New guy from Wyoming

[Popps]

New guy. Bought an insurance sale 34' Monarch Medallion several years ago. Never camp when it's cold until now. I have an Atwood 8535-IV DCLP. Motor runs and you can hear the spark fire . It burns for a couple of seconds and shuts off. Pulled the burner adjusted the spark gap, checked gas orifice (clean and clear) and put it back together. Stays lit for 5 seconds and shuts off. If I block exhaust it will fire and continue to run but does not get very hot.

Anyone have any idea. Searched the forum and found similar symptoms but not exactly the same

 

[Old_Crow]

I'll be watching this. 
I've got an 8520-IV that's doing the same.  I pulled  the burner and found over half of the flame spreader burnt away.  I figure the flame isn't hitting the igniter probes because of the big hole in the burner.  Part's on order.

 

[Gary RV_Wizard]

The most likely cause is some debris (e.g. rust flakes) or insect nests in the air intake or exhaust tubes. These furnaces are extremely sensitive to air/fuel mixture and any disruption in the flow will cause problems. Next is a damaged burner box, as Wally Crow described.
If the burner lights and runs several seconds before shutting down, the problem is the flame sensor, a safety device that verifies the flame actually lit. If the flame sensor doesn't report success, the controller board shuts off the gas valve.

All this and much more is covered in my library article on Furnace Troubleshooting