That is the sort of problem you get with a cracked timing key in the flywheel or a carb with no main jet.
But a full service would not hurt
So start with the mechanical things like valve lash .
Then check the key
The check the carb by doing the following.
Sitting outside your shed, manually open the throttle fully by moving the governor rod with our fingers.
Watch where you put your other hand .
Let us know what happens .