This could be for few different reasons. Check the following:

-make sure the exhaust bolts are tight on the cylinders, the crush gaskets may need to be replaced if not installed right
-check your fuel mixture ratio, I bet you are running lean if it is popping a lot, pull your spark plugs and see what color they are. If it is light brown your running too rich and need to adjust your mixture screws. you can choke the engine also and see if the popping decreases.
-one last thing (not the easiest to determine) is check to see what jets are used.