Club Cobra - Weber 48IDA Accelerator Pump/Discharge Valve Sizing

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Weber 48IDA Accelerator Pump/Discharge Valve Sizing

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.

Which accelerator pump nozzles do you have, push in or screw in?

They are push-in type. Stamped "50". There is a zero bypass valve at the bottom of the float chamber.

Was thinking of changing the pump bypass valve to a 50 and seeing if that makes any difference. Does that sound reasonable?

Adding a 50 exhaust discharge from a 00, is like adding a third pump nozzle, and so 33% will go back to the bowl at a heavy stab. At very slow applications, generally a lot less gets to the nozzles.
I wouldn't jump from 00 to 50 in one hit. I would start at 35.
If you buy steel ball valves, you can either drill the 35s to 40,45,50 etc, or solder them closed if you make a tuning mistake.
Once you find your happy spot, you can replace them with your new ones of your tuned size.

You can also try pulling out the brass tab in the return valves you have now and remove the check ball, this would give you some idea if the direction you are going is correct or not.

Any accelerator pump has 2 one way valves as either a check ball or silicon diaphragm.
One check valve allows fuel into the pump, but not back, and one allows fuel into the carburettor throat.
If either one is faulty, the pump generally doesn't work too well.
In the case of Weber, we have the ability to tune the VOLUME with the exhaust valve orifice.

Another point for tuning is the type of ball in the inlet valve.
Nylon balls seat much faster than steel.
Slow throttle application can still have fuel go back to the bowl BEFORE the fuel lifts the ball, especially with a steel ball.
Gary