Hi there
I had this issue once after rebuilding a 21 sp,it was primary side crank seal it was a new seal,I dismantled the motor wondering if I missed the groove in the crankcase that the seal locates in,I hadnt but when I examined the seal closely I could see the seal in the outer flat side
when it was moulded and the moulding tool was pulled away from the seal it left a fairly deep crater across the sealing face of the seal,
i replaced the seal with another new seal with a normal sized dot on the outside of the seal and that cured the problem,It was sucking gear oil passed the indentation in the seal,
If you are running premix there cant be many other reasons for that amount of smoke from just from the lower cylinder,
I hope this helps,
jonmac
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John