[C10 - C90 Bill]

Do you downshift in order? 4th Hi to 4th Low before going to 3rd Hi?

I'm also thinking Timing, if it's not a Carb problem.

Also, when adjusting the Carb, use a Vacuum Gauge and adjust it to the Highest Vacuum on the Low side (start with the Screws turned more in than out).

Would love to see the truck in action.