The filter on 400~450 should be a nylon mesh tube around 1/2" diameter about 3" long and fitted into bung in tank.
The OEM fuel tap was usually riveted together but I think some were similar to the earlier CB's with a filter bowl on the bottom.
The filter bowl was also a water trap.
The CM's had pretty 'slack' steering compared to the CB's. The only one I'm really familiar with is the CB250/400N.
The 250 was pretty overweight as it was basically a sleeved down 400.
The 400 did handle better than earlier bikes, they were designed by a different set of people
Honda pretty much moved everyone from motorcycle division to the automotive side in 1976~78 (also why some engines were such a disaster in what should have been trivial areas, CX500 cam chain tensioners and water pump seals, CB250N generators, CB900 Cam chain tensioners, ALL early 'Com-Star' wheels, etc. )
They should be able to rev way past red-line, 4,500 is where slide should be lifting.
Usual cause on the bikes with rubber diaphragms is the transition circuit makes motor run rich without the stock intake system (you can remove the filter foam but not the air-box assembly or even lid)
On later carbs with solid piston the inside of the vacuum chamber could just need a good cleaning (along with the piston part of slide)
They are also much more accurate on fuel delivery running a bit on the lean side even when set up 100% by the book
Have you checked the electronic advance with a timing light? I've never heard of a high speed coil failing but anything could be happening after30 years or so
The main reason 3 valve parallel twins are scrapped is failure of the generator/ignition system as there were no replacement for probably 35 years?