Hi my friend it can hapen for probably 4 reasons i am going to numerate in order of probabiliti.
1.- becouse your hi speed adjusment is too rich or too line try clouse all hi speed adjust and then open it 2 and 1/2 turns and try start the engine and se wat hapends.
2.- becouse your plug is bad check your plug and be shure is ok.
3.- Becouse you carb is taking air bubles from somewer. check this and be shure of this.
4.- I ting is realy not probabli becouse your fuel is bad or your low speed is bad adjusted wen you move the low adjust the hi adjust change and wen you move the hi speed adjust the low speed adjust have to be readjusted you have to find the perfect configuration for correct runing engine.
I can not explain wou to adjust this becouse you have to hear the engine and for the sound that the engine make when you trotle is the adjust you have to do for it.
I recomend you find information for youtube you can lern alot thear.