| Many of those were equipped with an idle mixture screw also, generally close to the spigot side, these are typically set at 1-1 1/2 turns out from lightly seated. The backfiring is most likely caused by a lean condition, often caused by an ubstruction due to long periods of non use. Make certain the carbs are clean and every orrifice allows flow. The pilot jets are the likely culprit, check the emulsification circuit the feeds them also. |