Soot from exhaust

That’s way too long. The technique is to open the throttle and close the choke simultaneously and quickly as soon as it starts. No need for 20 seconds.

My choke knob does not have a friction/lock to stay open.

I like to have my hands as free as possible to respond to any issue that may arise during engine start.

In my installation, I have found that two clothes pegs, placed on the choke knob shaft, holding the choke open, before start is initiated, works very well.

Cold Start Sequence:

Master on. Throttle fully closed. Both ignitions on. 

Press start button,

Engine starts immediately,

Remove one peg immediately. after start

After engine note /speed changes - remove second peg. Engine speed reduces.

Throttle advanced to maintain 2500 rpm for warm up.

The above all happens within a very few seconds.

Works for me/my engine every time😁