I just did the same job on my '04 Rancher ES and I took the safe route and adjusted the valves. No telling what the PO or shops did. BTW, mine were out of spec. I will probably go back in and readjust them after a few rides.I had to tear down my engine to replace the crankshaft and connecting rod, piston, piston rings, and had to get a new cylinder due to it was already bored out to max. So I went back to stock piston size. I got it together today and primed it with oil and everything and when I went to go fire it up all it would do is back fire. I took it back out and tore it down to make sure all the alignment points were correct and they were. My question is since I went back to stock does the valves need to be adjusted because of the smaller size? Or could anyone have some things I should check or try? Everything was built by the book.