The cause of the old 'runaway idle' in the 450 isn't even in the carbs(!).
It is in the spark advance. The engine runs quite hot, and those 450s that were ridden mostly in heavy traffic overheated the springs of the spark advancer and softened them a LOT. This makes the weights hit full advance by as little as 1400 RPM, so the first startup locks it into full advance, making idle below 2000 RPM impossible.
So...start by cutting off 1/2 turn of both of those advancer springs: when you look at them you'll see they are likely quite soft and loose. It usually takes more like 1 full turn cut off both springs, or more, before it acts normally again.
HOWEVER - this presumes the stock air filters and a functioning crossover tube with good seals is in place. If not, the bike won't idle, period. The CV pistons in those carbs are extremely fast, and they 'talk' to one another thru the stock airbox cross-connection pipe, such that when one side is flowing already this lowers the air pressure inside the box (intake stroke on one side) which makes the opposite side's piston move more slowly than normal to damp it's rise. Since the cam drives this engine with 2 power strokes 90 degrees apart, this cycle begins again at the next idle intake. The mass of the crankshaft being unpowered for 270 degrees then slows the crank speed slightly for the beginning of the next intake cycle. This has ALWAYS puzzled the Honda mechanics: a very smart Honda rep explained it to me in 1970, after which it made perfect sense.
The bottom line is: if the stock airbox arrangement is ripped apart and 'pod' air filters are installed (or foam replacements in the stock positions) the bike won't idle. Worse: if foam air filters are installed it will only start when cold, and is nearly impossible to restart hot (like at the gas station after fillup).
The bugaboo that is selling many of the 450s today is our gasolines. It burns so slowly now that the engine's cam timing is a little too aggressive. Try using Regular grade instead, and you'll find that old gas (like last year's lawn mower gas) will run better than new gas will. This will help. Also, add some oil to the gas in the tank, especially if it is new, to lube those big rings and valves: they rely upon the missing lead in the gas in this design.
