Well, I've often wondered if it was to replace the pressed-in "jet" in the float chamber, but have also wondered why (a) it's called an "air jet" in the manual if it's in the float bowl, and (b) why there is only one, unless it's to richen/weaken the fuelling to only one cylinder. (This is why I've often suspected it to be a replacement for one of the X/Y-piece jets.)

Whatever it is, we've never had reason to require it! Maybe if I physically saw one in real life, I would be more able to answer the question. As it is though, I wouldn't lose any sleep over what it is or what it's used for! As I say, it's only shown in one version of the MC21 manual... the least common version in my experience. I can safely say that I know of no-one who has utilised it.

I'm not saying it's useless, but I am saying that I expect what it does can effectively be masked with a different SJ/MJ/PWJ/JN/AS setup, which is probably why it was only made available for one year.
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Andy.
NSR-WORLD.COM

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