Suburban Furnace Gremlin?

[rdownward]

I have a strange situation with my furnace. As long as I am not below about 750 ft. elevation, the furnace works fine. Once I get down to below that, the furnace will not ignite. The fans go on and you can hear the ignitor trying to ignite the gas, but it will not fire up. I have checked and rechecked every part that effects the turn-on function and have replaced the gas valve in the furnace as well as the regulator valve at the tank. Again, the furnace works perfectly fine so long as we are above 750 ft. elevation but below that, it will not ignite. Since I live at around 3,200 ft. elevation, when I am able to work on the thing, it works fine. After replacing parts, etc. I can only tell if the problem is fixed when I physically get the coach down to near sea level. Anybody got any ideas as to what could be causing the problem? We rely on the heat pumps when the furnace won't work but they don't work very efficiently below about 40 degrees (and I don't like running the generator all the time when not hooked up to shore power) so we have been relying on portable electric heaters to keep things manageable when the furnace won't work. Again, any suggestions regarding what could be causing my furnace issue would be appreciated. Anybody know how to scare away "furnace gremlins?"

[RWeigant]

I've worked on these for years and the only thing I can think of is too much air, or not enough fuel. At lower altitude you would get a more dense air. Try and loosen the sliding air intake and see if it was adjusted for your altitude by someone in the past.

[rdownward]

Thank you R Weigant for the tip. I will check the air adjust slide as that is something I have not done in the past. I appreciate your input.