cossy timming marks the 2 arrows on the cam pulleys that face each other 1 all ways seems to move off slightly when I tension the belt so I put verniers pulleys all sorted now. but why is that .it only moved very slightly car run perfect even if it was like that but. why

 

because the distance between the pulleys isnt an exact number of teeth on the belt.

especially if head has been skimmed

 

becase there was slack in the belt between the 2 pulleys when you tightened it. it needs to be tight, then you release the tension on the belt.