Engine backfiring, hunting at idle, damp exhaust

Hi guys,
I just bought my 78 kz650 b2a not too long ago but its been acting up lately. I started noticing it struggling at idle with the choke off after it was warm and backfiring on deceleration. I pulled my plugs and cylinders 1&2 are running normal but 3&4 are black and sooty. Furthermore, I'm running a 2 into 1 exhaust on either side and put my hand up to the tailpipe coming from cylinders 3&4 and the exhaust felt damp and was cloudy when a light is shined through it. This was not the case for cylinders 1&2. I'm pretty sure its the stock set of carbs but I'm not 100% certain. Can anyone help me out to solve this issue? I have decent mechanical ability but I'm not really sure where to start as this is my first 4 cylinder.
Thanks in advance!

Do you have a factory Kawasaki manual? Gone through all the maintenance such as checking/adjusting the points and valves?

www.zedrider.co.uk/service--owners-manuals.html

I do have the manual, I checked the points this afternoon and they're getting old but usable and the clearance between them is within spec. Haven't checked the valves yet though