Several kinds of carburetors I have had to stop rebuilding. Not by choice. There are three problems.

1  The first problem is that Suzuki and Yamaha long ago ran out of critical internal metering parts for their carburetors. The Chinese aftermarket replacement stuff--float valves and needle jets and jet needles--are famously bad; I won't use them. So there you are. Unfortunate, but reality. Thus no more rebuilds.

2  Then there are many Honda carb models, most notably all the 1970s SOHC inline fours, but also certain V-Four and CBR models. Honda has finally reached the same place Suzuki and Yamaha have been for the past twenty years. No float valves. No needle jets and jet needles. They're gone. Therefore, no rebuilds.

3  The third part of this carburetor crisis isn't parts, it's people. We are now at a point in history where owners of the aforementioned motorcycles, and here I will add the GL1000, perhaps the poster boy for this aspect of the issue, just can't accept that their bike's engines have ignition systems that require diligent and frequent maintenance. They won't listen, they won't believe, they are living in the wrong place in time, and there is no fixing this. Ergo, no more rebuilds.

Recommended reading:
GL1000 ignition anamolies