Day & Night (Carrier) Natural Gas Furnace Model 394JAZ036065AAFA with intermittent pilot (3-wire model 740A).
Problem - Sometimes it works fine, but more often than not the pilot will not light unless I blow on it with a blow dryer while the electrode/ignitor is sparking. After just a couple of seconds of blowing, the pilot lights and then everything usually works ok.
Problem - Sometimes before the pilot has a chance to ignite or even shortly AFTER it has ignited, the ignitor shuts off after a fairly loud "clunk" sound. This can happen before the first spark or after just a couple sparks. Whatever makes the clunk sound seems to be shutting off the ignitor and the gas before it should.
I've already replaced the thermostat, the electrode/ignitor, the pilot assembly along with the aluminum supply line and compression fittings on both ends, and the gas valve, but none of that helped. I also inspected the circuit board inside the ignitor/lockout control box, but found no obvious burnt out components. I measured the spark electrode wire resistance at 0.5 ohms, which seems pretty good.
What could be causing these problems?