Join Date: Dec 2001
Location: Belleville, Illinois
Thanked 22 Times in 17 Posts
Re: Hesitation on acceleration
Actually, if the pump is working properly, any opening movement of the throttle will squirt in a proportional amount of extra gas. When I was in school close to 40 years ago, they taught us that the pump is needed because air, being less dense than gasoline, will accelerate faster when the throttle opens - the air velocity will increase faster than the fuel velocity, so the pump makes up the difference. If everything is working properly, opening the throttle will not cause a momentary rich condition.
Anyway, because there's a mechanical connection between the throttle and pump, they must move in unison, no matter what speed. If the change in the throttle is slow and small enough, the pump might not be needed, but you're going to get it anyway.
There are other things that will cause a stumble. For example, after the throttle is about 3/4 or more open, the mid-range circuit is pretty much out of the picture, as it works off the difference in pressure above and below the throttle plate. But, at low engine speed, there is not enough airflow through the carb to activate the power circuit, which is dependent on relatively high venturi vacuum. In that case, you can get an initial surge of power, followed by stumbles until the engine speed fights its way up.
And, as someone else pointed out, incorrect timing can also cause the stumbles. The loss of vacuum advance from the throttle opening, before engine speed is high enough to activate the centrifugal advance, can result in timing too retarded for efficient operation. In that case, the engine will stumble immediately and continuously until engine speed comes up.
It could also be that the vacuum advance is attached to manifold vacuum, when it should be on venturi vacuum. I guess that the reverse could also cause a problem.