Old topic, I know, but the information is relevant...
As regards the modification of the Zenith carb for the 4AO-84, most carbs I have tried have had a "stumble" coming off of idle, and when advancing the throttle suddenly. Guaranteed to get your attention!
I have found that the 10hp Zenith, for the 2AO-42 genny engine can be used, and is much more user-friendly. As it comes out of the box, it idles smoother, but will drastically lean-out past 2600 rpm. Drilling the lower (main) jet holes will sort this out. There isn't an exact jet size that will work, as no two carbs, nor any two engine set-ups, alike. So, a little cut n' fit testing will be in order.
As a side note, the 20hp carb has a 1 1/16" venturi, and the 10hp carb has a 1" venturi. As most of the folks I have dealt with, using the AO-84 in a light plane, are propping the engine to run less than its rated 3600 rpm, the 1 1/16" venturi carb will run a bit rich, at lower rpm levels, and then go lean at the higher manifold pressure (greater throttle plate angle). The smaller venturi on the 10hp carb helps offset this tendency.
Another item is to keep your eyes out for a Tillotson OM-25A carb. This is an older carb, for the earlier dash number genny engines. The neat thing about the Tillotson is it is easier to re-jet, and One can find repair kits for the carb.
