Hmmm. I think you have flying saucer problems.

The carb secondary barrel is vacuum operated, and if the flying saucer on the carb, pside mounted at an angle with a single vac hose, has a broken diaphragm, and when that happens, the truck is a REAL dog, and not just doggish, and will pull ok up to about 40 and then fall on it's face due to not enough air.

To test the thingy (mitsu calls it a "depression chamber") pull the vac hose from the carb end and apply vac to the thingy, and then open the throttle manually and you should see the secondary barrel (the pside one) open up. If the thingy won't hold vac, it's dead and you need a new one. When doing the test, have the engine warmed up so the choke is open and out of the way, and remove the air cleaner for access and view.