yes, a 20* drop thru the rad is really minimal, are you sure the fan is close enough to the rad? 1/2-3/4", and pretty centered on the rad .... also yes, likely timing could use tweaking. although the backfire on shut down could also be the heat causing detonation

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics