The ASII is a shorter body carb with a stronger signal for theoretically better response. In reality, from all accounts here over the last few years, it seems like more of a PIA to get right. The ASI on a 250 is absolutely fine, I'm fact I prefer it. It also fits better in the airboot. It just wants richer jetting specs, fuel economy is excellent on my 250s. The other nice thing is its a lot easier to get the top off for a needle change, but once your set its a pretty stable setup, I only turn the airscrew and swap the pilot seasonally. 250s are also less fussy than 300s anyway, plus you use more of the range and use it faster so any small jetting inaccuracies are not as noticeable as on a 300.
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