[n4mwd]

Unless you got a copy of the maintenance records when you bought the bike, you have to assume there was no maintenance.

Bogging down past 5K rpm usually means the pilot jet in the carbs are clogged.

I would recommend doing a valve adjustment and a thorough carb cleaning and chaining the oil and filter all at the same time. Make sure you read my blog (link below) regarding overtorquing the rockers or else you could destroy the engine. That is just routine maintenance.