The popping noise can be a non seating valve but usually if this is the case the engine uses a lot more fuel.
The popping noise with the motors I've had is mostly a sign of weak spark meaning the spark plug is faulty or the coil is faulty,the correct method of checking the ignition coil is with an oscilloscope .
I would change the coil for one that you know is good as long as the plug doesn't have a problem.
If changing the coil still doesn't fix the problem at least you have ruled that out as being the problem.
