There is no reason to tune a carb on the bench.  The only thing you can set is the idle speed and idle mixture.......revving and engine in a static condition will tell you almost nothing about the jetting.  You need to be able to accelerate and run the engine at full throttle under a load to test the jetting.

Also - running and air cooled engine without the bike moving down the road results in poor cooling, and low engine speed results in low oil pressure.

Why would you want to run the engine on a bench. The bench isn't going anywhere.

Exhaust is no problem. I can easily drill a hole in the wall, expelling the exhaust outside the building.

I am still trying to figure out the correct name for things so when I wrote AF-meter I meant O2 sensor. I thought AF (Air-to-fuel-mixture) was the same thing. I aim to install a Bosch LSU 4.9 in the exhaust and a slide potentiometer on the throttle control so I can log the values and check the O2 value from 0 to 100% of throttle.

Since I already have the motor mounted in my bench stand it is actually LESS hassle than putting back in the frame and doing the tuning there next year. Th only thing I can not do when tuning the engine out of the frame is put load on it but revving it to full throttle shouldn't be a problem.

You want to bench test a LS650 motor??? I thought the Finnish were a strange bunch...