Here is an extract from some info I got from one of the Cobra forums a while back. It may help to sort the problem you have with excess pump shot. A bit long so will post in a few sections
When you press the accelerator sharply, there is a mechanical spray of fuel to fill the gap until the engine catches up with enough air flow to get the Bernoulli principle going. The system is also designed such that if you slowly roll into the throttle, little or no fuel is sprayed by the pump jet. As you might imagine, this range is quite tunable as well. There are two primary tuning components – the pump jet and the pump bypass exhaust valve. The pump jet is just the spray nozzle that does the spraying. A large pump jet will spray a lot of gas over a short period of time. Conversely, a small jet will spray a smaller amount over a longer period of time. If that weren’t enough, there is also the pump exhaust valve. This is a relief valve of sorts that is plumbed in parallel with the pump jet. When the piston applies the pumping force, some of the gas will go through the jet and get sprayed, but some will be “wasted” through the exhaust valve and recirculated to the float bowl. This helps fine tune the pump spray.
Above is a pump exhaust valve (part # 11 in the diagram). Inside the valve is a steel ball bearing, and at the top and bottom of the valve is an opening. The valve sits in the float bowl just under the float tang. When the pump is not operating, the ball is at the bottom of the valve, allowing fuel to enter the piston chamber. When the accelerator is pressed very slowly, the piston does not offer enough pressure to lift the ball up into the closed position, and all of the gas will flow past the ball back into the bowl. On the other hand, when the accelerator is pressed sharply, the piston depresses quickly, forcing enough fuel into the valve to cause the ball to rise and cut off flow into the bowl, sealing off the escape route and thereby directing flow to the pump jet (#10 in the diagram). If you look at the valve closely, you will see a hole bored into the side. This is the exhaust orifice. It allows some of the fuel in the pump circuit to bleed back into the bowl, thereby reducing the overall pump shot. The valves are available with exhaust orifices in various sizes, including a valve with no orifice at all. That one directs all of the fuel to the pump jet.
The relationship between the pump jet and the pump exhaust valve is a complicated one. It may help to illustrate the relationship by looking at few hypothetical examples. For these examples, we will assume that the accelerator is pressed quickly enough to force the exhaust valve closed and that the volume of fuel produced by the piston always remains the same. This is a fair assumption since the actual piston displacement volume is not generally considered a tunable parameter, though there are ways that are beyond the scope of this discussion. Just for the sake of these hypothetical examples, we’ll assume that the piston will displace 1 ml of fuel with every pump and that it delivers that fuel at a constant pressure. That 1 ml of fuel will be delivered to the pump circuit and will be shared between the pump jet, which delivers the fuel to the engine, and the pump exhaust valve, which wastes the fuel back to the float bowl.
So, let’s look at the configuration that the IDAs have as they come from the factory. They are equipped with a size 50 pump jet (with an orifice of 0.5 mm in diameter) and a pump exhaust valve in size 00, which means that no fuel is exhausted. This means that the pump jet will deliver the full 1 ml of fuel to the engine. We’ll arbitrarily assume that this delivery takes 2 seconds for a fuel delivery rate of 0.5ml/sec. That is the fuel delivery rate of that pump jet. Now, let’s replace the pump jet with one that is twice as big. Before you reach for the size 100 jet, remember that the important aspect of a jet size is the area of the orifice. The jet size that most closely matches twice the area of the size 50 jet is a size 70 jet. Remember that area increases with the square of the diameter. So, the size 70 pump jet will flow twice the fuel as the size 50 jet. That means that the entire 1ml of fuel will be delivered in half the time, or 1 second. The fuel delivery rate for the size 70 jet is therefore 1ml/sec.
Now, let’s look at the scenario where we go back to the size 50 pump jet and replace the size 00 pump exhaust valve with a size 50 valve (which also has a 0.5 mm orifice). Now, what we have effectively done is to split the pump volume into two equal paths. The pump jet will still deliver fuel at a rate of 0.5ml/sec, but half of the total fuel will be bled back to the bowl by the exhaust valve, resulting in only 0.5 ml being delivered to the engine over a period of one second. So the net effect is that the delivery rate for the size 50 pump jet remains the same, but the duration of the shot is cut in half by the exhaust valve.
If we modify the above scenario by inserting a pump exhaust valve that is half the size of the size 50 valve (size 35 is close), then we find that ¼ of the fuel is exhausted to the bowl, leaving ¾ ml for the pump jet, which, as we have already determined, can deliver fuel at a rate of 0.5ml/sec, resulting in a pump shot that is similar to the one above, but lasts 1.5 seconds instead of 1 second.
So, as a first approximation, the pump jet determines the flow rate of the fuel delivered to the engine and the pump exhaust valve influences the duration of the shot.
This relationship can be illustrated in the table below. You can clearly see how the pump jet and pump exhaust valve are related and the effect the pump exhaust valve has on the duration of the pump shot.
Cheers
Mike