Don't touch that mixture screw.
mmm mmm..... nope.
Not planning to do that unless all other efforts fail....
I am hoping that it is primarily a timing issue..
When I was lining things up for the timing belt install, I noticed that the dizzy was lining up - BARELY - with the mark on the housing, but at the trailing edge of the dizzy conductor. This leads me to believe that it is probably running a bit on the "too much advance" side of things. The slotted distributor mount is maxed out to the farthest "delay/retard" possible, so there is no space to delay the spark.
If this is the case(too much advance), and correct me if I'm wrong (because I often am), I am guessing that the distributor itself is off by a few teeth, so the mark on the accessory gear (timing belt wheel for the dizzy) might not be correct.
If all this is true, my inclination would be to turn the distributor gear 2-3 teeth counterclockwise from the indexing mark on the belt, and center the distributor in the slot range, and re-check timing. (or, alternatively, re-install the distributor)
This is all predicated on the spark being too far advanced.
If the opposite is true, I have the whole range of the mounting slot to advance the spark
Did any of that make any sense?