Can I assume you have vacuum synced the carbs? (Gotta do this before, and sometimes after each metering/adjustment change.)
An engine may "stumble" on throttle twist from either being too rich or too lean. To tell the difference, you listen for sounds during engine power recovery.
If the engine stopped firing because the the mixture was too lean, as it comes back into power it will be a clean resumption. The effect will be as though someone switched off the ignition during the stumble event.
If the engine stumbled because the mixture went too rich, recovery will burble back to life as the cylinders will fire on 16th and 8th cycles before the routine every 4th, before the mixture stabilizes into consistent firing.
With mechanical slide carbs, attention must be paid to what the slide is doing at the time of the "trouble". It matters if it was being held steady, or if it was moving and thereby changing the carb throat vacuum level at the time of the "trouble".
While at a constant throat vacuum level (slides stationary), "burble" (missing firing cycles) implies jetting is too rich for that throttle position.
At low RPM, stumble from a throttle opening change with a clean recovery afterward, indicates the mixture was too lean at the time of the stumble. This could be from low vacuum in the carb throats, or a too-lean pilot circuit (and lack of an accelerator pump).
If you are fine tuning, you would be best served with marking your throttle, in order to relate engine run symptoms/sounds with carb slide position. This allows you to address the parts of the carb metering devices that directly effect the symptom you are observing.
Expert carb tuners, would alter jetting for track elevation, barometric pressure, temperature, and humidity on race day.
Among these variables, interaction would sometimes cancel each other, and sometimes augment each other or "stack up".
I say this because an air cooled street bikes should never be tuned for the "ideal mixture", because someday it will encounter a "stack-up" day/place and the engine will operate too lean with the associated danger of detonation. Because of this, carbureted street bikes of the era were tuned, on the rich side of ideal, and why some days the engine ran better than o other days.
The engine cylinder will fire with mixture ratios between 5:1 and 25:1. 14-15:1 is chemically ideal, and 12-13:1 produces best power.
Today's computer controlled and sensor laden fuel injection systems, can take all the variables and adjust mixtures for the current conditions encountered. Carburetors can not, and must be adjusted for the best compromise, while ensuring the engine can't be damaged.
This may sound a bit preachy, but, if you want to understand how carbs do what they do and how to tune them, the info can be useful.
The danger you want to avoid is making one section of metering in the carb too rich, while another section is too lean. The result can be an "average" plug reading looking ok, but a prolonged throttle setting at one position or another, will either foul plugs, or increase engine heat and even detonation events. Hopefully this explains why noting throttle position is important.
Hope this helps, somewhat.
Cheers,
