I've heard that if the butterfly valve in the carb is not completely closing, it will allow fuel to get into the cylinder and create an ignition. May check that the carb adjustment is correct.
As I have heard, the linkage in the black box should be adjusted (the 10mm nut on the inside, and the one on the outside) to have the linkage at the carburetor to just begin to try to open the butterfly value. By running threads in and out of the black box, the carburetor is affected.
I am no mechanic, but I have had this problem, and have had lots of advices on what to fix.
So, I thought I'd give you the 2 cents worth that someone gave me - pay it forward so to speak.