Opening the shutter leans the mixture; closing it enriches it.
Crud in the stack prevents the propane from flowing naturally into the combustion chamber making an over rich mixture as well.
Someone may have tried to fix an over rich mixture by opening the shutter.
There should be a "starting" shutter opening amount in your operators manual. If not, check the installation manual.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW