Why Does My ATV Backfire? [An Explanation With 4 Tips]

About a month ago (give or take) I went riding through some trails with a buddy of mine who has an older Kawasaki Prairie.  He was trailing behind me when out of nowhere I hear a loud boom, it kind of sounded like a gunshot. I asked how long it had been backfiring on him and he said it was something that just recently started.  

I figured this is something that happens to a lot of other riders so I decided to give some insight on why it happens and what you can do to prevent it from recurring.

Note: in this post i’ll be working under the assumption you have spark plugs, but if your quad has a carburetor the same principles will apply.

Why Does Your ATV Backfire?

Backfiring in an ATV engine, or any small engine, is usually the result of either one or multiple spark plugs igniting the fuel in the chamber when it is out of turn, specifically during the combustion cycle when the exhaust valve is open on that cylinder.

So does that alone cause the backfire?  Well not really, there is typically an underlying issue causing the backfiring “symptom”, here are a few:

Rich Air/Fuel Mixture

When you’ve got a rich air/fuel mixture (overabundance of fuel) it’s common to leave behind unburned fuel in the exhaust system.  When the spark plugs cross-ignite while the exhaust valve is open it will burn the unused portion of the fuel in your exhaust manifold and tailpipe causing that exploding effect and is followed with a loud BOOM!

To explain this differently, an overly rich mixture has an abundance of fuel that doesn’t burn up all the fuel in time for the exhaust part of the engine cycle.  When the exhaust valve on that cylinder opens, the influx of extra air allows the unburned fuel to immediately combust causing the dramatic “POP”!

When you’re running too rich, there can be a multitude of reasons why, but the first thing I would check is your air filter.  Having a high amount of fuel in your air/fuel mixture doesn’t necessarily mean that you’re intaking too much fuel, it could just mean that your engine just isn’t getting enough air.

It kind of sounds like the same thing, but it’s not really.  Your engine could be getting the proper amount of fuel but the air is getting choked off somewhere along the process.

Check the air filter to see if it’s all “gunked-up”.  If it is, then either wash it if it’s a foam air filter or completely replace it if it’s a paper filter.  Addressing this issue first should help balance out the air/fuel ratio tremendously.

Now that you’ve checked and cleaned the air filter for large sediment and deposits, start up your ATV and see if that has helped.  Make sure to accelerate and keep your ears open for any popping or booming sounds.

It would also help to know what the target air/fuel ratio is for an ATV.  To be clear, different models have different optimal ratios and those ratios will change depending on the performance you’re after – whether power, low emissions, or fuel efficiency.  In general, the Stoichiometric AFR (Air Fuel Ratio) would be somewhere around 13:1 though the ratio can fluctuate from 12.5:1 to 14:1 

It’s important to know what is meant by a lean or rich AFR so to put some relative numbers around this terms, a lower AFR than 13:1 is a richer mixture (more fuel) and a higher AFR would represent a leaner mixture (more air).

As an example:

  • 14:1 is lean
  • 13:1 is Stoichiometric
  • 12.5:1 is rich

Leaner AFR produces higher combustion temperatures and maximum power output usually hovers just rich of the Stoichiometric AFR, but unless you’re a mechanic preparing an ATV for a race, the numbers are kept at the Stoichiometric ratio.

But what if that’s not it?  You’ve checked and cleaned the air filter and the AFR is on point.

Delayed Engine Timing

This is also known as retarded timing, implying that the intake-compression-power-exhaust strokes of your engine are not in sync with the strokes of the cylinder head.  The head and the block must be timed exactly so that both are performing the very same stroke at the same exact time.

If both ends aren’t synced accordingly then the ignition part of the cycle will begin late in the combustion chamber and it will ignite the fuel when the exhaust valve is open (not what you want!)

You’ll find that these are the two most common reasons that backfires occur.  If you address both of these potential causes first, there is a high chance you’ll address the backfiring problem you’re experiencing.

Backfiring Vs Spitting

These two terms are somewhat synonymous but technically different.  As explained above, backfiring is when the fuel is overly rich and the combustion is released on the back end of the quad (the exhaust pipe), but spitting usually takes place in the morning or on the initial start of the engine and has the combustion come out of the carburetor itself.  Spitting usually ceases once the engine is warm.

When the spark is out of turn, the pressure from the unwanted burned fuel has to travel somewhere – since the intake valve is open during this stroke, the pressure travels right back up through the intake manifold and out of the carburetor creating a “spitting” or “coughing” effect and is sometimes followed by a flame.

How Can You Tell When Your ATV Backfires?

Diagnosing a backfire is pretty darn simple.  You won’t miss it unless you’re deaf or blind in which case you’ll still feel it.  When your engine backfires it creates an extremely loud popping sound and in same cases you’ll even witness a flame coming out of the exhaust pipe.  Though flames coming out of the exhaust isn’t as common, it still comes with the territory.

You’ll more frequently experience an engine backfire when you suddenly reduce your engine speed too quickly.  This was the issue with my friend’s ATV. Again, if you’re experiencing this, i’d check your air filter or your engine timing as explained above.

Tips To Prevent Backfire

  1. Regularly clean your air filter – clogged up air filters will have a significant impact on how rich your engine operates.  Having an overly rich fuel mixture not only causes your machine to burn through more gas unnecessarily, but it also affects acceleration and performance.  If you’re a rider that enjoys mud riding then you’ll definitely have to clean/change out your air filter more regularly
  2. Check your fuel injectors – are they working properly?  How long has it been since they’ve been changed?  How dirty are they? Common sense is your best tool here, but if you need others you can find them at most automotive shops near you.  You can go with sophisticated, expensive equipment that checks the firing of your spark plugs, or you can purchase much simpler tools like a quick probe.  It has a circuit that measure the solenoid clicking on and off in the fuel injectors. It’s super simple to use and the results are immediate.
  3. Add fuel injector cleaner – a high grade cleaner will help clean out any debris and gunk buildup found in your injector lines.  You shouldn’t need to use this very often at all, but every now and then it definitely helps as a precautionary measure.
  4. Go for the higher fuel grade – this is another tip that doesn’t have to be done every time you fill up the tank, but as a general rule of maintenance, it’s good to sparingly mix in a cleaner fuel grade to reduce the engine buildup of additives and other impurities that are commonly found in gasoline. Check your owners manual for specific restrictions and recommendations for your particular model.

Is Engine Backfire Bad?

I guess the question is how big is the backfire?  Most often you’ll hear the combustion as a mild cough-like sound usually not causing any damage to the engine or injury to the rider, but as the situation worsens then so does the after effect.  Letting the problem go unaddressed isn’t advised at all. If the problem gets worse and the backfire gets stronger, it could get strong enough to where the backfire causes severe damage, such as a crack in the exhaust manifold.  At that point, you’re looking at extensive repairs and costly replacement of parts.

Looking at the issue more microscopically, an engine backfire really means a fuel malfunction, which in itself is bad.  Having a consistent delivery of the proper air/fuel mixture into your engine is a fundamental basis for the modern engine.  

Keep in mind that any combustion that takes place outside of the combustion chamber itself will, by default, mean a reduction in power output from the engine.  You might notice the difference in top speed of your 4 wheeler or when you’re trying to tread over larger rocks or a steeper incline. Maybe you’ll notice the loss of output in the sand dunes, or worse yet, out on the race track.

Running extremely lean or overly rich leads to internal temperatures that can deviate outside the safe ranges the manufacturer has set for your engine.  To be blunt, you’re unnecessarily killing your engine and it won’t be long until you find yourself buying a new one.

The good news is that with consistent and proper maintenance, you should be able to avoid this problem overall.  There is no reason why it should ever get to that point to begin with, not only is it in your best interest to provide routine maintenance, but riding a 4 wheeler that backfires is uncomfortable and annoying!