do u know how to set the piston at tdc
u cant possibly set valve lash without a feeler gauge (or get it within sepecs anyhow)
id say u have attempted to set it at some random place in the cycle and have built up too much compression
remove the spark plug and that should do it but be warned if u have pulled and pulled then u have built up alot of pressure so the spark plug might come out with a bang
its not dangerous or too powerful but just be ready for it and dont use a spanner use a socket
to set the piston at TDC u must turn the engine over till the piston is at the very top of the compression stroke
as u turn the engine over watch the valves and remove the spark plug to watch the piston
every 4 stroke motor has the same patten
intake
intake valve down,, piston goes down
compression both valves up, piston comes up (TDC)
power
both vales up, spark fires and piston drops
exhaust
exhaust valve down, piston comes up
the other thing u may have done is tighten the adjustment screw down with the valve half down
so as u pull the starter string the cam is trying to push the rod up but the valve is already bottomed out
id advise u remove the spark plug and wide out the adjustment screw then buy a feeler gauge and complete the test properly