An exhaust emission control system for a spark ignition internal combustion engine comprises a first pressure sensitive valve for recycling a proportion of the exhaust gases from the exhaust to the induction when the engine is warm, being inhibited when the engine is cold, valves for admitting a small amount of fresh air to the exhaust manifold, a second pressure sensitive valve for admitting fresh air to the induction manifold downstream of the carburettor when the engine is warm, being closed when the engine is cold and when the starter motor is energized, a first fuel/air mixture enrichment valve in the carburettor sensitive to induction manifold pressure and de-sensitized when the engine is cold, a second fuel/air mixture enrichment valve sensitive to induction manifold pressure and de-sensitized when the engine is hot, and a vacuum advance delay which is by-passed when the engine is warm.