In error, I posted this in the 'What did you do with your bike today' area but here is the text and picture:
I don't believe that it is a Yamaha, unless the engine case, frame or VIN says so.
Yamaha was strictly into 2 stroke engines until the time that they made the 650cc twin, 4 stroke, Triumph knockoff. My best friend had one in 1974.
Here's some good info for you:
The bike sort of looks like a middle 60's-early 70;s GILERA.
All the Gileras I have seen have tubular frames, but the engine does look a lot like the engine on the 106 Gilera I have in my crusty basement. These were sold as Sears Allstates, back in the 60s.
Pictures I've seen of the early Yamahas also have tubular welded frames.
Your bike looks to have a pressed steel frame, such as Honda used on some of their early models. Can you confirm this? Maybe post some better or close up pictures of any badges or numbers you find. What's that feature on the right side tin?