I have a 1991 Rheem gas pack (self contained furnace/AC). The Ac works fine but when I turn on the furnace it either comes on or does not as if it has a mind of its own. When I adjust the thermostat setting to start the furnace the ignitor sparks but the burners do not turn. The combustion vent fan does come even if the burners do not ignite. I have gas and power coming to the unit.

Could be a bad flame sensor or could be it just need to be cleaned. When you open up the unit most of the time you will probably see much rust etc.

If it does not have a flame sensor it may have the old White Rodgers control.

I have no drawings for that unit available to me especially since you did not bother to list a model number.

Hmm spark but no burner,, dirty pilot maybe

I believe on that model the pilot proves the flame.
So we need to follow the sequence of operation to where it stops .

See the faq that should help

It sounds like the gas valve in the main gas pipe inside the unit, the pilot light doesn't operate through the same valve as your ribbon jets do so you will see the pilot light come on but not the jets/burners. I on the other hand just had mine replaced this past winter and now my AC blower won't work the compressor and outside fan works just fine but the blower that pushes the air ducts will comeon for 2 or 3 minutes when I initially turn on the unit but it will stop working, everything outside will still be running just no air from blower, any ideas on that?