Join Date: Jan 2017
Location: Paradise, Newfoundland
I was in the same position as yourself, the previous owner never broke a sheer pin touting the Yamaha quality, turns out both augers and the impeller were rusted in solid. The best approach would be to remove the side panels, tip the machine on an angle and liberally spray the auger shaft channel with PB Blaster or similar rust busting catalyst. Switch sides, spray again. Try the puller if you have some means of gripping on to it. If it doesn't budge, try a little Map gas. I used propane and was only able to pull one auger off despite repeated applications of PB Blaster, heat and brute force, the other had to be taken to a machine shop.
My guess is the reason you're not able to turn the augers by hand is a result of increased friction from rusted parts, worn bearings, parts out of alignment, dried seals, and perhaps old auger gear oil. As efisher said, the gear ratio makes it difficult to turn while the impeller can be turned easier which in turn rotates the augers. I was barely able to turn mine, if at all as they'd bind.
I'm not sure how the MTD models differ from my older Yamaha YS624 but during rebuild everything, including the new bearings, was generously coated with cold weather snowmobile/snowblower calcium based grease to displace any moisture and to prevent future rusting. New seals and synthetic oil was used in the worm gear casing. I'm now able to easily turn the augers which in turn spin the impeller.