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.

What if you put a line that will allow the bowl to drain back to the tank. Install a solenoid that will block drainage when the key is on.

Would make for a more automatic way to do the same thing.

I can just burn it off by turning the fuel pump off and driving off that last little bit before parking.
Its only about 100ml of fuel. I think that will only power this monster for about 1/4 mile.
I just don't think there is any way I can get the all down hill slope needed to return the fuel to the tank by gravity.
Driving home I could cut the fuel off, run completely out and just roll into my drive way.
Going to work and the store not so much an option, I could burn off a good portion of it.

Most carbs with accelerator pumps will provide fuel to the accelerator pump until there is nothing left in the float bowl. The idle circuit will also pull fuel until the bowl is empty.

The main circuit relies on the fuel level in the float chamber to operate properly.

The previous statement about float bowl fuel level changing is not correct. the fuel level remains very constant in the float bowl, probably within 1 millimeter since the float level controls the activation of the needle valve that refills the float chamber. That is a very precise control mechanism and it needs to be for the main jet to consistently supply a precise mixture.

You can argue the precision of the mixture all you want, but consider this, fuel economy did not change much when delivery transitioned to fuel injection and Honda's CVCC designs are still top dogs in economy and in their time period were good enough to delay the adoption of catalytic converters.

Precision measurement of the actual amount of fuel in the float chamber, followed by letting the engine idle with the fuel pump shut off will give you a very good idea of the idle consumption, which in most V8s is about .5 gph.

Once you have converted that consumption into a time period that the engine will idle on the fuel in the float chamber and you know the volume of the chamber, you will have that time recorded and can calculate other scenarios using time (to empty float chamber) to calculate the consumption based on the idle consumption time. Half the time twice the fuel and any point above or below the idle consumption time period can be easily calculated as a fraction of X/idle fuel consumption time.

regards
mech

The greatest change in fuel level in the float chamber is when you mash the throttle and use the fuel capacity of the accelerator pump. My guess is it would be less than 10% of the capacity of the float chamber, but the replacement of the fuel by the needle would be almost instantaneous.

regards
mech

As long as you can pressurize a separate container, like a burette, you can precisely measure the amount of fuel consumed.

A small hand pump to pressurize the burette will provide the same fuel pressure as long as you maintain the pressure within specs.

Fill the burette, pump the pressure up to the maximum specs recommended and maintain the pressure with the hand pump. This way you can test anything you want without dealing with float chamber variations that would affect the main circuit operation.

Want a lower float chamber level for lean burn? Let the pump pressure in your burette drop off below the minimum spec and the fuel level in the chamber will drop and you can control that very precisely.

As far as the evaporation you saw when rejetting, ASSuming it was a hot engine, most of the fuel evaporation was due to the heat produced by the engine. Try it over a hot engine, then a cold engine to confirm my statement.

regards
mech

Yes that is what I was thinking.
For testing time or distance with the fuel in the bowl I would hold the gas in the same position as to not add more fuel with the accelerator pump.

Then for fuel burn off I figure I would be pushing the petal down a little here and there just driving normally and maybe more as it ran out of fuel, draining the accelerator cylinder. I figure it would get 50% to 90% of the gas out of the bowls. And therefor cut evap 50% to 90%.

When the carb starts to heat up it starts off gassing through all the little vents, I can see the refraction of light through the gas, looks like a little propane leak.