Heil / Honeywell "smartvalve" gas furnace issue

The culprit: Heil propane furnace (H 9MPD1 series) with Honeywell SV9451 smart valve, running on LPG

First problem: no pilot produce during ignition cycle. I took the tube to the pilot off, and carefully cleaned the jet (in the past, speck of rust or corrosion have blocked it, and caused a similar problem). This didn’t work. Next step was to adjust the pilot flame using the set screw. At this point, I got a nice, steady pilot, and the furnace came on as expected. I watched it over the next few cycles just to make sure everything was OK, and then noticed a different problem.

The problem now is that on some cycles, especially when the furnace is starting up after not running for a number of hours, only three of the four main burners ignite. Doesn’t happen everytime - when I see this, I shut down immediately, wait minute, and then start up again, and typically all four burners then ignite. The one that fails is the one farthest from the valve, at the end of the gas feed pipe. When it does ignite, it produces a strong blue flame just like the other three.

Any ideas?