It's really hard to troubleshoot without seeing the bike and being the one testing it. Knowing what the mech. has actually done? With that being said and how new the bike is, I would definately check the pipe for an ubstruction. A friends kid was trying to help him work on the bike by putting tools in his pipe, you never know? Someone else mentioned a broken baffle, I think it was the same pipes? You said you had spark, but is it at the right time? Or all the time? Maybe a bad pick up coil? Someone else also mentioned the coil wire on the rear cylinder being loose?