'03 YZ450 w/ 13.5:1 piston

I ahve an 03 yz450 with a 13.5:1 piston that I just installed in it. The carburetor is completely untouched other than a MSR fuel screw. When I fired the bike up for the first time after the rebuild, it appeared to be running very lean. It would backfire out of the pipe every time I let off the throttle. What would be a good baseline to start my jetting from when switching to a hi-comp piston? The bike also as a full pro-circuit exhaust, and a powernow. My elevation is around 1100 to 1200. Thanks

-Paul

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