Don T - ok, well it seems I was wrong about the fuel delivery since you're back to backfiring again. Based on everything else you've done, and it seems you've gone over all these things at least twice, I think you're left with no choice but to see if you lined the cam/crank up correctly. It sure sounds/acts like it is out of time but it reads like it is in time - so it must be the cam. It probably starts because you've got everything in "like new" condition, but it won't throttle up because it goes way out of time. It's got to be the cam. (I wish one of the wiz/bang engine experts would pipe in here. I'm such a novice and way over my head with this stuff. I just know something about a mark on the cam that is supposed to be aligned with something on the crank, or vice versa, but there is a pic in the manual.)
Bill J - gol dang it, get the Creeper. They are hard to come by. It's a necessity if you're gonna be pulling out any tree stumps or fence posts. It's like having an extra set of hands for some jobs. You can put the tractor to work while you assist it. It's like getting 4-wheel drive in your vehicle. If you get it you know you got it and if you need it then it's there to use. If you don't get it and need it, well, then you wished you had it. And for what it's worth - get the dang spring assist to. These are all the little goody items.