120 Main Jets? BS34 carbs

Just put in new K&L carb kits. BS34/Stock KZ1100A-2 w/airbox and 83'LTD exhaust.
I have a question about main jets. The shop book calls for 120 main jets, and 37.5 pilot jets...so those are the ones from the kits I put in.

The spark plugs have carbon fouling, although not bad.

I assume the engine is running rich/correction (too much fuel)? Would changing the main jets one size smaller help? What size...117.5?

Suggestions anyone?

Lean is not enough fuel. Usually with carbon fouling you are running rich. The main jet really only affects wide open throttle, seems weird that you'd be fouling after a kit. Have you got your air screws adjusted properly?

Had the air screws all at two turns out, now two and a half and three.
Pics of plugs 1-4 (left to right) Plug/cyl 4 is the issue. Could a faulty plug wire have this effect?

Yeah the first three look fine. Could be plug wire, could be plug, could be float level in that carb. Those would be the first things to check. Swap the plugs and the wires from cylinder 4 to cylinder 1, see if the problem moves to cylinder 1. If it stays rich in cylinder 4, then it's pretty certain to be a carb problem. If it moves, put a new plug in. If it goes away great, if not then its your wire or cap most likely.

Would also check and assure valve clearances are within specs, especially on #4. And test compression on #4.

And use the clear tube method to determine fuel level in #4 carb float bowl, which might be too high.

Good Fortune!

Also obstructed Air Bleed at Air horn/ Carb bell-mouth, can cause rich running.