back fires

[Guest joe land]

M 1921 Old 43 AT back fires on acceleration ....... tried adjusting timing. cleaned carb ... zenith up draft 

[chistech]

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. 

[Tedd Thompson]

  Or a badly burnt valve if it is constant usuall pops back through the carb though.... Tedd 

[Guest joe land]

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?

[Guest joe land]

BTW it only doe's it after so many RPM

[chistech]

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)