My completely stock 1978 F-100, 300 cid has been taking much longer to start recenty (only during cold start). So I suspected the YFA-1 carburetor. I removed the air cleaner and worked the throttle and could not hear gas squirting in the carb so I suspect a faulty accelerator pump. I removed the carb for better access. After I removed the bowl cover assembly, I removed the pump diaphragm to see if it was bad...no tares so I put it back together per my Ford shop manual and an old YFA carter rebuild kit instruction. While the carb was still on my work bench and with gas in the bowl, I worked the throttle and the pump does not squirt gas. Now if I press on the pump diaphram shaft and release, it will squirt gas. Am I missing something here? Everything seems to be connected correctly, even the pump connector from the throttle to the pump lifter link. I would take a picture of my carb, but I have no camera. Maybe someone can send me a picture of their carter YFA-1 with the bowl cover off. Any suggestions?

 

Here's a shot of my old one I keep in the shop. The acclerator pump would not contribute to a starting problem. To diagnose a hard-start issue, I would first look at the choke.

 

Thanks for the nice picture fmc400. Your carb looks like mine from the top, but do you have another pic with a side/angled view of the accelerator pump assembly? The problem I seem to have is that there is not enough movement of the pump shaft when I work the throttle from full closed to full open to actuate the accelerator pump. In other words, the pump lifter link moves but it doesn't contact the pump return spring retainer and adjustment screw plate in order to move or activate the pump diaphram to squirt fuel down the carburetor throat. Doesn't the engine need a couple of squirts of fuel for a cold start...even with a properly adjusted choke?

 

I have found the following website helpful when working on Carter YF(A) carburetors. Scroll down to the "Ford Service Manual" section. It is for a Carter YF, but the accelerator pump systems between the two generations are relatively similar.

http://www.tocmp.com/manuals/Carbs/Carter/YF/index.htm

This question has come up before. I remember my pump not moving much either when I worked it manually, but it never seemed to have a problem with fuel when I had it installed on the truck (I'm running a 4-barrel now, so my old carburetor is stored in the shop). I remember someone mentioning to me at one point that engine vacuum is involved. I am not sure if this is the case, or not.

Looking at my picture, you can see the small discharge hose that runs to the throttle bore. Fuel travels through this hose, pushes past the check ball, and out the discharge nozzle. The fuel then sprays against the venturi. The check ball acts as a one-way valve. It prevents air from being drawn in through the pump discharge nozzle when the accelerator pump returns to rest. The weight simply keeps the ball pressed onto its seat. Based on this, it would make sense to me that fuel is dicharged by the pump rod being lifted, hence the name "lifter link" for the piece that runs up through the middle of the throttle body. Unfortunately though, I am not 100% confident in my explanation, this is just a guess. I'd really like to spend some time studying this carburetor more. I'd be interested to hear any of your input as well.