Guest joe land Posted July 4, 2017 Share Posted July 4, 2017 M 1921 Old 43 AT back fires on acceleration ....... tried adjusting timing. cleaned carb ... zenith up draft Link to comment Share on other sites More sharing options...
chistech Posted July 5, 2017 Share Posted July 5, 2017 Backfires with black smoke or no smoke? If no smoke try choking slightly while accelerating and see if it helps could be a lean condition. Is it a muffler/exhaust backfire or more like a coughing type through the carb? Is the motor cold, hot, or doesn't matter? Bad cap or wires will also cause spark to jump to wrong terminal. Link to comment Share on other sites More sharing options...
Tedd Thompson Posted July 10, 2017 Share Posted July 10, 2017 Or a badly burnt valve if it is constant usuall pops back through the carb though.... Tedd Link to comment Share on other sites More sharing options...
Guest joe land Posted July 11, 2017 Share Posted July 11, 2017 Thank you for your replies. Back fires with no smoke. Tried choking..... however does seem to be a lean issue ... carb is a Zenith it says on us.pat.Dec.29.08.june27-16 there is nothing on this carb. too small? Link to comment Share on other sites More sharing options...
Guest joe land Posted July 11, 2017 Share Posted July 11, 2017 BTW it only doe's it after so many RPM Link to comment Share on other sites More sharing options...
chistech Posted July 13, 2017 Share Posted July 13, 2017 (edited) Ck your plugs for color and their gap. If it's a lean carb caused issue all will be whiter than normal brownish. If one or two are white could be valve or valve spring issue. Coil could also be suspect. Point gap correct? Not sure how your car advances the spark but it's probably manual by the operator? Edited July 13, 2017 by chistech (see edit history) Link to comment Share on other sites More sharing options...
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