Std Pipe To Open (poor Performance)

hi team t135/t150,

im using a standard pipe for my snipey and it runs good with my 62mm bore but when i changed my pipe to open pipe, a lot has changed. every gear is like getting choked or limited and i can hardly reach 100kph.what could be the problem?i have racing cdi, vsprings,cams and 28mm carb...im getting confused since big elbows are meant for big bores right?

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