If you crank the ambient strength of your visible prop high enough to cast light, all you're going to get is a flat appearance, losing the look of depth.

     Give your visible flame prop a gentle ambient value, maybe 0.7-1.3

     Use an unseen prop (un-tick visible in camera) with high ambience (10?  30?  70?) to cast light.

     For intensely bright things like flame or lamps, I like to also add a faintly visible aura.

     There are a couple of threads here in the forum on this topic.  I have a simple torch model in the freebies which uses this technique.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5