One tailpipe smoking especially under choke

81' kz305 Smokes very lightly when riding and very heavily (out of the same pipe) when starting with choke. Could this be the valve seals? They are original. What's the reason for smoking so much when the choke is fully engaged?

Might be seals, but I suspect one carburetor is running really rich. When you apply the enrichener (choke) the mixture gets even richer, hence more smoke.
I would start by checking the fuel level in both carbs using the clear tube method described in the factory service manual. Once you are sure the levels are correct, I would then check the valve adjustment, then fine tune the carbs. 
If the problem persists, you will likely need to dive into the carbs. A sticking float valve or a jet that has come unscrewed can cause an overly rich condition.

Have you done any service or repair recently?

I see that those carbs have the dreaded rubber plug under one jet. It is reference number 16012 in this schematic.  www.partzilla.com/catalog/kawasaki/motor...-a1/carburetor-parts

If that plug has dislodged or is missing, that carburetor will run very rich.