Unlikely pre-detonation unless you are running 50% nitro. YS are relatively low compression engines.
Have you set the valve clearance lately? Did you replace the plunger with the diaphragm? It's also possible you are a bit lean.
What prop are you running and is it tight? I chased an engine problem all day once that turned out to be a slightly loose prop.

Another possibility is the rotary valve, did it get located back correctly when you had the carb off?
The rotary valve must go on properly located on the crank pin.