Soot in fuel servo

The fuel shop just called regarding my sticking fuel servo. The unit
has 900 SNEW 10 years ago. The shop said they did a full overhaul on
the unit. The only oddity they found in the unit that may explain the
hanging is soot in the air side of the unit. Since air flows across the
unit I'm assuming the soot could only have come before the unit,
meaning the boot or through the filter. Has anyone else had problems
with soot in the fuel servo? I noticed the unit looked "dirty" in the
throat when we took it out even though it had just been cleaned before
the previous flight.
I didn't see any leaks in the boot.

-Robert
M20F