My furnace cycles on but only about 20% of the flame is on. When I blow on it, the flame spreads acoss the rails. Once its goes off after warming up the room, and re-cycles on,it does it again. Because it does not fully flame up, there is a slight gas odor. What could be causing this>

Could be

Low gas pressure

Dirty burners

Defective burners (usually rust or corrosion)

Blocked flame jumper rails (here again dirt or corrosion.)

It might be time to have your furnace professionally serviced by a local HVAC company.

What causes low gas pressure?

What causes low gas pressure?

Lack of gas,, could be dirt clogged,, could be at the main outside meter regulater,, the gas valve in your furnace,,