Slow engine cranking

I just bought this bike off the original owner. It's a 1981 650 csr. It is in good shape and only has 10,000 original miles. So I took the battery out and charged it, along with pulling the carbs and cleaning them drained the gas tank and changed all the fluids. But when I go to start it, it turns over slow, real slow a crank and a half then thats it. I even tried a brand new battery...same thing. I don't know where to look next. Any help would be great, I am very wet behind the ears when it comes to this, but I want to learn.

Have you checked both batteries under load?
Did you try checking the cables and connections?

I would try those and rule them out before I tried looking at anything else.
You might want to ask the original owner if he/she had this same prob, although he might not give you a straight answer:unsure:. If you know him/her well enough, then they might be able to clue you in on the prob.When was the bike running last with the original owner? If he stored it, was it running at the time? Just try to get as much info as you can, if you can.

I hope that you get it going soon,
ride safe