Here is a picture from my Haynes Workshop Manual, your issue is a simple fix by means of adjusting the pilot screw mixture on the underside of the carb.
At idle simply screw in the pilot screw until it is LIGHTLY seated, don’t wind it in hard as it will damgebthe tip. Back the screw out according the manual reference or until it starts to run correctly.
Correct fuel mixture is identified by looking at the spark plug colour, your after a nice beige/Tan/Tea type colour on the tip of the spark plug.
Hope that helps
Edit: Just realised the manual reference says ‘turns in’ from standard setting. Standard setting is 2 1/4 to 2 1/2 turns out.
At idle simply screw in the pilot screw until it is LIGHTLY seated, don’t wind it in hard as it will damgebthe tip. Back the screw out according the manual reference or until it starts to run correctly.
Correct fuel mixture is identified by looking at the spark plug colour, your after a nice beige/Tan/Tea type colour on the tip of the spark plug.
Hope that helps
Edit: Just realised the manual reference says ‘turns in’ from standard setting. Standard setting is 2 1/4 to 2 1/2 turns out.
