I assume your leaving it outside overnight? They need warm air in the airbox to help vaporize the fuel. If you are able to come up with an electric heater that blows warm air, aim it under the airbox for 10 min before you try and start it. Don’t put it directly under the box incase it drips fuel. If there is any wind you need some sort of wind break. Note of caution, don’t twist the throttle or move the collective up while the heater is under there. (Dont need any fuel dripping) Of course make sure the heater isn't blowing hot enough to melt anything, or blister any paint.
Make sure you have everything organized for the flight and last thing you do is remove the heater. A few twists of the throttle with the collective up to prime it, and start normally.
Many moons ago I was unfortunate enough to be stuck overnight in the bush due to snow and zero ceiling. We woke up to 3 inches of snow and temps well below freezing. I tried the Robbie a few times and it didn’t show any intention at all of starting. We were about 150 miles from the nearest road. We lit a fire (away from the heli) and stuck a shovel filled with sand in the hot coals to warm up the sand. We then pressed the warm sand against the bottom of the airbox with the shovel. It started immediately.
The heater trick is kind of a last resort if you cant roll it inside overnight. Darn primers never seemed to make much diference anyway from what I can remember....
DMNH