Idle speed is adjusted using the black plastic knob underneath the carbs (clearly visible in the photo you posted of your carbs).
Exhaust system is pretty much personal preference. My '79 CB750L had a nasty 4-2 system on it when I purchased it. I dumped this in favour of a Marshall/Jama 4-1, which sounded nice, looked cool and suited the engine. I then decided I wanted something like the original 4-4 that the 750L would have had as standard, but original systems for this bike are as rare as rocking-horse sh*t, so I got a used 4-4 system from a 900 custom. Fits perfectly at the engine end and just needed a small bracket making up to re-locate the prop stand rubber stopper. A system from any of the 750/900 dohc's will fit.
Maz
'76 CB500T
'75 GL1000 'Wing
'79 CB750L
'90 FJ1200
'93 GS500E
In a little place called Bexley, Kent, UK (Just south of London!)