Fuel in the bowl
 

Converting over to electric fuel pump on the carb should allow me be driving along, cut the fuel pump off and do a time/speed comparison from the time the fuel is switched off until the engine sputters indicating it has consumed the 200 or so ml of fuel in the bowls.
I don't know how much volume the fuel bowl holds but I can find out next time I pull the carb. It just needs to be consistently full when I turn it off.
The way the fuel floats are it should give a very stable fuel level in the bowl.

I could also use it to cut evaporative emissions. Once I learn how far it goes on a bowl full of fuel I can turn off the fuel, keep driving that last little bit home and could pull into the drive way with an empty or almost carb.
Usually the engine heat conducts up into to the fuel and off gasses the lighter components of gasoline to the surrounding environment.
It could make cold weather starting easier too, when I start it flip the fuel pump on several seconds before cranking and get a bowl full of nice fresh volatile gasoline.

Has anyone tried this?

Haven't tried it, just thinking about potential pitfalls... If your carb has an electronic mixture valve / jet, running the bowl out might screw with your fuel trim for back to back testing

It a very non-electronic 600cfm edelbrock performer.
Electronics are external, such as fuel pump, wide band O2 meter, HEI distributor.

Now I'm thinking you should be figuring out how to use your O2 output to cycle the fuel pump on and off #oldskoolleanburn

I thought the lower the level of fuel in the bowl the leaner you ran? I'm no carb expert though!

Evaporative emissions, yes, but also just plain wasted fuel. That's a mod that'll add up fast- every time you park it.

I believe this will be the case. As long as its consistent it will work.

I just want to use it to compare various conditions, cold engine versus hot, higher and lower speeds, trailer versus trailerless, intake air at various temperatures, different levels of ignition timing, aero mods.

If it leans out too much too fast that will pretty much make it useless for testing different levels of ignition advance, but also might allow me to find a lean burn sweet spot.

Yeah if nothing else I will have a novel way to reduce evap, make it less stinky and get it to start faster making the battery and starters life easier.

Using the electric fuel pump is kind of must, that way I can relocate the fuel line away from the engine and remove the possibility of vapor lock. Which is a strong possibility here at higher elevation and also because its normal to see 80'F+ while winter gas is still in production.

I think using an electric fuel pump, turning it off, running the fuel out of the carb bowl would be a very good idea.
While changing out jets today about 10% of the fuel evaporated right before my eyes, when I turned the engine off heat flowed up the intake runners to the carb its self and heated up the fuel pretty good.
I am thinking the fuel bowls holds some where around 100 to 120ml of fuel, not 200 like I was thinking. Next time I pull one of these carbs and dump the fuel (well I save it, remember I am too cheap to dump perfectly good fuel on the ground). Or when I install the electric pump and run it out of fuel before a jet change I will manually fill it back up and measure the fuel that goes in it to that normal level.
Every time I pulled the top of the carb off to change jets the level was at exactly the same place, so that is good.

Interesting idea. Couple of thoughts. The fuel bowl has a high level and low level and will not always contain the same amount of fuel, the difference may be a trivial amount. Also will the pressure in the fuel line allow the bowl to partial refill even after the pump is shut off. These concerns are only if you are tiring to produce a amount of fuel used. If you go with your original idea of time and you throw out the top 20% and the bottom 20% of the results for each test you should have a good idea which ones work.

As for wasting fuel if you kill the pump while still moving and roll to a stop I see no additional waste of fuel.