Thread: 40 backfire
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Old 01-09-2013, 07:58 PM
spectre6000 spectre6000 is offline
Join Date: Mar 2012
Location: Coal Creek Canyon
Posts: 299

The closed throttle means there is very little fuel coming from the carb, yet the engine is still turning due to the movement of the car through the transmission. The mixture in the cylinders is too lean to consistently burn, so some of it is making it to the exhaust where it is meeting the hot gasses from cylinders that did burn and igniting in your exhaust. The holes in the exhaust obviously aren't helping and may make any tuning attempts moot, but you may find turning your idle mixture screw out a quarter to half turn will rectify this to some extent.
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