On the old Simmons carb compare bench the manometer read from 0 to 10.0. a good one on mine would be anything over 9.0. A carb used as a standard was compared to the carb you were testing so no two builders would use the same numbers as standard, one builder may test the same carb as another builder and get different results. -- Chuck
It is an air flow number for the Simmons Carb Comparator. The Simmons simply "compared" one carb to another. A 9.1, if compared to a pretty good carb is actually very good. However, if 9.1 is compared to a stock, never reamed carb (if memory serves me, such a carb would be in the 7.8-8.1 range) it would not be nearly as good. It is simply air through the air horn and bore and is not a measurement of fuel flow. CMac
It's not a measurement of airflow as much as it is a comparison of airflow.
Think of a scale from 1-10 with 10 being the best.
If your very best carb flowed 10 - then a 9.1 would be pretty decent.
Now, it would not necessarily be in the top 10%, as other carbs wouldn't fall below about 7 or 7.5 typically if they are all built the same.
It works quite well for it's intended purpose (much like a dyno - to "compare" rather than to match numbers from one machine to another.)
At one time there was a sticky post on this site for building your own homemade flow bench - very similar to this unit.