It is on LP.
First, I don't believe there is a 280 in Garland. If a range with 4 burners and a 36 inch griddle, It would be a 10- something. Rguardless. The first question I have to ask is did the mixture change suddenly or over a long period of time? Suddenly problably means that the pressure regulating valve is out of calibration or broken. To determine this requires a tech with a manometer to measure and calibrate.
Other causes are dirty burners or air intakes to the carburetors. Pinched gas lines Clogged venting. and even low O2 in the kitchen from inadequate venting.
Safest and best solutions is to bring in a gas tech. Your LP supplier will check it for nothing as they are required to check all installations every other year. Once you know manifold pressure is 11 inches, air mix can easily be calibrated by rotating the air cover on the burner.
BTW, I only know of one restaurant at 7200 ft. Colorado rockies?
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