walter, check the needle valve and float in your carb fuel bowl, make sure that it is not stuck and clean. Check the jets and passageways for any debris. Also check for the gas tank lid, make sure it is vented. Check your fuel lines and fuel filter and make sure they are not pinched/cracked or dirty. Finally check to make sure your timing is right. Just because you have a spark does not mean that it is at the right time. Hope this helps, good luck.