Wow. This thing is really kickin my butt. I've been wrestling the deck out from under and reconnecting the belt a couple times every mow. To your point earlier, there is no side to side play in any of the mower blade pulleys, nor in the idler arm pulley; all 4 are brand new and tight. The other 2 idler pulleys, if that's the correct term, one (#52 toward the right side of the deck as you look down at it and facing forward), has just ever the slightest bit of play - almost negligible. The other one, #39, that the tensioner arm pivots on/about, is snug.
Every time the belt jumps, it jumps off the DRIVE (#12 or #11) pulley.
For a couple of weeks, after putting on new spindles, blades, and deck belt, it was doing okay. But I could only mow at the 3rd lowest position. As luck has it, I prefer to have it in the 2nd lowest ...and every time I put it in there, it usually always jumps. Sometimes not, (maybe after running in the 3rd position a while), but, for the most part I can't use the 2nd lowest position any more.
Today is a disaster. Went about 50 ft in the 2nd position and it jumped. Got it back on and started in 3rd position, jumped immediately. Thought the tensioner arm had too much up and down play, so I took it apart and put about an 1/8" washer in there to snug it up, which it did, put it back together, belt jumped. Got another used belt that was in good shape, put it in, didn't get 2 feet and it jumped.
Help me. Please help me.
. Thank you.