Hi there,
New member here. Sorry to jump in the thread, but I've also got a Kawasaki J300 and I've been using the Downtown 300 manual as I can't get hold of the Kawasaki one. So thanks for that link, I'll download one now.
As for the J300, I'm loving it. I've only had it about a month, it's a 2014 model with 13,000km on the clock and it runs like a dream. I'm in New Zealand and am commuting to and from work on it every day now. It's been a revelation. I should have owned one years ago! The previous owner put on the larger windscreen which seems to work well to shelter me. The only thing I want to do remodel the seat. I'm just under 6ft2in and while it's fine for the short commute to work, it's not the most comfortable riding position. I need to be able to slide back an inch or so but the low backrest prevents that. I'll attack the seat in the coming weeks.
I've recently given the bike service. I did have a transmission shudder at low revs when taking off from lights, but a bit of clean up of the drive belt area and a de-glazing of the clutch bell and pads has improved that about 80%.
Great forum too by the way. It's proved invaluable to me so far!