Engine Jerking At High Speed

[cedinko11]

Hi.I have a dirt bike with loncin engine and it just worked fine for 1 year with no big problems...but my old spark plug broked so i bought new(not the same cause i couldnt get it) and now engine jerking at the high speed...top speed before was 95km/h now its around 50km/h...write any solution if u had same problem and ill try it.Thanks for help.

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[nickv1980]

ahhhhh wrong plug........ just thought id throw that one out there !