My Furnace Is Not Working. I Do Have 24 Volts And When I Turn My T-stat To Heat The Igniter Lights ,I Have A Small Flame But The Gas Valve Doesn't Seem To Be Working. No Heat. Just Flame

Model #, age, efficiency would be very helpful. What do you mean by small flame, a pilot light? Is there a green, yellow, and white wire going to the pilot assembly?