We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Ernie Bilko
saying something like:

has been producing a lot of smoke and soot when it runs, now it has
started cutting out - no doubt the photocell is filthy. I was about to
get it serviced anyway but as it has been running fine for 11 years
with just a regular service I was wonder what would have caused it to
burn this way. Why is it producing such muck and what is likely to be
needed to fix it.

Worn jet, worn pump, crap on the fan blades, other air restriction, low
motor speed, flue restriction, take your pick.

Check the basics first - it might just be dirty blades.
--

Dave