Clean and gap your plug to 0.9 mil.
Fire the bike up and take it on a run to get it hot. Bring it home and put the centre stand in a chock of wood to lift the rear wheel clear from the ground.
The carburettor has two screws
The large one for idle speed and a small one for air mixture.
With the bike running turn the idle screw fully clockwise.
The engine speed will increase and the back wheel will spin.
Next turn the small (air) screw fully clockwise (which will block the air off)
Quickly turn it counter clockwise one full turn.
This will prevent the engine from stalling due to air starvation.
Now, here's the important bit!
Continue to turn the air screw counter clockwise in half turns as you listen to the engine.
The aim here is to make the bike rev as high as possible.
Pause for up to twenty seconds between adjustments to listen.
Eventually you will let too much air in and the revs will drop, at this point turn the air screw clockwise to build the revs up.
Once you find the spot where the engine is revving its highest, turn the air screw a quarter turn clockwise to slightly richen the mixture.
Then put your screwdriver back into the other (idle screw hole) and turn it counter clockwise until the engine is just ticking over normally
Your mixture is now set. Any future adjustments to idle speed will only involve the large screw