It's my impression that the decompression system causes the intake valve to slightly open just before TDC at slow cranking speeds. Is there some way some part of the decompression system is "sticking" due to being dirty (sludge? carbon?), or can ethanol (E10) fuel cause any problems? Can improper valve lash add to the problem? Are parts of the decompression system accessible from inside the valve/cam cover, or is further disassembly required?