Engine should run at the same rpm on either cylinder if plugs are disabled on the other, IMO. Do you have access to an inductive timing light for a car? Or a low scale tachometer for auto tuning. You might be able to better judge engine speed between cylinders with one of them.
When synchronizing carbs you remove vacuum line from petcock and plug it. There is enough fuel in the carb bowls to run the engine for several minutes while you perform the sync. If needed, you could fashion a vacuum supply for the petcock with a large syringe from a vet applied to the hose too. There are directions here someplace on how to do the sync, but I`m on my way out for the day and don`t have time to look for them right now.
Realize that the vacuum to the petcock is static or non moving once the vacuum is applied to the diaphragm. It does not *flow* in volume like the air in a vacuum cleaner hose for example. It is more like a rubber sucker stuck to a frig door or a window pane.
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