Congrats on your CB750F2N purchase.
I now have a 1979 cb750 limited but had a blackbird in the mid 2000's. I liked the blackbird but it can't match the look of the cb750's engine that's a work of art unto itself. Enjoy your bike.
I rebuilt my stock carbs two months ago and put pod filters on. Plugs were white when I checked them as it was running too lean. Since then I've gone back to the airbox that made it run better and now I've re-jetted the carbs to 110 main jets (was 102 stock size) and pilot screws from 1.5 turns...
They are not great 100% reproductions but they worked for me.
Sounds like a vacuum leak. Check the carb boots by spraying carb cleaner on the joints while the bike is idling. If it revs up then you have air leaking into the engine at that spot. I had the same problem with mine and needed new rubber carb boots as mine were dried out and didn't seal anymore.
Those are the 1979 style floats that use adjustable float needle, you can use yours.
Check the 4into1.com website they have carb kits. This one has both types of float needles.
Vintagecb750.com has kits...
cb900 had 32mm Keihin carbs. cb750 has 30mm Keihin carbs. I'd stick with the 30mm cb750 ones or buy a new aftermarket set.
Any used carbs will need rebuilding so don't spend too much on them, I wouldn't pay more then $150.00 on a set because a good rebuild kit is going to be at least $80...
I would disconnect and clean all the wiring connections with contact cleaner first. Could just be a bad connection.
Camshaft lobe positioning will tell you if it's on the compression stroke. Turn it over by hand with the valve cover off and watch how the valves open and close on that cylinder.
But I think these motors use a wasted spark system that fires on both the compression and exhaust stroke. So the...
ebay is probably your best bet for side covers and gas caps. I've gotten parts from vintagecb750.com.
Getting a shop to do it maybe your best bet. Or if you feel it something you can do yourself get a Clymer manual and take your time.
Either way you may want to spend a few dollars on new cam chains instead of trying to get the same old chains to work as they could be stretched to a point that...