I fired up my heater for the first time this season and it runs in pulses, like it's too rich and won't stay lit, on-off, on-off in 1-2 second "puffs" - is there a mixture screw or something that I need to adjust? I can't find my original paperwork on this but it is only 2-3 seasons old. thanks

Heating & Cooling - Mr. Heater - MH75KT