I recently bought an XT350 project and in the process of putting the top end together, ran into the same problem. I even tried having all cam chain guides in place and the tensioner bolted in. The cam chain still jumps when I try to turn the engine over. I've examined each of the guides and they all look like new. I'm convinced that the cam chain is stretched. At this point I am going to order a new chain and a new tensioner.