its due to low current in plugs..
mostly mechanic plug ka gap kam krdete hain jis se almost masla hal hojata h, but phr bhi back spark kbhi na kbhi ajata h pick uthate way.
solution is apko coil tapet cover se hata kr tapet cover k side pr lgwani hogi, or coils ka current chk krna hoga, acha electrition janta hoga baleno ka mis fire ka issue kese sort krte hain.
mind it jb tk coil tapet cover k uper rahainge mis fire kregi chahe ap new dalwa lo