf and it is the same way, the arm only moves a small amount.
I don’t have any single action pumps around to compare them to but they have to have more movement to do any sort of pumping I’m sure.
Anybody run into this before?
Denny Graham
Sandwich, IL

Funny you bring that up Denny, I was thinking about that a month ago when I was rebuilding my engine. I put the 216 fuel pump on my 235, because I didn't like the way that the 235 fuel pump felt. It was the same deal. I did however, put my 235 fuel pump on my bench with one end in a bucket of water, and see if I could get it to draw up some water. Sure enough, it worked just fine. From what my understanding is of it, it only needs so much movement to function, I would suppose they factored in cam lobe wear, and decided that to make the fuel pump function longer (Which we know they concidered the operators expenses at that time) It didn't matter if I pushed it up to that "Limit" or if I pushed it a bit past. It seemed to deliver the same.

In normal operation the diaphram hardly moves due to surface area and amount of fuel used between strokes.
Tony

Are you suggesting that the diaphram only moves "On Demand"
Wouldn't the float needle tension have to be equal or greater than the diaphram force, in order to prevent flooding out the carb bowl?

Its normal for the double pump lever to be almost impossible to move. Note how much thicker the casting is where the pump bolts to the block. Does this indicate something? Like the casting had to be stronger to support the greater effort required.

Denny.....don't try to fix it if it ain't broke!

Actually the diaphragm type pumps do only pump on demand. They have springs that transmit the movement to the diaphragm. When the pressure in the discharge exceeds the spring pressure then the fuel diaphragm does not move. Only when the fuel discharge pressure drops (when the carb needs more fuel) does the pump actually pump more gas. If it did not work that way it would either pump until something broke or you would need a pressure relief valve and line to return gas to the tank. The latter is how modern fuel systems work.

Ok guys, I eat a big lunch today and leaned my big old belly into the pump with the arm against the work bench and with about a hunderd pounds of force the arm moves about 1/4" to 3/8". Neve saw an AC type mechanical fuel pump that you couldn't operate with just hand pressure.
This thing has to be real hard on cams and you can be sure that when this one goes into my engine it's gonna start life with a healthy slab of high-pressure grease on the rubbing block!
Now all I have to do is find the plumbing schematic and I can install one of them in my buddy's 216. Mine is gonna have to wait till I get the engine back since I decided to go all the way and have the short block completely rebuilt this fall.
Denny Graham
Sandwich, IL

If I remember correctly the arm for the double pump is wider that for the single pump. This will give more surface area. At any rate the pressure on the cam will be much less than the pressure the lifter has on the camshaft.

Yeah, spose your right Gene.The spring pressure in a powerglide engine can be over 150lbs. Without going into calculations I would guess that would put the pressure on the lifter at around 200lbs. Never really thought about it much before.
Well, that settles that debate, time to move on.
Thanks guys.
Denny Graham
Sandwich, IL