Accelerator Pump
-Written by Steve Phillips for Hot VWs magazine-
Eventually, it will start pulling fuel and the RPMs will go up and as they go up it will pull more and more vacuum, then things will start to be happy. As you can see, we need RPMs to create vacuum, but we need the fuel to create that RPM and it is controlled by vacuum. In other words, you need the chicken to lay the egg, yet the egg makes the chicken. This is where the accelerator pump comes in. As you open the butterfly, the pump squirts fuel to give the motor RPMs which in turns creates the vacuum to pull the fuel. You can also see why it must be a mechanical thing and not a vacuum controlled circuit. “So, what does that have to do with gas mileage?” Again, it’s the only circuit that is mechanical and that dumps raw fuel. Raw fuel doesn’t like to burn. A mixture of air and fuel is what burns efficiently. As I explained before, we cannot take this circuit out. We must have it. The trick is making it efficient.
To make things more efficient, first, look at a stock carb. I’m going to explain how I take a look and modify things as if the carb is off the engine. It also can be done with the carb on the engine. There will need to be gas in the bowl of the carb so don’t drain it. First thing I do is pull down on the throttle arm. I look at the squirt of gas coming out of the accelerator pump squirter. From the factory, it usually will hit the middle of the butterfly. I don’t like that at all. I rotate the squirter, so the stream of fuel goes down the throat as soon as I open the butterfly. In other words, move the squirter towards you. Keep pulling on the accelerator arm until you get the squirt as close to the side of the throat without touching the side. What we are trying to do is make it so that squirt goes down the manifold as soon as possible without hitting anything. This will get the fuel to the heads faster causing the RPMs to go up and creating that vacuum we need to get the other circuits to start doing their thing. Once we have that, it’s time to start driving and adjusting how much squirt. Look at your accelerator pump linkage. Are you able to adjust it? Some early carbs don’t have an adjustment (they were made when gas was cheap). If you have the ability for adjustments, follow these next few steps. Take the car for a drive. Is there hesitation? If there is, you will need more pump. Adjust the pump to give you more fuel. If no hesitation, back off the pump. Go drive it again. Keep adjusting the pump linkage until you pick up a hesitation. Then, adjust it back to where the hesitation went away. This is as good as it will get. You’re done! (I am assuming your jets in your carbs are correct). For those of you running IDF(s) style carbs or even most aftermarket carbs, the accelerator squirters are not movable. So, they are what they are. What I do is find a socket that fits the nut on the accelerator pump linkage and scribe or paint a line on it. Then, back off the pumps a full turn and go drive it. Keep doing that until you pick up that hesitation, then go back in a half a turn.
Ever wonder why your gas mileage is better on the freeway verses in town? A lot of that is due to the accelerator pump. We need the pump to get the RPMs up to get the car moving. This circuit is not efficient because it’s dumping raw fuel. We can’t live without it, but we can limit it. By limiting it we are not wasting fuel. By not wasting it, you will get better mileage and better mileage means you can take the girl friend out to dinner.