I have a K&B .61 that I was having similar problems with. It has the black Perry carb with the brass wheel on the high speed side. I broke the carb down and found a very small piece of trash in the spray bar of the mid/low adjustment. I searched high and low to find out how to adjust the Perry carb. I finally found some forum postings like this and it shared that you will get an enormous change with very little adjustment 1/32 of an inch. After looking at how this carb works, I understand it better.

The spray bar has a VERY fine slit with a small hole at one end. By adjusting the wheel you align this with the barrel. A very slight change in the wheel and it doesn't line up. The forum said that the wheel adjustment is up to 1/8" either side of the whitness mark @ 90 degrees.

Once cleaned and properly adusted, mine starts and runs great.

Hope this helps someone.