German, I assume you have a carb on yours. If so, try adjusting the automatic choke first. Turn the black plastic choke cover to align the marks; one is on the cover, the other is on the housing. This is a starting point, you can fine tune from there. Also make sure the choke linkage is free and lubricated. Make sure the choke plate moves as you move the choke cover. If it does not, the bi-metal choke spring may be broken, out of place, or sticking.
The second thing to do would be to make sure the idle mixture screws are adjusted properly. To do this, turn them both in until they bottom (counting the turns for reference). Then turn them back out until you get the fastest idle, about 2 and 1/2 ..........3 and 1/2 turns. The best way to do this is with a vacuum gauge. You just turn them out until you get the highest manifold vacuum.
Hope some of this helps.