Rob. 20f is cold, but not cold enough to hurt anything imo. warming the motor deffinatly helps when starting in cold though. but i m gonna say your needles are a bit rich. mine was when new. turn your High Speed Needle, HSN ( screw in brass coller where the gas input hooks to, clockwise about 1/8th of a turn to LEAN it up a touch. counter-clockwise will Richen the mix. when you pull the ignitor, your getting more fuel than needed, and the fuel is cooling the glow plug and stalling your motor. you may need to do this a couple times. but i figure once or twice should get you running when you pull the ignitor.

theres a GREAT online guide on tuning, but i cant remember the site and have the lost the link i had. if i find it ill repost. the NX-15 is a gr8 motor imo. mines been working well for near 4 gallons.
Timmahh