this is likely due to a dirty heat exchanger backing up the gases in the chamber. To fix this, remove the vent pipe, then remove the top chamber cover and look inside there and see if rust hasnt covered the tops of the chamber exits. If so, then either take a vacuum cleaner and suck up that rust. Also look inside the lower burner chamber where the actual burners are and see if this is also covered with rust. Do the same thing there, vacuum it out or brush them out with a steel brush. This is not that difficult an operation and shouldnt take long to remedy
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