5 Reasons A Motorcycle Wont Start But Backfires (Explained)

A motorcycle backfire is a loud noise coming from your bike’s engine, usually while the bike is already running, due to the presence of uncombusted fuel in the exhaust pipe.

Backfiring is generally the result of poor tuning or improper air/fuel ratio.

That said, in more serious situations, you might find that your motorcycle won’t start but backfires when you go to crank her over.

Here are the most common reasons why!

1. Carburetor Clogs

Another frequent cause of motorcycle backfires on older, carbureted model motos is clogs within the carb.

A carbureted motorcycle responds to the rider’s input by jetting fuel into the combustion chamber, which then combusts to thrust the pistons into action.

A backfire can occur if the fuel backs up and fails to combust in time. If the pathways of the jets get clogged with debris or bad fuel, the jet backs up into the carburetor and your acceleration delays. If this process happens at a start-up, you will experience a motorcycle that won’t start but backfires. 

  • To be clear, carb maintenance is a routine part of owning a carbureted motorcycle. If it’s been a while since you’ve rebuilt or detail cleaned your carburetor, and you’re experiencing a backfire in any situation, this is an excellent place to start troubleshooting.
  • An unkept carburetor can clog with debris, corrosion, or even fuel. If it hinders the fuel flow, your air mix will start to run lean, meaning it has more air to fuel than your particularly tuned engine requires for efficient operation.

Before we get into the specifics of why we’re starting here, we need to clarify something important; most people, including myself, refer to both an after-fire and a backfire as backfires, but there is a distinction.

What’s the Difference Between a Backfire and an After fire? 

An after-fire is technically the term to describe the pop that results from the late combustion of fuel shooting out of the exhaust pipe, usually due to an air/fuel mixture overtly rich in fuel. A backfire is a result of unburned fuel rushing back into the air intake and combusting there, causing more of a bang than a pop. 

What’s Worse, a Backfire or an After-fire? 

After-fires in the exhaust pipes are more common, including at start-up, but a true backfire in the intake manifold has the potential to seriously damage your engine. This is why carb maintenance is crucial to the longevity and performance of carbureted motorcycles.

To best serve our readers, this article will refer to both situations as a backfire, as most writers do. That said, in this section, we are referring to the more severe, true backfire that happens in the intake as a result of a clog in the carburetor.

  • If you’re experiencing problems with your carbureted motorcycle not starting but backfiring instead, the carb is the first place to start troubleshooting immediately before the intake combustion causes severe damage.
  • The solution here is to rebuild your carburetor and inspect, clean, or replace any clogged jets that might be backing up with fuel while you’re starting your motorcycle.
  • Use a quality carb cleaner to break up the clogs and reassemble your carb, following the service manual instructions for your make and model.

And finally, make carb cleaning a routine part of service maintenance to prevent this damaging backfire from happening when you’re just trying to fire up your motorcycle.

2. Poor Fuel Quality

Most motorcycles call for premium-grade fuel for ideal performance.

Failing to meet this basic expectation and running on poor-quality fuel could cause your motorcycle to have starting and backfiring problems.

Running expired or contaminated fuel through your motorcycle will certainly cause starting problems, often accompanied by a backfire. That said, the service manual of every motorcycle specifies an idea fuel octane or grade to use. Failing to use the appropriate quality of fuel for an extended period can result in your bike backfiring and not starting. 

Contaminated or poor-quality fuel expires more rapidly when exposed to engine heat, and bikes get hot.

Expired fuel affects your engine’s performance, taxing its power load and causing wear, and causes corrosion in your tank and fuel lines.

Corroded fuel has a considerable impact on carburetors, and we got into that in the previous section, but fuel-injected bike riders aren’t off the hook either.

  • Contaminated fuel negatively affects fuel injection; while some dirty fuel particles burn up enough to start your engine, others quickly escape into the exhaust system without burning. 
  • If they combust within the exhaust system, a backfire will occur.
  • If the injectors weren’t able to combust enough of the fuel to run the engine, the back pressure is enough to kill the bike.

How To Stop Bad Fuel From Back Firing and Causing Starting Problems:

Preventative maintenance and proactive service. Use the fuel grade suggested in your motorcycle’s owner’s manual and gas up at trusted establishments to be sure you’re using fresh, clean fuel.

Additionally, be sure not to allow old fuel to sit in the tank for long periods of inactivity without a proper winterization or additive treatment.

And finally, it’s best practice to store your motorcycle with a full tank of fuel, as the empty space in partially filled tanks allows air to enter. The moisture in the air can corrode the inside of your tank, the fuel system, and the fuel itself, which will then cause the bike not to start but backfire.

Related: Motorcycle Ignition Won’t Turn | Here’s Why (SOLVED)

3. Excess Fuel in Air/Fuel Ratio

We brushed over this earlier; let’s slow down and review a bit more thoroughly.

Running rich is what we call an air/fuel ratio that has too much fuel and not enough air.

Running a rich fuel mixture pummels the combustion chamber with more fuel than it’s designed to ignite. This often results in the excess fuel flowing into the exhaust chamber and combusting there, shooting out of your tailpipe and popping.

Every motorcycle engine is tuned in a particular way, based on the riding style and environment in which the machine was designed to perform ideally. If this ratio is altered, even an electronic CPU-governed tuning might be unable to adjust in time to prevent a backfire at a start-up.

If enough fuel enters your exhaust system and combusts therein, the back pressure could be enough to kill the bike altogether, preventing it from starting up. 

Air is just as crucial to starting your motorcycle as fuel is. Your combustion chamber only has a capacity for so much mass, meaning that if you are running a mix that’s excessive in fuel, it’s at the cost of air.

As a result, you’re pumping unburnt fuel into your exhaust and allowing it to burn up there as an after-fire, commonly referred to as a backfire.

Additionally, you’re also starving your ignition process of the air required to start your bike, which can cause the bike not to start but to backfire instead.

Related: 10 Reasons A Motorcycle Won’t Start Even If Battery Is OK (Solved)

4. Shoddy Aftermarket Part Installation

We all appreciate the joy of motorcycle customization, some of us more than others.

There are myriad aftermarket air intake and exhaust enhancements that increase throttle response and acceleration, torque, and engine power to make your moto sound just like the movies.

That said, if the pipes your motorcycle is equipped with aren’t compatible with the rest of your exhaust system or intake system, or if your air/fuel ratio hasn’t been tweaked to accommodate for the change in equipment, your bike might not start and can even backfire instead.

On a carbureted bike, the carb itself has to be adjusted to function seamlessly with an update to your air intake or exhaust systems.

Even on modern, CPU-regulated fuel-injected motorcycles, your Electronic Computer Unit will have to be updated with the specs of the new accessories so it can tweak and regulate your air/fuel ratio and combustion process and ignition sequencing, etc.

The carburetor and the injection system serve the same purpose, to maintain a regularity between the air your intake is sucking in, the fuel your injector or jets are injecting, and the exhaust your pipes are pushing out.

Improper accessory upgrades are probably the most common cause of backfiring in any situation, including when you’re trying to start your motorcycle to no avail.

This is why some regions have laws that prevent pipes that are shorter than a particular length, as pipes that are too short are prone to backfiring.

While we appreciate the aesthetic logic that goes into bike customization, the straightforward physical fact of a pipe less than twelve inches in length is that it doesn’t leave room for baffling of any kind. A baffle governs the turbulent flow of fuel like a muffler does for a car; without it, backfiring is regular and may even throw a wrench in the starting process. 

It’s not just shorter pipes that are the issue, either. Any new exhaust or intake parts installed on your motorcycle require a new tuning to adjust and maintain the flow of air and fuel and the ratio between the two.

This means adjusting your carb or flashing and updating your ECU. If you’ve recently installed new exhaust parts on your bike and haven’t made the proper tuning adjustments, chances are this is the reason your motorcycle won’t start but backfires.

5. Problems With Engine Timing

Timing issues can develop on modern and vintage bikes alike, whether your electronic timing is off or your points are off time.

Timing issues can cause frequent backfiring and starting problems on most bikes.

If your motorcycle’s engine timing is off, the compression and tension are thrown off kilter as well, sometimes resulting in a backfire at start-up that may be enough to prevent it from starting. 

If your bike uses electronic timing, a computer unit analyzes and governs your engine’s timing, also referred to as your bike’s ignition sequencing.

This makes troubleshooting bad timing as simple as running diagnostics and reprogramming or flashing the unit to optimize the ignition sequence, which affects both starting and combustion.

The points need to be adjusted manually on an older bike that uses an analog points system to time the ignition.

Either way, timing adjustments can be complex, as the process varies from bike to bike. If you’re unsure of your skills as a home mechanic, there’s no shame in taking your bike to a pro, as improper timing adjustments can make starting issues and backfiring both worse and can cause severe damage to your bike’s engine.

Sources

6 Reasons Motorcycles Backfire When Starting

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