some motors you cannot run on muffs--PERIOD!! if you want to set idle, you can get it close either way, but to get it right, it has to be fine tuned, IN THE WATER, IN GEAR!!!!
my 120 would not idle below like 1300 rpms on the muffs this weekend. I filled up an old horse trough we had and submerged the lower unit and it idle around 700rpms or so with no changes. Couldn't believe it myself. Only Johnson I've messed with so far that would do this. The 76 115hp, 88 spl, and 200hp merc don't do this so I guess it depends on the engine.