Hey everyone, I've been digging around the forum a bit and have found a ton of useful information but now I need some help!
I purchased a 1973 CB750 a few months ago with 20,000 miles and have been trying really hard to get it running like a top but have been hitting my head against a wall the whole f'in time! Who ever said 4 cylinders was a good idea? 
On to my problem, I'm getting a lot of popping/misfiring/missing from the bike. Started out missing at all speeds. Particularly on cylinders 2/3. I was getting inconsistent spark so I just sucked it up and bought the ultimate pamco ignition (after replacing my points and condenser and getting the timing nearly perfect). The reason I got the ultimate kit was so I could get new coils along with the new ignition system, and plug caps. That on top of a complete tear down of my carbs. I put all new float needles and seats in the carbs and adjusted the float heights to 26mm. Synced the carbs. I adjusted the valve tappet clearance, 1 turn out on the air fuel mixture, inspected the jet sizes (40/110) and slide needle positioning (2nd from bottom) and everything is within stock spec. I have a stock air box with a brand new air filter from 750supply.com. Stock pipes, new plugs.
After doing all of this service, I eliminated backfires/ popping that was happening at idle and it pulls strong (at least compared to my tiny 360 twin). Where my problems are is in the mid-range. The damn thing sounds like a machine gun in the mid-range when holding the throttle constant or on deceleration. The top end isn't perfect but it's definitely working well. I pulled my plugs after riding it around town for a bit and 1/3/4 are a bit sooty which is to be expected since I was idling a lot, but 2 looked normal/lean relative to everything (a bit tan). I'm fairly certain that most of my problems are coming from cylinder 2 but I don't know why or how to fix it. That was the cylinder that was giving me the most problems at idle with backfiring, but now I suspect its only doing it in the midrange now. I sprayed the carb boots/manifolds with carb cleaner and couldn't find any air leaks, I'm stumped.
Things to continue checking:
1. The clamps on the rubber carb mounts: are they stretched, like with the screws almost pulled through the clamps? If so, go get 8 of the 2.5" size radiator hose clamps and use them: your hoses are very hard and ARE leaking, despite what you might think.
2. If you used the typical Keyster aftermarket fuel valves in the carbs, the 26mm float depth is too shallow. Try 25mm instead. While this seems counter-intuitive, it will make dark plugs happen (takes a much longer post to explain the reasons, but 'tis true...).
3. The air screws: try setting them to 7/8 turn, especially with #110 mainjets: those mainjets are mighty rich for a K3. More K3 bikes came with #105 than with #110, as the K2 proved the #110 was too rich by about serial number 2007000, and it became #105 instead. Almost all of the 750s after that came with #105 until the "F" series bikes started, but many got "improved" over the years by the owners, in pursuit of K0-like power (the K0 had #120 and #115 jets, but also had many other different things to go with them).
4. Trueblue and Pamco Pete also have it right: they are aged (mature?) carbs, so check for odd defects and corroded air passages that narrow the air holes or let unauthorized fuel slip through. The pilot jets get this malady, too.
What is your compression? Have you adjusted the valve lash? Tight valves on cylinder#4 will cause the #2 plug to look leaner than the rest of them, when all would otherwise be too rich (as one example).
4 cylinders really IS a good idea: when 1, 2, or even 3 of them quit, you can still ride it home. (Been there, done it!
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