Help me understand how a diaphragm carburetor works

[Machine_Elf]

@Francis_Vaughan

I think the missing element is that the large volume containing fuel under the metering needle valve is compliant in some way. The fact it is drawn as a significant volume rather than just a simple pipe is a clue toward this. So it stores pressure as fuel is pumped into it. It may be as simple as a tube made of rubber or flexible plastic, or could be constructed with a plunger and spring at one end. Probably endless possibilities.
But if there is some springy compliance in the volume, the system should work.

I think you’ve hit it with this. Take a look at this video, cued to 2:33 where he starts taking the carb apart:

2 Stroke Carburetor - In depth Fault Finding, Testing, Service & Repair

He starts by taking the cover off of the pump side of the carb. At 2:53 the cover comes off, and the area of interest is the exposed part of the diaphragm nearest his ring finger. That’s covering the bottom of the chamber just upstream of the metering valve (the round pocket nearest his ring finger at 3:00). At 26.33, he shows the inside surface of the cover that clamps the pumping diaphragm to the carb body. The rectangularish depression closest to his pinky finger ends up directly over the chamber upstream of the metering valve. So yes, there is a diaphragm there that’s providing compliance, allowing the pumping diaphragm to squeeze in a bit of extra fuel during the piston’s downstroke, and pushing the fuel past the metering valve when venturi vacuum pulls down on the metering diaphragm.

Looking at this some more, it’s not so much the large overall volume of that chamber that matters - rather, it’s the large cross-sectional area, which facilitates the use of a large diaphragm that can store a substantial volume of fuel under pressure within the volume of its own deformation.

So it looks like that’s the missing element in the video I linked to in my OP: a useful degree of springy compliance in the chamber just upstream of the metering valve, allowing it to behave like a hydraulic accumulator.