I have a York LP Gas furnace with 3 burner tubes. The tubes are burning back into the venturi, near the orifice. The system has been cleaned and checked for leaks. The burners pop when they first light and then hiss loudly when burning back in the venturi. Any ideas on what causes this and or a solution?

Two initial possibilities.
1. Check the color of the flame when the
burners are on. Usually the popping is
caused by an improperly adjusted flame.
The flame should be a bright blue. If
not then the burners are receiving
too much or too little air along with
the gas.
2. A more remote and more dangerous
condition is a partial blockage in the
heat exchanger or chimney area.
Improper venting will cause the gas to
draw back into the house area rather
than being drawn up through the
chimney.
These are two areas that are easy to check.
If you are unsure of your ability then have a
qualified serviceman do it for you.
GOOD LUCK!