Having a similar issue with my '86 Venture. In addition, the tach doesn't work until the dropped cylinder kicks in around 4500 - 5000 plus.
Where it kicks in at higher rpm, I'm guessing that it's a potential carb issue, maybe plugged jets.
Can anyone tell me which cylinder the pick up is on? If I remember correctly, 1 is driver side rear, 2 driver side front etc.
Dave