When you figure out how to tune a 3 circuit carb let me know. There are only a couple of times where I would use one. E85 would be one IF the carb was 1200 CFM or more. A highly oxygenated fuel would be the other, but again it would have to be a very big carb. And since I haven’t done an E85 carb I’m not so sure you’d need a 3rd circuit for that either, if you can get the main jet well big enough.
From what I was told by a guy who designs carbs, that 3rd circuit was done to help small inch engines with a pair of big carbs on it. Even with the best boosters that had they couldn’t get enough signal and there was a big hole in the fuel curve. Since the 3rd circuit is a plain tube located in the venturi, it will pull fuel a cover that hole. The other time he told me he uses a 3rd circuit is in tractor pull applications especially on oxygenated fuels and big carbs. Carbs upwards of 1600 CFM each. You can’t get enough fuel through the Jets so they add that 3rd circuit and enough emulsion to choke a horse.
Other than that I can’t think of a reason for slobbering fuel from a plain tube when you can get it through the booster and get properly atomized fuel.
Also remember that pump gas today is blended for EFI, not for carbs. So booster choice becomes even more critical.
