cam lobes down piston at TDC flyhweel aligned with fixed indexer - sounds like you did all of that. But, did you align the marks on the cam chain sprocket flush with the head? _____________0_____________ if not... the valves will actuate - but at the wrong time and may result in bending a valve stem. I like to dop a short awl into the plug hole to ensure I am at TDC when I install the cam and camchain. Helps to verify the position of the piston in my mind, even though the flwheel indication is correct. Afet I connect everything I turn it over with the cam cover off at least 3-4 times - returning to TDC to verify I am dead on prior to buttoning up. Last thing I do BEFORE installation into the frame, is to adjust the valves on the bench and then with the plug out, turn it over again from the flywheel to ensure it is suckin, compressin, and blowin, gets kinds a wa-pop sound (even with plug out) That way I know when it goes in the frame it is ready to start. I would ensure your carb is fresh and clean too and that the floats are set okay.