I need a parts schematic and users manual for an armstrong sx90 furnace?

I have an Armstrong SX90 propane furnace. The ignighter gets gas and lights for a short time but the burners do not get gas to them. I have checked all the drain lines and they are free from obstructions. I cleaned the vacuum port to the burner box. I replaced the ignighter. Any Ideas?

Need parts schematic / users manual - armstrong sx90 furn.? M. # guk 075d10-3a

Where can I find a good schematic or diagram of the parts within a Heil furnace? Also, when purchasing parts, do they come with installation directions? I always want to try the initial install myself rather than paying for service calls first.

I was just informed by my HVAC guy that my 10 yr old Armstrong Ultra SX90 furnace has popped a rivet on the heat exchanger. He says he is seeing this more regularly with these furnaces. He said he can replace the heat exchanger and told me to think about replacing the whole furnace because in the...