The needles are the fuel metering needles and they rise and fall depending upon your throttle position. They have a corresponding emulsion tube jet that exposes the emulsion jet holes depending upon throttle position. They serve to mix the fuel and air into a bubbly mist that is drawn past the needle and jet opening which atomizes into the throat of the carb and down the intake. It would pay you to spend some time reading the factory manual section on your carbs and looking up the carbs for your bike for the parts diagram (exploded parts diagram) on some of the dealer and aftermarket websites. South Sound Honda has very good prices on Honda parts at good prices and their shipping is based on the weight of the total order, very reasonable. Ordering many items you will need for basic maintenance and upkeep is a good idea to spread that shipping cost across many parts.
You want to clean and reuse your brass parts in the carb rather than replace them with an aftermarket rebuild kit. The aftermarket parts are not copies of the OEM Honda and they are notorious for causing running and tuning issues. Buy genuine Keihin parts for your carbs if you have to buy new brass parts. Many times a PO has drilled the existing jets larger (emulsion tube holes or even slow or pilot jet, etc. and this can cause you to pull your hair out once things are clean and you can't get the bike to dial in the mix in close to stock factory recommended settings. (If you are at high altitude...over 5000 ft...then you may need to rejet. But, usually the stock jetting for a stock motor is the right setup when using factory airbox...best setup for a stock bike.)
Get your bike's valves adjusted properly, points adjusted properly and ensure your spark plug caps aren't well beyond their life (resistors are built into the stock caps and they go bad with time), Cut off about 1/4" of wire at cap end and rescrew the plug caps into the freshly cut wire.
If you are seeing consistent soot deposits on all plugs then you likely are OK on the plug caps.
As PeWe and Scott S said, do the clear tube check to verify your float levels are set correctly and adjust them until they are right.
Sync your carbs and then you will need a bunch of new plugs to do plug chops if this doesn't clear up your plug fouling issues. Plug chops are a technique of checking your jetting and adjustments for low, mid, and high speed operation of your motor. You can't just throw a set in and warm up the bike and run it up to mid or high speed, hit your kill switch and then pull your plugs for a reading as the warm up and low speed work will obscure how the fuel mix really is. Look up plug chop in the forum and you should be able to find out how to perform a plug chop and you can post photos of your plugs to help confirm your reading the plugs and what to do next...
The item you circled and pointed to is not your needles, they are buried inside the carbs and the emulsion tube main jet and carb needles are paired.
Your air screw or idle screw adjusts the air bypassing and mixing in to adjust each carb beyond the base setting. As the name implies, it affects your idle mainly but it also impacts your low speed mixture and a bit into mid-speed.
I don't understand some of your logic to wait until winter to tune and clean the carbs, as long as you aren't running too lean (you aren't) you won't damage the bike but you will burn more fuel and it will not run as designed. I understand part of the logic of being able to keep riding but sometimes delaying maintenance can be a bad decision.
Download the factory repair manual and read it, it will teach you a lot if you grasp what you are reading about. HondaMan has a book on the CB750 that is an excellent resource and can teach you a ton as well. It can be bought through Lulu, PM HondaMan about it. I believe he is slowly working on a revision for the book. Lulu often has discounts, so take advantage of one of their discounts they run from time to time and get 10-15 or even 20% off the normal price.
Good luck and welcome to vintage bike ownership.
David
