How to fix bugs rpm unstable

I bought this bike from a friend , I have replaced the ignition coil , a month before I broke all magnets in electric coil , and I was glued magnets without replacement new part . the use is normal , but the throttle feeling unstable , there is a delay in it , how I can fix it . Below is a video of me on the issue of RPM . sorry because my english is not good so I use google transtale
many thanks
[video=youtube;nEcBEGepMRk]https://www.youtube.com/watch?v=nEcBEGepMRk[/video]
 
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