This isn’t hovercraft but is relevant nonetheless. I’ve been fiddling around with an outboard engine for weeks, an almost unused tohatsu four stroke auxiliary motor. It would never run well, ok flat out but very poor idling. I bought it as ‘a bargain’!
I always suspected the carb, and had cleaned it a few times. It’s hard to find info on these small Keihins and so it’s difficult to fault find, some of it could be worked out but not with any certainty. Lots of chatter online about the idling circuit getting gummed up with fuel residue. There was a hole that couldn’t be blown through even with the airline.
Anyway, I was on the point of buying a new carb when I decided to strip the whole thing to remove all the plastic and rubber before dunking it in acetone overnight. That did the trick! It shifted the lacquer and the idle circuit now blows free. Just like that!
The only reason to mention it - we do get similar things with Briggs carbs sometimes. Acetone might be worth a try next time. But you do need to get it properly apart otherwise all the seals and gaskets will be junk!