Advice: What is backfiring?

You know and love it - that cool café racer cruising up to the lights, exhaust popping and banging satisfyingly as it slows. Sadly, that super hipster backfiring isn’t exactly best for the bike. It is actually a symptom of poor fuelling and, taken to extremes, it can lead to overheating, a damaged engine, valve train, exhaust and muffler system, or a gummed and sooted-up intake along with the potential for intake fires. Not good.

A tiny bit of popping on the over-run is OK. Poor idle, serial backfiring and unreliable starting all signal engine alarm. Here’s the theory.

An ideal engine squirts petrol and air into each cylinder in the ratio of one part petrol to 14.7 parts air. This is called a stoichiometric mix (SM), and means all the fuel burns perfectly and there is no petrol or oxygen left over, and your bike runs like warm custard. The SM is adjusted by your injection system (or carbie on old bikes) at all speeds. Mess with the air intake, the muffler, timing, or let anything spring a leak and this ideal air/petrol ratio will go out the window.

Too little air, and unburnt fuel builds up and randomly ignites in the pipe and muffler giving those slowdown pops - or massive bangs if things are really off. Too little petrol and the engine struggles with full burning which leads to overheating and auto-combustion (knocking).

The good news is that today most bikes come with sufficiently clever Engine Control Units (ECU) that use smart air, fuel and burnt gas sensors all over the engine. In most cases, the bike will have airflow, air pressure and temperature sensors as well as fuel metering on the intake side, along with a lambda sensor (thanks Dr. Günter Bauman) to measure oxygen levels in the exhaust gas. Using these, the engine’s computer calculates the best possible burn ratio for any given throttle opening and you get max power, good fuel consumption and virtually no backfires. Better still, any SM ratio changes that creep in (or you introduce with that fancy pipe or third party air filter) can be adjusted automatically.

That’s the theory – but what can you do if you bike backfires all the time? It’s usually caused by either mixture or sensor problems. Here’s a few pointers.

Check the lambda sensor

This sits in a constant stream of red-hot exhaust gas all its life, usually under the engine where road crud attacks it from outside, and as a consequence, it fails. Get it tested at your local repair shop – replacement ones aren’t that expensive though. If it is blown, replacing it should fix the problem. If you don’t want the expense, get a blanking plate and sensor eliminator off eBay but be prepared for other poor fuelling issues if you do.

Look for exhaust leaks

These are easiest to spot and fix – just run the engine and listen and feel for wind with the back of your hand, or smell for leaks all along the plumbing. If you don’t want to burn your hand, use a piece of thin paper. If it bends, there’s gas blowing out. With the leak located, fix it by tightening the clamp, replacing the seal, or if you are a trailbike rider, bodging a bit of tin can over the hole using circlips. Neat bikers will of course replace the faulty part.

Look for intake leaks

These are harder to find, and usually happen deep under the tank or seat. Taking the tank off is the proper way. There is another way; using a can of instant start, spray around the plastic intake ducts (but not the main air intakes). Do this when the engine is cold to avoid singed eyebrows. Then start the engine and see if the backfiring stops. If goes away, you have an intake leak. Tracking it down means doing the instant start spray thing on selected areas, then trying a restart each time. Not an ideal technique, but it can shorten the time compared to a complete tank/intake strip-down.

Fixes for intake leaks usually mean replacement gaskets and seals, plastic welding, or duct tape over any obvious cracks. If you are lucky, fixing up the intake and exhaust tracts will cure the backfire, and give you better consumption too.

Injector problems

Very occasionally an injector will go wrong and this can cause fuelling problems. Run the engine from cold for a few seconds, then check all exhaust pipe temperatures. They should all be the same; any that are unusually hot or cold could signal a dodgy injector. Sometimes fuel lines and pumps can fail erratically; both can lead to backfiring and bad running, and are really tricky to locate and fix – it’s usually a workshop job.

ECU programming

One last thing: If your bike is an import, the ECU could be set up for a different (more humid, hotter, or drier) climate so might not have the right stoichiometric ECU curve. Two options here – get a Power Commander add-in with the right map for your bike and pipe, or go to the local dealer and get the ECU reprogrammed. Either should help the problem.

This article was originally published on January 30, 2020.