Most gas grills are only rated for 550, so if yours truly is rated for 650, it probably requires fine tweaking of the gas to air ratio.
However, a grill getting that hot will require a lot of propane. You maybe pulling propane out of your tank so fast for such a large burner system that is is cooling the propane tank down, which reduces the ability for it to vaporize and reduces the total flow out, resulting in lower temperature.I have experienced it a lot with camping grills and cheap or not properly balances grills at friends places, even ones that do not get as hot as yours is supposed to get (just huge so requires a lot of burners). Next time give the side of the propane tank a feel, if it is overly cold, this might be your problem. You may can solve it by just when cooking having the propane tank in direct sunlight. But you also might need a bigger tank than those little 20lb tanks you get and exchange at the home store and gas stations. Upgrading to a 30 lber might be required. Luckily, those are readily refilled for cheaper anyway than the 20lb exchange.
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