i went to adjust the mixture of the front carb. then when i was about to turn the nut, black oil was all over my fingertips. i check the rear carb for leakage and there was none. where did this oil come from? and how can i fix it?

Do you have the hose running from the block breather tube to the PCV valve in place?

Typically, it's not oil. It's gas seepage with dust and dirt to darken it and make it look and feel like oil. Also, the front carb is near the block breather tube, which can leak and be caught on the front carb area.

When you choke it too much, the overflow will travel down the fuel valve and collect on the adjuster nut and tube. Don't worry about it.

Dave