A flow bench is basically a fancy manometer. If you'd like to make one yourself, just make a U tube of acrylic that's clear, with a vacuum port on one upward leg of the U (quite a bit above fluid level.). Make gradations on the U, and fill partially with a colored fluid until centered on the gradations. From the vacuum port, run a flexible line to a test wand that's rigid and can be positioned where you'd like it. Use that same reference point for both carbs. Run a known CFM (or constant RPM for both carbs, and measure the relative readings. We don't care so much what the absolute reading is, just the relative comparison (much like a dyno.) Can also be used to synch motorcycle carbs.