Not getting full power, backfiring during acceleration

Have a John Deere 737 ZTrak with 23hp Kawasaki motor (approx 600 hours). I recently replaced a front main seal that had failed. When reassembling, I ensured coil clearance from flywheel magnet (.3 mm). Engine started and seemed to run fine, but didn't seem to reach full rpm. When clutch is engaged for the deck, there's an immediate loss of power, almost dies. Power slowly rebuilds, but again dies with any load. Seems as though the governor's not opening the throttle all the way. The plugs have clean, light brown color, look to be burning well. Fuel filter recently changed. Gas has Stabil fuel stabilizer in it. Air cleaner looks good. When coming off idle, engine backfires. Noticed when idling, a small squeak, almost sounds like a sheet metal thing. Any ideas would be helpful.