Originally Posted by
~McSmooth~
What we have found that the factory needle settings on this engine is a little on the lean side for the high needle, and rich for the bottom.
What happens during break in, is that when the engine is rich, you naturally start leaning out the high needle. This is what causes the the burn mark on the piston you describe, because we're all making that high needle TOO lean.
Start by leaning the bottom needle first, and you won't get the burn mark. Be very careful with that top needle.
Good general advice but this did not happen with my engine...it was never too lean on top. Unfortunately there is more for OS to address than factory carb settings and I think that this is fairly widely understood withing the racing community.