Furnace short cycling

[Timondirt]

The furnace on my RV was short cycling. Fan comes on, Burner comes on then after about 17 minutes the burner turns off and the fan keeps running. Before the temperature set point is reached. About 1.5 minutes the burner would come back on. This short cycling would continue until the set point was finally reached. After researching, I found that the Thor did not put enough vents in and the mixing box was overheating, which shuts down the burner until it cools enough. Added one 4 inch vent and now the burner runs until the set point is reached. Also found the ducting gets up to about 135 deg F as normal operation.