Hello again, I can't see that valve from here but if those 'screws' marked in and out, are actually 1/8" pipe plugs, (actual measure about 3/8" with pipe threads inside) then those are likely the gas pressure test ports
You can try connecting a U tube manometer to each of those as mentioned earler... do the hook up with gas valve OFF to the unit, then turn the gas on and see if the manometer reads 7 to 11" column of water at the IN plug, if so, thats the right plug for testing inlet gas pressure.
If thats not low, then test the outlet gas pressure with the U tube manometer hooked up first, then light the pilot and turn the unit on so the gas valve opens, and read that pressure, generally tthat should be in the 3.5 inch water column range,
I suspect it will be much lower, and that the cause will be a defective gas valve, Try adjusting the flame height knob, if that does not increase the outlet pressure the valve is bad... or the valves inlet screen is badly clogged, there is a very thin chance of that however.
There is some risk here, this work must be done by a trained gas appliance specialist who would use an electronic type gas pressure instrument instead of a U tube in most cases.... with a U tube if you drop the open end, all the water drains out and gas will come out the end of the tube, and if you do not shut the gas off within two or three seconds you could have gas entering the room. if you left it that way long enough it would be an explosion hazard.
I do not think you or your service man are likely to make that mistake but I do have to caution you,
The orifice at the fixed air mix valve is fairly large, it is not likely to be clogged,
I suspect the gas valve, and a reading well under 3 inches water column...
Notice page 17 with advice to call the dealer if the flame conditions are not right. That is because the manufacturer does not want home owners to work with the gas valve... the pressure tests i mentioned should be done by a trained gas appliance person. If that person finds the outlet gas pressure below 3 inches water column and the inlet gas pressure is over 7" water column, then the gas valve is bad and needs factory attention or replacement,
Let me know what you think, we can go from there,