I am working on my grandson's 300ex, we were at a mud park and he slung his timing chain. My son took it all apart and it sat for 2 months so I came and got it and put new timing chain, guides and piston & rings on it, got running but when I shift to first gear it just into gear but won't go. Took the clutch cover back off and with nothing hooked to the shifter it only goes into neutral and first. Any ideas on what to look for ? OK it does shift when I rock it but I can still roll it in every gear.