They are air correction solenoids.

20 years ago, when we knew no better, they were dubbed power jet solenoids, and the name has stuck ever since. There's no doubt references still dotted around the Main Site and all over the forum misnaming them. I change the references as I come across them.

The power jets are controlled by atmospheric pressure, not electronically, on the NSRs.

The solenoids are linked to throttle position, and RPM on the MC18. I believe they are also linked to gear position on the MC21, but can't say for sure.
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Andy.
NSR-WORLD.COM

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