Which are the major causes of burn exhaust valves in any gas engines?

Wrong fuel/air - ratio, disturbed cooling water flow to water cooled exhaust gas valve seat 8speaking of big engines).

improper combustion, overheating, valve not seat properly, wear tear with passage of time, improper material. deposit build up and aging fatigue.

In sufficient Combustion air, Condensate in the Fuel Gas, Choking of Air Filters, sticky valves...are the main causes of burning of exhaust v/vs in gas engines...

in combustion engine the force applied on piston makes chemical energy  then into usefull mechanical energy.

1/Anything that causes incorrect valve seal Cooling issues (improper cylinder head cooling)

2/Quality of fuel? (carbon deposit on the valves)

3/Concentration of heat at a particular point on the valve during opening (for exhaust valve) due to improper design.

I THINK THE CAUSE IS MISS ADJUSTING OR LEAKAGE  OF MAIN FUEL VALVES ACCORDING ITS LOAD AND PRECENTAGE OF COMBUSTION AIR

Improper air fuel ratio, poor valve seat or sticking valves and or material defect, out of timing

Main reason is air /fuel ratio problem , lubricant oil is condition is very bad condition or low .Improper operation & semitalent opertors may be done

The cause of the exhaust burnt valve is the excessive mass flow of fuel that damaged the internal hard ware of the engine.

The valve material is absolutely design to a specific heat rate that can withstand its own specific temperature might not exceed the heat rate, if the temperature exceed it maximum heat rate it can cause damage to the internal ware. Gas fuel pressure need to be maintained at its own design temperature, heat rate, pressure base on the vintage of the unit circle parameter. Corrosion also play its own role but is minimal for inspection period. 

The exhaust valve can be burned due to the occurrence of corrosion in the valve base, which acts as a non-lOCK

The exhaust valve is thicker than the exhaust valve and is made of high temperature bearing material SO IF IT MADE BY BAD MATIRIAL  IT MAY GET BURN EASLY , or it can be caused by a difference in timing, non-adjusting of tacks or non-fuel. Matching

escaping of cumbusion gases caused burning the edge of the valve