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The idle mixture screws on these should regulate air. So if all adjustment screws are set equally, it seems logical to expect a blockage in the idle circuit in the carb of the affected cylinder. Could be the jet, and/or the circuits in the carb body itself. I usually spray carb cleaner into on end of the circuit and check that the cleaner exit the other end to verify if it is blocked or clear.
A few of things on the SOHC CB750. As mentioned the spark plugs. Check the spark plug caps, they are screwed on to the plug wire. Unscrew and cut no more than 1/4" for a fresh end put back on plug cap. The wires are typical short so trimming to much will give you grief. The plug wires cannot be extended they are molded into the coils.Now to the carbs. If you have a cold cylinder pull the carbs off. pull them apart on the bench. Everything. Clean the passages and make sure that the emulsifiers are pulled and cleaned. If parts are needed, gaskets and needle valves and seats only buy keihin carb parts do not buy aftermarket kits. More money yes but less grief.Make sure the float levels are properly set. Fully assemble on bench put the carbs level in a flat pan wide enough to hold are carbs. Hook up a gas supply and check to make sure the floats and needle valves are working and there are no leaks out of the overflow tubes. If all that tests good put back on the bike. If you have a center stand put the bike on the center stand and turn on the gas. Give it five mounts and check for overflow leaks. When they are apart polish the pin that hold the float in place. Once there is not issue, fire up the bike and again check for your cold cylinder.When you park the bike overnight or longer period, turn off the petcock and run the carbs dry. this help keeps the ethanol from reacting with the pot metal of the carbs, and keeps the corrosion free.If you have spark the issue on cold cylinders on these SOHC motors is always carbs.Here is an excellent SOHC forum. http://forums.sohc4.net. Hondaman is the guy that is the technical guru.Hope this helps.
Have you said whether you have checked the carb balance ?Is the slide for #3 lagging behind such that it does not provide fuel/air at idle, but chimes in to some degree at more open throttle ?Can you detect the cylinder “coming alive” suddenly as revs rise, or does it just blend in ?I guess I’m asking, at idle it noticeably running on three cylinders ?