I could be wrong, but it sounds like the furnace
is trying to light
the main burner, but does not have enough air. You may need to remove the whole furnace and clean the air intake that runs down the back of the furnace.
At this point it should be noted that the odds of successfully removing the furnace are probably less then 50/50. The outside vent is installed from the outside. There are two concentric ducts. On the inside is the exhaust tube. Surrounding that is the combustion air intake. These are supposed to telescope so that if you remove the rivets on the outside, the whole exterior vent slides out in two pieces. The trouble is that the exhaust vent get very hot, and this causes it to rust. Typically fusing the two pieces together. The outside combustion air duct will likely still slide, but since the inside exhaust part will not fit through it, (without doing some damage) the combustion air duct is stuck as well.
Some have had luck using PB Blaster and letting it soak in for some time. I currently have all of mine soaking:
PB Blaster Penetrant | Canadian Tire
You can down load the manuals for your furnace in the document center: