Motorcycle Repair: motor stumbling at speed, air screw, cb400a

Question
Hey, I have a cb400a and a cb400I the carbs have been swapped so I'm not sure witch goes with witch, but here is my problem the 5 speed bike runs really well but almost too lean, then I swap the carb to the hondamatic and it is way too rich and will only run 30 mph or so before it stumbles. This is using the same fuel, fuel tank, and filter. It is firing like it is supposed too also, but it does blow black smoke and really smut up the plugs. The only adjustable thing i see on the carb is the idle air screw. Please help if you can thanks.

Answer
Take a look at the main jets between those two carbs.  If they are the same, I can't explain the difference in performance.  I've never swapped the carbs between those two bikes so I can't explain the differences.  There are only four things that control the amount of fuel getting into the engine...the float level(which is adjustable), the main jet (which is replaceable), the idle jets (which are replaceable)and the needle jet.  The idle air adjustment controls the amount of air going through the idle circuit.   I'm sure you can find a balance of the main jet size and float level to get the five speed running good.  The automatic is still a puzzle to me.  I'd appreciate hearing back what you find for main jet sizes between them. Maybe it would give me a clue.

Good luck
Rich