Look at the timing chain. There is an adjuster on it.
It is located on the cylinder and should be protected by a cap. Rear nearside of the barrel.
Take the cap off
There is a rod in the center and the rod and threaded part should be flush.
If they are not flush, loosen the big nut close to the cylinder and the turn the smaller nut at the end of the threaded part until it is flush with the inner rod.
Tighten up the big lock nut and the replace the cap.