Empire DV-35 low heat low flame

Yellow flame is a sign of contaminated burner and/or air starvation and proof the appliance is producing excess free carbon and carbon monoxide.

Propane is a carbon-rich heavy gas that will soon produce free carbon if combustion is imperfect and all it takes is a spider web or some miscellaneous fluff to reduce the amount of air flowing into the mixing chamber.
I am sorry but I am not familiar with your heater and have no knowledge of an air flow bolt but the mixing chamber/burner must be removed for thorough (periodic/regular) cleaning and a vacuum cleaner often isn't enough. I find compressed air much more suitable...

This normally is caused by low gas pressure, or some blockage in the burners. Do not run this heater until it is fixed.

low gas pressure, clogged burner orifices

The reason the control valve is opening slowly is the the Thermopile needs to be replaced. The Thermopile generates millivolts of electricity from the heat of the pilot flame, which then opens the gas valve when heat is called for. Also, check the pilot flame to make sure it's engulfing the thermopile. If it's not the pilot needs to be cleaned.

The Empire Comfort Systems, Thermopile part number is 672063

If I can be of further help, please let me know through the leave a comment link.

Could be the reg on the gas line not fully opening to let full gas to heater.

Whats the main flame like...strong or just weak? check the shut off valve and see if there might be another one around too. Probably a dirty clogged orfice or something blocking the gas from full open. maybe the main valve is going bad...try a few light taps and see if any changes. hows the wall thermostat? old? try bypassing that and see but i doubt it has anything to do but ya never know. good luck

no service on gas valve by diy er it a bad connection elecs or the valve is sticking on the seating , i would say to change gas valve i would not do diy on this valve ....

The thermocoupler is no good. It's that bulb sitting in the pilot flame. when it goes out it shuts everything doen in the main valve. It has to be kept hot by the pilot, that's how it know whether or not it is safe to open the main .

sounds like heater needs to be cleaned . I would recamend that you call a heating and cooling company to come out and take heater apart and clean. You can take front panel off and make sure you are getting go airflow through bottom of heater. If you feel comfortable with cleaning the heater yourself you can buy a burner brush at your local hardware store. If you decide to do this let me know and i can try an and help you with what you need to do. If you have the manual it will also tell you what to do to clean.

Either a restriction in the gas train, low gas pressure or a bad gas valve. Check the pressure on the gas line when on pilot and again when the burners are running. Depending upon what type of gas it runs on, you should have at least 4" W.C. if Natural Gas and 10" W.C. if Propane. If gas pressures test good and all valves are open, you are looking at replacing the gas valve.