like 7 said it could be a lot of things lean air/fuel mixture a crack in the rubbery part of the intake boot can do that over time. Hence the reason it blew before you had the work done. or as simple as a bad gasket any where in between there and the motor, (however unlikely) if the owner before you dident use all new top end gaskets. Wellllllll since you already got the head off I'd sudjest taking the cylinder off and pinch the top of the rod without the piston on since your gonna have to replace it (most likely) with your pointer finger or pinky cause the finger will fit snugly like the bearing only your gonna feel if it pull left or right if you go slowly, also looking to see if the rod wiggles even minutely side to side. Not up and down while at TDC Top Dead Center of full stroke. If it dosent then check the pin bearing in the piston and see if it wiggles up and down. But looking at the vid I'd say your bike ate the ring up a bit thus allowing that wiggle. Also look at where the rings are at cause the ball bearing lodged in the piston is at it's a marker as to where the gap in the piston ring is supposed to be. Also look to see if the rings were put in upside down every piston ring in existance should have a T for top stamped in it in the middle of it. Cause some are made with a rounded off edge to it. Last and finaly least with the cylinder off if it needs bored you can feel with the tips of your fingers in the cylinder mabey a quarter inch or more a groove if you feel it your gonna deed to bore it out some if not your gonna nead a hone to get the rings to seal up nicely. Also a break in typically consists of idle running for 30 minitues mid throttle for 20 min and 10 minitues of upper throttle changing the plug each time just to ensure your jetting correctly and that it will fire up without a hitch. Then after all that let it cool down for a day conservitly 6 hrs then leak down test it before and after aswell as a compression test afterwards a shure fire way to tell if your rings are sealing is if you get a spike in compression after you spray the wd40 in threw the spark plug hole about 6-10 kicks are needed or untill the needle on the guage stops mooving up. if the bike is still slugish after the break in only go about half throttle for about 30 minitues and do another compression test, that I just explained about. Do all of this and mabey a vid or 3 and your gonna fit in great arround here good luck.
