I done the same type carb swap on my '83 200K, but didn't have that issue. I used a OEM flange manifold. Did you use one of the aftermarket (Chinese) manifolds?
As for finding cheap parts, it's not worth the problems when OEM is still available. What's 'too much money,' is point of view. What's seemingly expensive now, will just get more so once it becomes NOS. $80 for a major part that can't be found anywhere else isn't 'too much money' in my book
https://www.ebay.com/itm/YAMAHA-AIR-...0/253026376322
Owning and repairing trikes is no longer a short funds hobby.
The story of three wheels and a man...