There should be a check valve in the canister, so changing position may very well be the problem. Try disconnecting it and see if that helps.

This car was likely a "Lean Burn" Chrysler when it was born. The Lean Burn system was a huge advance for it's time. It used a small computer to measure temperature, manifold pressure and rpm, and adjusted fueling and advance according to a pre-programmed curve. The early versions were passive analog, they went digital around the time your truck was built. Like a lot of electronics, it was unpopular with mechanics and so most of these systems were "simplified" by going to old style carbs and distributors. To make it even tougher, Chrysler kept it's dealer network alive during their lean years by buying back spare parts inventory. So the parts you'd need to make this factory stock have long since been scrapped, and diagnostic tools and expertise lost. As a result, a generation or two of Chrysler cars are real heartbreakers.