Toyota Repair: Toyota 4aF timing, air fuel mixture, toyota 4af

Question
(1) What is correct degree for ignition timing on 4-AF carb engine (1600cc) (1990 vintage) at sea level. Some say 10 degrees before TDC , others zero degrees.
(2) The vehicle backfires against trailing throttle when going downhill. Can valve timing be out ?

Answer
Between 8 and 10 degrees works,engines normally backfire because the air fuel mixture is too rich and there is too much fuel going into the engine on deceleration when there should be almost none, there could be a problem with the carburetor, backfire can also be caused by an exhaust leak.