NormK, I think you just found that the GCV160 was designed for bar blade use which more like a heavy flywheel. Most here have heck kick back without the blade being installed.
Now I do know you guys are more into swing back blades so most likely as you found by adding extra is the root cause.
As always things work great when new but as wear happens things can go AWOL.
Just got thru fighting a problem on a Briggs that came with a broken ACR. Mower ran great until the customer got home. Then it start heavy surging. I actually tore the engine down three times trying to find the problem with no results. Finally for some crazy reason I decided to try a new spark plug. It is was the root cause of the surging. I felt so stupid...
