If it were my bike... And i got the comp. numbers that u have. I would have the cylinders bored. New pistons, and rings installed... If u were gonna port it then would be the time.. i would also split my cases to ensure my crank and tranny is in good condition. U dont need to split cases but i would want to just to ensure everything lookds good... If ur scared to do the topend urself get a clymer manual and it will walk u through it step by step.. Otherwise We on Hq should be able to help with what is not described in there. Find urself someone to bore ur cylinders and u can do the rest... Trust me its not hard at all.. I dont think u have a carb problem.. and no the petcock wouldnt cause the issue u are having. But low compression would.. Just my .02