vanguard 18hp engine

I have a briggs and stratton 18hp vanguard. I have installed a kit to let me run from propane, natural gas or gasoline. I understand that gasoline has a cooling effect on the exhaust, that is lost when i'm running vaper. Exhaust temperature runs about 850 degrees F. on gasoline. As I adjust the blocking valves on my kit, the exhaust temperature varies with my setting and so does the power output of the engine. My question is, What is the temperature range I can run the exhaust on this engine without burning an exhaust valve???

Thanks