I have set my firing order on my 1987 Chevy s10 truck at 123456,and it still having firing issues what could be causing it

Clockwise, 1, 6, 5, 4, 3, 2. Looking from the front, #1 would be located at the 5:30 position on the distributor cap. Cylinders 1, 3 and 5 are front to back on the drivers side of engine, cylinders 2,4 and 6 are front to back on the passengers side.

U first need to know if its chevy or Buick. V6 one has reverse firing order. Both are very similar an both used in same vehicles.