I have a coleman evcon model 7970c856 presidential manufactured in 1991. I have had no repairs other than replacing the pilot light thermocouple. My problem is that the furnace takes way to long to get hot enough to activate the blower motor. In times past, when the gas valve opened and the burner lit, you could hear the robust flame. It appears that the thermostat works fine, the gas valve opens and burner lights immediately, but the fire on the burners is not as robust or hot as it used to be. I pulled the bruner out, it is remarkably clean , i uses a wire and canned air to clean it and the gas nozzle. When i placed it back in the furnace, it does the same thing. Why would the gas flow to the burner diminsh? I noticed the performance diminishing toward the end of last season, as it took longer to get hot enough to start, and would occasionally shut the blower off, get hotter and kick back on. The forced air at the end of last season was not as warm as in the past. Now , it takes 5 min to slowly heat up, blower kicks on, then kicks off in 60 seconds, warms again and briefly kicks on and repeats. If you turn the fan on manual, burner lit, the air warms so little its hard to notice the difference. I originally thought limit switch, but the issue is the fire on the burner isnt hot enough to get the temperature up and keep it up when lit. Can a gas valve wear , and decrease the gas flow presure to the burner?