[scotth1981]

Day & Night (Carrier) Natural Gas Furnace Model 394JAZ036065AAFA with intermittent pilot (3-wire model 740A).

Problem 1: 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 2: Sometimes before the pilot has a chance to light, 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 before it has a chance to work.

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?

[scotth1981]

Here are a couple easier related questions: How long should it take for the pilot to ignite once the spark starts? And

How long should it take for the lockout control to shut off the spark if the pilot fails to ignite?