engine sound up and down

I have 2 video here , if i pull out one side spark plug power , it's running good . ( left side or right side , any side if I running one side engine sound is good ) . but if I plug in both side you can hear up and down engine sound as second video . I am very sure distance of both ignition coil to flying wheel is same . My friend said is not coil problem anybody can figure out which parts have problem ? Thank you.

I can't upload video or link here , if anybody want to look the video I can email to you thank you.

Your carb is dirty and can not supply enough gas to both cylinders. You can try to run some Seafoam in your gas or pull it off and clean with compressed air.

Thank you so much , I will try with Sea Foam first . Thank you :thumbsup::thumbsup::thumbsup: