MotorCraft 2150 carb. I noticed hesitation, in some cases almost to the point of stalling the engine, when stepping on the gas pulling away when the light turns green. There does seem to be something wrong with the accelerator pump or the mechanism that works the accelerate pump arm. Isn’t the accelerator pump arm supposed to be out, and only pulls in when you step on the gas ? For some reason that arm is pulled in all the time. It never goes out.
The configuration on my carb is the accel pump arm is connected to the throttle via a metal rod; but it’s not a direct connection, the throttle end of that rod doesn’t connect directly to the throttle arm (the thing on the carb that moves when you step on the accelerator pedal, and which is connected to the throttle plate shaft). Instead there’s a spring that goes around the throttle plate shaft, wedged between the throttle arm and the carb, and that spring is connected to its own arm, which the accelerator pump rod connects to.
So the problem seems to be when I step on the gas, the throttle arm is moving like it should, but the arm the accelerator pump is linked to doesn’t move. So I get no accelerator pump action. I don’t see how moving the throttle arm would move the spring’ed arm in the first place. But it must have moved before, as I had no acceleration problem before. If it is by friction, I don’t see anyway to adjust the amount of friction. I’m sort of thinking the problem is there’s a broken spring in the accelerator pump, and that spring is supposed to return the accelerate pump arm to the “out” position.
What do you think? Anyone seen this configuration before? I can’t decide if it is the accelerator pump that is broken, or the spring mechanism over by the throttle shaft is broken.
That’s a likely cause @Tester. But the carburetor seems so simple to me after rebuilding so many that it I just can’t get my head around someone being confused about it. I would let that make me feel superior but I recall going to a funeral with a new cell phone and not knowing how to turn off the ringer and a 10 year old next to me figured it out and shut it off in less than 5 seconds. Talk about feeling stupid.
Exploded view of figure 2, that’s the MC 2150. That’s a pretty good diagram of the situation. I’ll try to use the terminology on that diagram.
The problem is that the arm on the accel pump cover is always “in” (or pulled to the right on the diagram.) And I don’t understand why. From what I gather, the pump squirts gas when that arm moves from the “out” position to the “in” position. (Left to Right on the diagram). That “out” to “in” movement pushes on the plunger on the diaphragm, against the return spring, and pushes gas through the elastomer valve into the carb.
When I press on the gas pedal the throttle shaft lever moves backwards (clockwise in that diagram) . So that action should pull the arm on the accel pump cover “in” and squirt some gas. But it doesn’t, because the arm is already “in”.
The other complication is that the arm on the accel pump cover isn’t directly connected to the throttle shaft lever ass’y. Instead it is connected to an extension on the accel pump overtravel spring. So I don’t see the mechanism of how pressing on the gas would pull the accel pump arm “in” , even if it wasn’t already “in”.
I’ll try @Tester 's suggestion, take the accel pump cover off, see if that return spring is broken.
If the diaphragm is ruptured or leaking, gas leaks out from the pump cover…The problem probably lies in how the pump over-ride lever and pump over-ride spring are operating or not operating…Is the spring broken or the lever jammed…?
Well a rebuild kit for it costs about nothing. You don’t have to use every part in it, just the accelerating pump diaphragm and the all important return spring. Another thing is to see if anything is just rusted in place. Anybody can fix this problem. The great thing is you can just remove the part first and look at it. Good advice from the other posters.
you can buy the accelerator pump diaphram separately and it easy to change, just be careful because the spring will go flying when you remove the pump cover
I haven’t had time yet to do the surgery suggested. One reason I’m not so gung ho to take it apart is last time I replaced it, the threads were starting to strip inside the casing where the 4 small bolts go in. The casing is aluminum I think. I believe I had to use at least one over-sized bolt to make it hold until the sealent set.
There’s no gas leakage from the accel pump area. Nothing on the outside anyway. That’s the part that’s confusing. I’ve replaced that diaphragm three or four times over the 40 year life of the truck. I have this fuzzy recollection of thinking – since when I’m driving the truck I’m seldom in much of a hurry – that I could improve mpg by preventing the accelerator pump from working … hmm … I wonder if I actually did that last time I did the replacement? … lol …
You could tap the stripped holes out to the next size up and bore the holes out in the cover to allow for the larger screws.
The carburetor body is probably made of pot metal, not aluminum, so any tapping of stripped holes should be done slowly and with care to avoid cracking the casting.
If he is or has thought about disabling the accelerator pump to get better mileage he is not going to spend $200 on a carb. I don’t know about his truck bt they used to be adjusted by putting the linkage in different holes.
An interesting tidbit: my '90 Toyota P/U had a 2-bbl carb with an intentionally undersized accel pump to improve emissions and mileage. For drivability, they added a vacuum operated aux. accel pump that used a vacuum switch to turn off the vacuum once the engine was warm. that small accel pump was just big enough to handle moderate acceleration, but bogged down if you really romped on it. But, since I was getting 29 mpg out of it and had decent power, I lived with it.
The early Motorcraft 2bbl carburetors had an adjustable stroke on the accelerator pump. On trucks with manual transmissions it was best to run at the richest position.
I don’t know about his truck bt they used to be adjusted by putting the linkage in different holes.