Spark plug condition and backfiring

[Dave in Denver]

I have a 1930 Tudor, 4 banger with Snyder 5.5 head, fed by a zenith updraft and sparked with a petronix distributor. Living in denver at 5000 feet.

Been trying to get this rebuilt engine running. I am certain my timing is off a bit, but the real issue seems to be the spark plugs. I gapped them at .035 and started the car, adjusted the timing (approx 20 degrees advanced) and the carb but at anything over 1200 rpm it backfires through the carb.

To check things, I pulled the plugs. #1 and #4 are very black sooty, while #2 and #3 look normal.

Question: might I have some sticking valves on #1 and #4?

Or what else might be happening.

Thank you in advance.

Dave