Small Engines (Lawn Mowers, etc.): Exhaust valve opening on compression stroke., compression stroke, horizontal shaft

Question
Hi. I already asked and question and got an answer. Both are below, but I am still confused. Why would the exhaust valve be opening momentarily before TDC on the COMPRESSION stroke? That would let compressed un-combusted petrol/air escape into a hot exhaust!  Many thanks. Michael

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.
I got this 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

Answer
hi  

firstly  12 hp ohv  horizontal   is this a tecumseh or briggs  vangaurd engine

secondly  has the engine been apart at any time that may make you the cam timming may be out?

if a briggs ohv  you need to set the clearance to 0.003" inlet and 0.005" ex at 1/4 inch past tdc on power stroke to eliminate decompressor action
which does open the inlet valve slightly near tdc ( on briggs engines ) it is supposed to be there.

on tecumseh engines i set engine to tdc compression stroke ( inlet opens and closes fully  then tdc the piston)  set the clearance to 0.004"  it is supposed to be there

make sure that  on briggs  the alloy p/rod is inlet  (btm one ) and ex p/rod is steel  ( top one)
and both are straight no bends   

if after this  and it still does not work right or trouble running provide valves are not sticking and close  fully and seal combustion chamber  the the  cam may an issue  


cheers  paul