I recently bought an 84 F150 with a rebuilt 351W in it. I believe the carb is a stock Motorcraft 2150. The engine sputters and tries to die under medium acceleration, but will accelerate slowly at light throttle and will also accelerate OK if I floor it but still seems sluggish. Engine idles smooth and seems to run OK otherwise.

This sounded like a failed accelerator pump to me, so I got a rebuild kit and replaced the accelerator pump diaphragm and the rubber check valve behind it. This has had no effect whatsoever, engine still stumbles when trying to accelerate with medium throttle pressure.

Can someone explain exactly where the gas should squirt out when the accelerator pump operates? I can't really tell from the pictures I've found so far. The engine does not react if I push the accelerator pump's piston in while it's sitting there idling, but I don't know if it's supposed to do anything at idle or not.

I'm also wondering if there is gunk in the passageway that the accelerator pump should be squirting the gas through. Maybe running some Gumout or STP through it would clean it out?

Anything else to check? Maybe a vacuum problem?

Thanks,
Chuck

 

If it was me I would rebuild the carb,but if you look down into the carb w/choke open you should see the gas squirting out when you open the throttle.Your supposed to change the filter every 15`000mi

 

Pretty sure there is a little fuel filter right in the base of the carb. Sounds like it might be plugged and thats where I would start. At idle and light throttle its getting enough fuel but anything past that its starving. Just a guess

 

I think the accelerator pump passageway must be blocked, or the check valve malfunctioning. There is a zinc alloy piece with two small nozzles that squirt fuel at the booster rings in the carb throat. If you remove the air horn of the carb (top casting) you will be able to remove this. Under it, in the accelerator pump passageway, there is a short piece of steel rod and under this a steel ball. One or both of these are probably stuck, preventing the fuel from getting through.

Check for gasoline squirting with the engine off, it's easier to see.

 

OK, I finally had some time to pull the air horn off today and see what's going on. I do see gas squirting out of the nozzles when I work the accelerator pump by hand, also if I work the throttle by hand. I did pull the ball and weight out of the accelerator pump passage with a magnet. They look clean and did not seem to be stuck. I put everything back together and it's still behaving the same. It will accelerate slowly at light throttle. Give it more and it tries to die, but push through that (nearly to the floor) and then it will pick up and pulls hard like it's getting plenty of gas at that point. I have also noticed that if I'm rolling along and let off the gas, it will sputter again coming down through the middle of the throttle range. So now I'm wondering if it's not starving at medium throttle but actually flooding. Any idea what could cause the mixture to be out of whack in the middle but OK at either end?

 

Perhaps clogged transfer slots? They are little slots that are exposed when the throttle is opened just a tad, like just off idle.

Try removing the idle adjustment screws completely and spraying in carb cleaner with the straw stuck in there as far as it will go. This sometimes forces junk through.

 

I took the carb off earlier this week and did a complete rebuild on it. Put it back on this afternoon and now it's running like a champ.

 

It would be a good idea to read the spark plugs after a couple of hundred miles and start keeping a maintenance log.