My spark plugs looked terrible when i did a plug shop after I changed from br9 to br8.
How can one spark be leaner than the other (rear cylinder)?
It also have a small bit of molten metal on it.
I guess I need to tear it down and check pistons?

You definitely want to look at your pistons. Depending on state of tune of the motor, you can have staggered jetting (Example: 145 mainjet on rear and 140 on front). An air leak on the left side of the engine could contribute to a lean condition (left bearing crank seal, carb manifold).
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MC28SP, '16 KTM 500EXC, '16 Beta Xtrainer, '07 BMW 1200GS, '96 Ducati 900SS