Air-valve propane carburetors like the Impco 425 supply fuel based on air flow and not load. The only external fuel mixture adjustments on this type of mixer are full-throttle power and idle mixture. Ideally, you put a CO analyser in the tailpipe and adjust the power mixture screw so that the full throttle CO reading is 1.5% to 2.0%.

It is always safer with propane or natural gas to go lean rather than rich. Rich mixtures burn exhaust valves because gaseous fuels do not have the evaporative cooling effects of liquid fuels. Since the air valve only senses air flow, you will likely find that your fuel economy will improve if you adjust the power valve to be on the lean side. The only danger from lean fuel mixtures is a lean misfire. Lean fuel mixtures (as with gasoline) burn a bit slower which means you will need a bit more spark advance. Once you've found a lean fuel mixture that still runs satisfactorily, try adjusting the spark advance to improve your fuel economy.

The best guide to setting up either the Impco 425 or the OHG X-450 mixer is Franz Hofmann's Diagnostic Guide. It is available on his web site for a very reasonable price.

Obviously, for a dual quad setup, you need to make identical adjustments to each carburetor.