Small Engines (Lawn Mowers, etc.): Tecumsah 12HP, horizontal shaft, compression stroke

Question
Hi. This is an OHV 12HP engine, horizontal shaft.  When setting the valve clearances and turning the chank to TDC, I notice the exhaust valve is momentarily lifting as the piston is approaching TDC on the compression stroke!  It is only opening a tiny bit, but to stop it I would have to have the valve clearance at about 0,018"
I am guessing there is either a decompression mechanism to help cranking (unlikely?) or a faulty ground cam.  Before stripping the bottom end would like your opinion.  Many thanks.

Answer
It is the mechanical compression release...it is supposed to be there.  

Valve gap spec link:

http://outdoorpowerinfo.com/engine_specs/tecumseh_valve_gaps.asp

If you pay very close attention to the valves you will notice both valves are just slightly open near TDC, this is the valve overlap.  Overlap exists on almost all 4 cycle engines and is a tell tale sign that the mechanical timing is correct.

Eric