I am looking at an 04 LTZ250 that has an aftermarket Yoshimura exhaust. The quad is very clean. My only concern is that the tip of the muffler has some black soot (not oily). When revving the bike moderately I see a small puff of black sooty type smoke. The seller suspects the quad was not re-jetted after the exhaust upgrade. This was supported by the fact the plug was fouled and needed a new one for it to be started for me to ride. Is the puff something to be concerned about or is it just the mixture is wrong?

Thanks,
Jon.