Weber 48IDA Accelerator Pump/Discharge Valve Sizing

[Pasha Saleh]

I am slowly dialing in a set of 48IDA's having changed choke sizes, main jets, float heights, etc. It is 95% improved from where I started and one of the only lingering issues (other than the 6.5mpg fuel economy!) is a stutter/stumble on acceleration through about 3,500 rpm that feels more rich than lean. I suspect too much fuel from the accelerator pumps (or perhaps too much duration or both) because the car will also flood from a cold start given a single shot of throttle priming. The flow when looking down the venture and twisting the throttles is very strong from the squirters (haven't measured the quantity) and I thought I would experiment by changing the sizing of the discharge valve.

What's in there right now is a size "50" accelerator pump (squirter) and a "zero" discharge valve, meaning none of the accelerator pump flow is being discharged back into the float bowl and all of it is coming out the squirters. Weber makes these discharge valves in size 00, .35, .40, .50 -- all the way up to 1.0. Other than getting a bunch of them and trying them one-by-one, any recommendations on where to start? It's a pain to pull them out each time as the carb tops have to come off, floats removed, etc.

 

[Mike Pass]

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

 

[Mike Pass]

Keep in mind that these are all relative numbers based on the fictional scenario where the pump piston moves 1 ml of fuel and a size 50 jet will flow 1 ml/second. The actual values aren’t as important as the relationships of the numbers. The bottom line is this – the pump jet determined the rate at which the fuel is delivered, and the pump exhaust valve determines the duration of the pump shot.



Tuning this circuit can be a real challenge. Part of the reason has to do with the method of fuel delivery. All of the other methods involve at least some mechanism to help the fuel vaporize. Remember that liquid fuel does not burn, only gasoline vapor does. The idle and progression circuits use turbulence and emulsification to help the fuel turn to vapor before it reaches the combustion chamber. The pump circuit uses nothing. It shoots a squirtgun-like stream of fuel down the middle of the carb throat. If the engine is cold, much of the fuel will go unvaporized and therefore unburned. Even in a warm engine, much of the pump shot may go unburned because of lack of vaporization. It may seem counterintuitive, but too much fuel in the pump shot can actually result in a lean condition. That’s because liquid fuel does not contribute to the air-too-fuel ratio since it cannot burn. A smaller pump jet will produce a smaller stream of smaller droplets that will more readily vaporize in the fraction of second available before the fuel enters the combustion chamber. As a result, you could start out with what looks like a lean stumble in the range that is solved by going to a smaller jet.



Another influencing factor is the rate at which you roll into the throttle. If you snap the throttle open, you quickly close the pump exhaust valve. On the other hand, if you roll into the throttle slowly, the valve won’t close at all and the pump shot will be recirculated back to the fuel bowl. Of course, the more slowly you roll into the throttle, the more orderly the transition from the idle circuit to the mains, and therefore the less need there is for something to fill the gap. The goal, then, is to tune the pump circuit such that it is there when needed, but not there when it’s not needed.



Yet another factor to consider is the engine load when you stomp the go-pedal. If you’re in a low gear, the engine will rev very quickly, which will quickly start the air moving in the carb throats, thereby engaging the main circuit fairly quickly. In the other hand, if you romp on it in high gear, the engine will rev much more slowly, requiring a longer pump shot to tide things over until the mains kick in. Unfortunately, the pump circuit has no idea what the engine load is, and it will happily provide the identical shot in each case.



As you can probably surmise, is not possible to tune the pump circuit to be perfect under all conditions. This region requires compromises. Fortunately, the time an engine spends in this region is very short, so the engine is quite tolerant of the fuel mixture being wrong for a short time.



If you have an oxygen sensor, hook it up and take the car for a ride. It is best to pick a repeatable condition under which you’ll test the pump circuit. I like to use a flat stretch of road in 2nd gear. Pick a starting speed that puts the revs somewhere around 2000 RPM, and then stomp on it. Pay close attention to what the engine does, and to the AFRs if you have the ability. With a perfectly tuned pump circuit, the engine’s response to the throttle should be almost instantaneous and there should be no stumbling or hesitation. If the engine instantly flops or hesitates, you should probably start with the pump jet. If you have an oxygen sensor, it will give you a hint as to which way you need to go (though remember that oxygen sensors can lie). If you don’t have an oxygen sensor, pay attention to things like popping. If you hear popping through the exhaust, it’s a hint that you are too rich. If you hear popping through the carbs, you may be too lean. Replace the pump jet with the next size rich or lean, and then repeat the exercise. Remember, we are interested in that instant immediately following the pedal going to the floor. Keep experimenting with pump jets until you get the engine to instantly pick up when the accelerator is floored. Don’t worry at this point whether some stumbling follows. We just want the engine to instantly jump when romped.



Once you have a pump jet that causes the engine to instantly respond, you want to make sure that the shot duration is right. I find that the best way to do this is under the same conditions as above, but in 3rd gear instead of 2nd. This will place the engine under load for longer and will therefore test the pump exhaust valve more thoroughly. Ideally, the engine should still respond instantly when the throttle is pressed sharply, though it may stumble after the first half second or so of response. If you don’t get an instantaneous engine response – even a short-lived one – then you’re not done with the pump jet experimentation. Here again, an oxygen sensor can give you a hint which way you need to go. The idea is to build on the instant response you tuned above and make it last long enough for the mains to take over. So, if there is a stumble after the initial response, it is because the pump shot is either too short or too long. Once again, your ears may give you a clue if you don’t have an oxygen sensor. If you decide, or guess, that the pump shot is not long enough, try a smaller pump exhaust valve, and vice versa.

 

[Mike Pass]

Once you feel like you have the 3rd gear conditions properly tuned, try 2nd gear again and make sure it still works well. This is often an iterative process as you find a combination that works for some conditions and not others. In any case, I recommend changing the pump jet first, and then modifying it by trying different pump exhaust jets as needed.

Cheers
Mike

 

[Chris Kouba]

Pash- You may want to resolve your temp issue before digging heavily into this. If you're too cold, you may be trying to burn unvaporized fuel, which may feel like you're running rich as you're experiencing. I'd figure how to get your motor up to temp before ripping through carbs and such.

The temp issue should be fairly easily (if not necessarily conveniently) resolved. This seems like a massive bucket of worms (which also happens to be impacted by engine temp).

 

[Pasha Saleh]

Great point and well-heeded, Chris! I will do that for sure.

 

[Pasha Saleh]

Mike, thank you for that gem of information! That is exactly the sort of explanation I had been searching for unsuccessfully to demystify yet another facet of the IDA. It really is a miracle that someone came up with this device and so unlikely that it actually works! I called Pierce Manifolds and they told me their 48IDA V8 kits are equipped with size 50 pump jets and .50 exhaust valves. Mine had 50 pump jets and 00 exhaust valves so I ended up ordering their .50 exhaust valves to baseline it from what an experienced and reputable Weber outfit recommends as a starting point.