The only throttle position any of my quiet lines felt "smooth" at was wide open throttle. Anything else always had a lot of vibrations and I always did the upgrades and mods to every QL I purchased. (the term smooth is open to interpretation.)
Did you check and replace the drive shaft bushings? If your ISO mounts were that bad, I would almost guarantee your bushings are shot because your drive line was way out of alignment.
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DWayne
1973: 128, ag tires, 3pt. lift, spring assist, lights, 42" Deck
10" moldboard plow
2016 XT1 42" deck 18HP
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