[Juanlap]

I have search everywhere for the answer but nothing conclusive
I have just fitted z40 cam to yx140 but can't get timing marks perfect it's about half a tooth out ,I have put the mark below the timing mark on the head it will start but backfires through the carb a few times and kicks back but will run
Now I'm going to retard the timing a tooth so it sits above the mark on the head I,m sure this will sort it but if I slotted the cam sprocket I could get it perfect or am I being stupid
Oh before you ask yes the woodruff key is in good order