Incandescent Christmas light help!

I have two incandescent c9 strands and I took some bulbs out at the end of the strand when I ran out of roof line. I have them plugged into a lightbulb adaptor for a porch light and about 2 hours into them being turned on I started smelling burning rubber and things looked a little hazy on the porch. I turned the light switch off for now, but I’m wondering if I’m not supposed to run incandescent strands when I have some bulbs taken out of the strands - or if it’s possibly my janky lightbulb adaptor situation not being able to power 2 25ft incandescent strands. Any help from someone more electrical savvy than me appreciated!