Impco still makes some dual fuel mixers but the selection has become much smaller. While a many mixers can be made to work on a particular engine, the main consideration is starting. Gas-valve mixers need a minimum air flow to lift the gas valve off of its seat in the mixer and approximately 6" wc (water column, ~0.2 psi) of internal mixer vaccum is required to lift the air valve off its seat. The cranking RPM of an engine can be as low as 200-300 RPM, which is about half of idle RPM. An oversized mixer may be very difficult to start and a primer or start assist valve may be required to provide enough fuel to start.
Undersizing the mixer improves starting and low-speed operation. However the gas valve has a limited amount of travel and reaches its full-open position when internal vacuum reaches approximately 13.8" wc (~0.5 psi) and this corresponds to the rated flow of the mixer. Since the gas valve cannot lift any higher, higher than rated flows mean that the fuel mixtures become increasingly leaner with a corresponding gradual reduction in power.
Impco only publishes the rated flows of their mixers at a specified manifold depression. The manifold depression is the resulting manifold vacuum from the pressure drop across the mixer at full throttle, which automotive applications using 1.5" Hg and industrial applications using 2" Hg. A variety of mixers can often be used on a particular engine size, it is the maximum RPM seen by the engine in normal operation that should be used in sizing the mixer. Although some engines can have reline of 5000+ RPM, this speed is likely rarely attained in normal operation. For automatic transmission vehicles, a reasonable maximum engine speed is the transmission's full throttle shift point.
The recommendations in this article are based on a maximum speed of 4200 RPM and a minimum idle speed of 425 RPM.