Gene,
If you are reading 20 degrees on the start box,
then the thermocouple should be good. When
you push the start button do you get ignition
on? Then when you push the air and gas switch,
do you get a pop? If there is a pop does the
temp stay the same (20 degrees) or increase?
If you got a pop and the temp stayed the same,
unplug the glow connector and the temp should
increase and after 150 degree there should be
a fuel ramp and start up. I have several 750
and have had the same problem.

Johnny Woods