Arrived on site to perform diagnostic. Homeowner states that system is not heating. Cycled system for heating. Unit initiated heating cycle, but never ignited. Pulled out heating section and found that igniter was completely burnt up and the wires were also burnt. Everything was completely charred up. Replaced ignitor. Cycled system for heating. Unit kicked on and started heating. Flame is not burning correctly, and is rolling over and burning new ignitor. Verified that gas pressure is set correctly, set at 3.3inwc. Roof hood is completely charred up, indicating improper combustion. Flue needs to be checked at roof. Sending a smaller tech out due to weaker mobile home roof. System is not operational upon departure.