What type of carb is fitted? A photo will help identify it.
Is engine throttle controlled or does it rely on just the governor?
If it has a throttle cable going into top of the carb, there should be an adjuster where the cable enters the carb, there is also usually some adjustment at the throttle lever end.
Is the tick over screw set correctly this prevents the throttle butterfly closing completely, if it has one (depends on type of carb).