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Oh the carbs are off the bike, and completely disassembled on the workbench. I'll probably just make a jig to hold each individual carb and keep them level as I check them. Sounds messy, I guess I should do this step before I clean and paint the carbs. Thanks for the info.

Remember - do it outside and no smoking! Don't want to burn down the house. Ed

Instead of gasoline, may use mineral spirits, a liquid of similar density, and odorless is available.
May also use common household rubbing alcohol after determining target height in a known correctly adjusted carb.

May use only one screw to attach the float bowl for testing the level following a tang adjustment, which saves time and doesn't affect the test.

Some carbs such as Mikuni 26mm manual slide carbs on KZ900 are easily held level on the workbench (or kitchen table) without needing to use a simulated engine attachment device.

Good Fortune!