I am getting a knocking noise from my ATV when running. I purchased it two months ago and have slowly been bringing it back to life. Top end rebuilt with new piston, rings, and cylinder honed. Also recently rebuilt the carb with a rebuild kit. It is finally running but is running only on full choke and barely running with a knocking noise. I used a mechanics stethoscope and it sounds like the knock is coming from the cylinder or cylinder head. Any suggestions on what the knock may be? Please watch the video. 2008 Yamaha Big Bear 250. Thanks!

 

Is it definitely a knocking or possibly an exhaust leak? I'm kinda deaf, hard to tell.

 

Old Polaris Tech, please see the attached pictures which looks like there isn't a flush connection to the cylinder with the exhaust. I unbolted it a few times to try and make it flush but I can't seem to get the bottom in any further. Does this have to be flush? Also are the black deposits on the exhaust valve cover evidence of an exhaust leak? There also is some oil around the bottom of the exhaust vave cover... I started up the engine and poured a little baby powder next to the cylinder - exhaust connection and it appeared to blow away... I really appreciate any input.

 

Once you compress an asbestos exhaust pipe gasket then loosen it,you'll never get a good seal again like it was when new. Same thing when people try to reuse a head or base gasket.I'd replace the exhaust gasket and by the looks of it order a new o-ring or gasket for the exhaust valve cover that might be leaking.