5 Reasons Motorcycles Misfire When Hot Or Cold (Explained)

A misfire happens when one of your motorcycle’s cylinders doesn’t fire.

To visualize how this affects your bikes performance, think of your pistons as the legs of a bicycle rider, pushing down on the pedals—on a misfiring motorcycle engine, one of the cylinders skips around, forcing the other to compensate, and cutting your enYou may also like to read our article gine-power-output until the pistons sync back up.

 And the fact that a misfire can happen while the bike is hot or cold makes troubleshooting that much more difficult. 

With that bike pedal image in mind, consider how important preventing misfiring is to your motorcycle’s performance, and start by asking yourself, what are the reasons motorcycles misfire when hot or cold? Read on to find out!

Here Are a Few Reasons a Motorcycle Misfires:

An improper air/fuel ratio, compression or timing, restricted air or fuel, a clogged carburetor, and a faulty spark are all common culprits behind motorcycle misfiring when hot or cold. Read on for in-depth analysis of these issues.

1. Carburetor Clogs and Complications

A clogged or faulty carburetor can cause a motorcycle to misfire regardless of the temperature of the motorcycle.

Before fuel injection, older motorcycles used carbs to maintain the combustion process that powers a motorcycle’s engine function. One of the primary jobs of a carburetor is to maintain the proper ratio of air to fuel within its chamber. 

This requires regular maintenance, adjustment, and proper tuning, which is why fuel injection eventually rose to replace carbs as the primary combustion force of modern motorcycles.

If you have a carbureted motorcycle and you’re experiencing misfires, inspecting the carburetor is a safe first troubleshooting step. 

But what exactly are you looking for when you ‘re peeping at your carb? 

3 main carburetor culprits contribute to misfiring:

1. Poor Carb Tuning
2. Vacuum Leak (Air)
3. Fuel Leak

Poor Carb Tuning

If you’re lucky, your carb might need a quick re-tuning. It might be a change in air quality or elevation. Regardless, if your carb’s tuning goes out of whack, your air-fuel/fuel ratio dips hard in one direction, and your cylinders misfire.

It could be that the air/fuel screw needs a readjustment, or, on a bike with multiple carbs, it might mean that your carbs are out of sync. 

A carb tune is a simple job for an experienced wrencher, but it’s not a job I’d attempt unless I had the right tools and a proper moto-know-how. Improperly tuning your carb could make it worse and could cause engine damage in extreme situations.  

A decent moto-mechanic can re-tune a carb in no time, and the labor is generally less than 80 bucks for a simple carb tune.

Another plus about taking your motorcycle to the shop is that the mechanic will let you know if a poorly tuned carb isn’t the culprit. They’ll be able to quickly determine if there’s a different carb problem contributing to your chronic moto-misfiring. 

You may also like to read our article on 5 Reasons Motorcycles Make Whining Noise

Vacuum Leak

If your carb has a vacuum leak, it can cause the motorcycle to misfire when hot or cold.  

A vacuum leak can cause several complications for your bike; one of the most basic is misfiring. 

Vacuum leak means your intake boots are leaking air; you guessed it, the air portion in your air-fuel ratio takes a hit.  

Time, weather, and corrosion can swarm together to make your intake boots brittle. Eventually, they start to crack and leak air. The boots are located between the motor and the carb, so if they’re leaking, the air delivery to the carb is inadequate. 

If a cracked air-intake boot is your problem, it’ll have to be replaced; an inexpensive venture. 

Some carburetors have multiple vacuum ports for upgrades or for cross-utilization with other models that equip extra accessories. 

When not in use, a vacuum port needs to be plugged. Otherwise, you’re losing air out of the open port. 

This unplugged port leak is easily fixed by buying a vacuum port plug that fits your carb’s ports, plugging that sucker in to keep you on track to the carb without losing the flow.

Fuel Leak

The third way a faulty carb can cause a motorcycle to sputter is by leaking fuel.

A fuel leak deprives your bike of fuel, affecting your bike’s air/fuel ratio and causing it to run “lean”(more on running lean later).

Carb-contained fuel leaks are generally the work of a cracked or splitting gasket inside the float.

The float or float bowl is located down in the bottom of your carb, and it’s a lot easier to spot and smell a fuel leak down in there than to find an air leak in the intake boot, for obvious reasons.  

Suppose you see and smell fuel leaking in the float bowl; it’s probably just a worn gasket. You can replace it yourself for super cheap or take it to a pro, and you’re still not paying much. Most mechanics suggest you replace them all when replacing one float bowl gasket, and I’m inclined to agree with the pros.

Related: 9 Reasons Motorcycles May Keep Stalling (Solved)

2. Malfunctioning Ignition Cycle

A motorcycle with ignition issues will experience misfiring, either when hot or cold. 

If your motorcycle is fuel injected and therefore doesn’t have a carb, or if you’ve troubleshot your carb and it’s good to go, your ignition is the next place to look. 

The ignition issue could be caused by a bad spark plug or spark plug wires, or it could be a failing ignition coil.

• Always check the spark plugs first. Pop the rubber boot off and pull your plugs out to check for any buildup, discoloration, or corrosion.
• Check to make sure the gap between the electrodes isn’t off—there’s a specific tool you can find at any auto-parts store designed to measure your spark plug gap.  
• Then, inspect the point. It was down eventually, and once it does, your ignition is thrown off. 

A bad spark plug will trigger misfires within its respective cylinder, along with a slew of other potential performance problems. Here are things to check if the ignition cycle has issues:

Spark Plug Wires

If the plug itself appears healthy, check the spark plug wires. Especially on older bikes, it’s not uncommon for the spark plug’s connection head to attach to the spark plug wire via threads.

If those threads develop corrosion, or if enough debris gets stuck in between the threads to affect the connection, this could be the culprit. 

Again, this little troubleshooting tip is only relevant to older bikes with those threaded connector heads. If your bike is of a more modern design, your spark plug wire and its connector head are probably one solid piece.

If you’re an old-schooler with a threaded connector head and wire set up, you may think it’s the reason for your malfunctioning ignition cycle and causing your motorcycle to misfire. At the same time, both hot and cold, clean your spark plug wire free of debris and corrosion and thread the connector head back onto the wire. 

Whether your spark plug wire is threaded or not, check it for splits and cracks in the rubber boot or in the wires inside. Split or exposed wires can arch the plug-spark to your moto-frame, detouring it away from arcing all the way through the spark plug as it should. 

If the spark doesn’t permeate all the way through the plug, your motorcycle will misfire; cracked spark plug wires need to be replaced. 

Ignition Coil

A bad ignition coil is another cause for a malfunctioning ignition and can cause the bike to misfire, hot or cold. A bad ignition coil won’t spark hard enough to hit the spark plug, triggering that misfire. That said, it’s far more common for bad spark plugs and split items to be the root cause rather than a bad ignition coil. 

We suggest beginner mechanics whose plugs and wires are good, and you think the ignition coil is the problem, have it looked at by a moto-pro mechanic—high-voltage ignition coils are hazardous to handle.

Related: 9 Reasons Motorcycles Won’t Start When Hot (Explained)

3. Clogged Air Filter

If a motorcycle has a clogged air filter, it can misfire when hot or cold. 

If your air intake filter is full of dirt and debris, it’s not pulling in the amount of air it’s intended to, and the air/fuel ratio runs rich. 

Running rich happens when the carb or combustion chamber is starved of air and receives too much fuel as a result.

Like we mentioned elsewhere in this article, a motor is engineered to run on a specific ratio of air to fuel, and if it’s off, you’re bound to experience misfiring. 

Air filters are to be changed in adherence to the owner’s manual of your motorcycle to prevent this from happening. Your filter is either inside your airbox or put in the open, mounted on the side of your motor. A filter is cheap and easy to change.

Like my customized Indian Chieftain, some motorcycles have upgraded reusable filters that need to be cleaned out. 

If your bike has a reusable filter, pop it off the intake, spray it with some Green Cleaner or degreaser and brush it with a soft toothbrush to get that crud off, and reinstall it onto your intake for crisp, renewed throttle action, and more importantly, to prevent your motorcycle from misfiring.

4. Engine Timing Thrown Out of Sync

If your motorcycle’s engine timing is off, you’ll experience misfiring whether the bike is hot or cold. 

Having your timing thrown off generally only happens if you or the previous owner has rebuilt the engine or tried to fix the timing themselves and made it worse. It could be the culprit of an opened intake or exhaust valve that’s causing your motorcycle to misfire. If that’s the case, have your timing inspected and, if need be, adjusted by a professional mechanic.

A distributor cap regulates the spark plug timing on some motorcycles with multiple spark plugs, ensuring the spark sequence functions in accordance with the pistons. If the distributor cap is broken, the timing gets thrown off, and the air/fuel mix is off. Notice the pattern? 

If a broken distributor alters your air-fuel ratio, your bike may experience a misfire.

Related: 6 Reasons Motorcycles Backfire When Starting (Explained)

5. Loss of Compression

Combustion forces the piston down to generate motor power. Then, as air and fuel rush into the now empty combustion chamber, compression pulls the piston up to start the cycle repeatedly. 

If compression escapes from your cylinder, the ignition spark won’t happen at the right time, breaking the ignition cycle and causing your motorcycle to misfire when hot and cold. 

What causes lack of compression?

It could be a blown head gasket, but more often than not, it’s caused by a leaking valve in the exhaust, maybe due to heat damage.

If your motor’s running noticeably hotter than normal, or if multiple cylinders are misfiring, chances are your problem is the head gasket. 

Was this article helpful?