Originally Posted by yondering
Woohoo, just took my 520 EXC out for a test ride after being down all winter. It now has a 540 12.5:1 kit and 8/33 cam. It's a noticeable improvement over stock; the power is still smooth with no hit, but it feels like the whole power band jumped up a gear. Before it would pull power wheelies in 4th easily, now it does the same in 5th.
Be kind to that motor, that was my favorite combo but I abused it and paid the price.
That 8/33 cam is all mid and top. On loose dirt/gravel you'll blow right to the limiter if you hold it, even in 6th.
Carry a can of octane booster. They didn't have super at that little gas station in the middle of nowhere or you just don't trust the gas? Dump it in the tank. If you run that 12.3:1 motor (12.5:1 but you unshrouded the valves) WOT on bad gas, this happens.
Combustion chamber ran too hot, valve seat came out, piston hit the valve...at 8000RPM...repeatedly.
As for the popping, lean out the fuel mixture. I think I went down one or two on the main. I know it sounds counterintuitive but the bigger piston creates a bigger vaccum signal so the stock jetting is too fat.