I had some similiar issues with my truck lately and chased my issues to the vacuum advance -distributor. The last thing i tested after months of dealing/ tinkering with it. i tested mine like the my manual says and it failed. Luckily i had a spare that tested fine. since replacing it with a good one, my gas mileage is better, no back fire and smoother throttle acceleration. I'm running a weber instead of the stock carb though.
