Strange Warm-Up Symptoms

[jimfj]

I would look pull the fuel hose and put the petcock on prime. Drain the fuel into a glass jar and see what color the very fine particles are. You stated no fuel filter. I am betting there is some rust in your tank some somwhere.

I just finished cleaning a rack of VM carbs off my 77 550 that were covered in the nasty brick red stuff. Not big chunks, just super fine dust. I believe that when the contaminated fuel burns away it leaves this crud. i would also pull a float bowl and look at the little recess where the drain is located and I would also bet some has collected there.