Craftsman 4 stroke weed eater

Overhead valves on this engine had no lash..I adjusted them to .006..problem is I can't get the exhaust valve adjusted. I get engine tdc, set lash, tighten rockers, check lash and its on the money. However, when I pull starter a few times. Put engine back to tdc and check lash again..exhaust valve rocker is flopping around. Tried this several times with same result. At first I thought maybe bent push rod, but I removed it and it looked ok..I rolled it on flat surface and its not bent. Can anyone offer a suggestion as to what might be going on. I've never had one of these little 4 strokers apart b4..so I dont know how it is set up. Im hesitant to tear engine apart because I can't even locate parts for these engines. Can't find gaskets or any internal engine parts, except on ebay. So if anyone has any idea why the exhaust valve isn't maintaining its adjustment, id appreciate it.