Yes the good old airbleed carbs, many beginners will see these, they are on the plain bearing 2 stroke engines very often, on a side note, they are very prone to issues when the tank is too high or low in relation to the carb. But the concept is the same as any low end fuel circuit, except instead of having the air volume fixed and the fuel adjustable, the fuel passage is fixed and the air volume is adjustable. Many are correct in the fact that the pinch test will get you close, but a little more time spent tweaking the idle so it can do so reliably for at least 3 minutes will assure you that it's going to drop to a nice steady idle, yet not flame out due to a lean transition. Main needle has some effect on it in flow to the idle circuit, so if you touch the high needle, just verify the idle is still correct. They are not hard to adjust, just limited in range of adjustment, so you have to pick a good compromise between high and low. To date, the only carb I've tuned that requires alot of patience and testing to get right is a 3 needle carb seen on many high-end italian R/C car engines. Those have a needle for high, low and mid.