you could be right by thinking the length of exhaust is the problam because we had a few c90s to play with and we made an exhaust for mine using diferent length bmx handlbars and a kr1s end can and we found the short one was doing the same as your describing but the longer pipes cured it and in the end we settled on an in between that gave briliant midrange and sounded like a crf
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ive seen more tarts than mr kipling