The only people who attempt those single carb conversions are the ones who cant work on their dual carb set-up. So why would they have the knowhow to install a non stock carb??
Thats junk.... Looks like the manifold isn't fully seated in the forward cylinders insulator.
I'd say just pull that crap off and get a set of stock carbs. Or you can try cleaning the carb thats on there, and try to find a filter element that isn't gona be stuck another 4 or 5 inches out to the side...
1981 cb750c with 900f forks, cams, oil cooler.