Honda mowers have crazy long model numbers, of various types/options and engines for the same main model line or model ID for lack of a better term.

So the best way to get the service manual is the Honda website, where you put in a bunch of information , serial number and model.... a DNA sample etc etc to get the manual for your specific mower.

There is a recall of the honda mowers for models HRB216TXA or HRB216HXA .

Something to do with safety if the crankshaft is stuck with enough force to damage the safety blade off system, or or something like that.

So if your in this model year range it may have a replacement under recall Possibly solving your problem in the process since these parts may be replaced anyway.

Meanwhile if there is some kind of clutch issue slipping,......... could try using the bag and the highest setting while walking extremely slow may at least allow you to get it cut..... we have some kind of Zoysia grass and that stuff is THICK requiring this technique esp. during the summer.

Not assuming you have not tried this, just sayin.

output shaft prob. have a belt way up there somewhere. ( landscaping while in highschool, even the big fancy riding mowers all belt drivin).