5 Signs Your Motorcycle’s Spark Plugs are Bad

This site contains affiliate links to products. We may receive a commission for purchases made through these links.

One of the most critical parts of a motorcycle’s engine are its spark plugs. Spark plugs ignite the fuel in the combustion chamber allowing you to go on your leisurely weekend rides. Like all mechanical parts, spark plugs can wear out. Since they play such an important role in the engine, it is important that you recognize the signs and symptoms of failing spark plugs on your motorcycle. Spark plugs and ignition systems have improved over years, but without proper maintenance they can cause problems for your engine. Here are the common symptoms of a motorcycle with bad spark plugs.

Misfiring Engine

The most common symptom of bad or failing spark plugs is a misfiring engine. When your motorcycle is running properly, it will have steady sound. However a misfiring engine will disrupt that rhythm. Many times this only happens for a few seconds and then the engine returns to its normal operating cycle. A misfire issue is caused by a failing motorcycle spark plug that is not igniting the air/fuel mixture at the right time.

A spark plug ignites the air fuel mixture in the combustion chamber by sending an arc of electricity between the ground electrode and the center electrode. This arc of electricity is what ignites the fuel causing your engine to work.

If a spark plug does not arc at the right time it will mess up the timing of the engine. The ignition in the cylinder is coordinated very closely with the piston’s position. So igniting the fuel at the incorrect time will reduce horsepower, engine efficiency, and the overall smoothness of the engine. Spark plugs could even be the reason your motorcycle does not start. If you’re having issues with your motorcycle not starting, check out are article on 4 Reasons Why Motorcycles Struggle to Start.

Flooded Engine

Flooded engines don’t start. Sometimes they can be caused by an excessively rich air fuel mixture, but other times, bad spark plugs can fail to ignite normal air fuel mixtures leading to flooding.

A common symptom of a flooded engine is the engine cranking over and over but failing to start. This is often accompanied by a strong gas smell. This is caused by a combustion chamber filling up with gas but not igniting.

Sometimes everything can be working just right. The carbs are delivering the proper mixture. The intake valves are letting the mixture in to the combustion chamber, but the motorcycle is still failing to start. This can be caused by the spark plugs not working properly.


Backfiring is a common sign of failing spark plugs. This usually sounds like loud pops coming from the motorcycle’s exhaust. This is caused by spark plugs that are intermittently sparking.

This happens occasionally when the exhaust valve is open and the spark plug sparks randomly igniting the left over air fuel mixture in the combustion chamber. Because the exhaust valves are open when this occurs, it is very loud. Usually this problem is intermittent and regularly occurring throughout the motorcycle’s operation.

Another cause of backfiring is when the air fuel mixture is not completely burned by the combustion cycle. This left over air fuel mixture then leaves the combustion chamber through the exhaust valves. When the fuel its the hot exhaust manifold it ignites. This causes and extremely loud bang sound.

Smelly Gas or Gas Out of the Exhaust

When the spark plugs are malfunctioning and not sparking at the correct time, leftover gas will remain in the engine combustion chamber. Eventually this gas will get expelled to the exhaust during the exhaust stroke of the engine. If the exhaust manifold is not hot enough to ignite the gas (causing a backfire), the gas will shoot out of the exhuast.

Gasoline coming out of the exhaust is never a good thing. In fact it is a sure sign that the combustion cycle is not properly igniting the air and fuel mixture or the fuel system/carburetor is not delivering an appropriate mixture. This can be caused by failing spark plugs. Be sure to check the condition of your spark plugs if this is happening to your bike.

Its worth mentioning that not all fluid that comes out of your exhaust is gasoline. In many cases it can actually be water caused by condensation. Be sure that the fluid is actually gasoline before tearing apart your carbs or engines.

Worse Fuel Economy

Motorcycles are typically more fuel efficient than their gas counterparts, but failing spark plugs can drop their fuel efficiency. This happens because your engine fails to ignite all of the fuel in the combustion chamber allowing it to escape out the exhaust valves.

How To Inspect Spark Plugs

An easy way to check if its time to replace your motorcycle’s spark plug is to pull them out and look at them. You can tell a lot about the condition of the spark plug by looking at.

bad spark plug
Spark Plug in a Rich Air Fuel Mixture

If the electrode area is black and sooty, it means that the engine is running rich. However, if the spark plug is very clean and not discolored it indicates a lean air fuel mixture.

The other thing to inspect is the spark plug gap. Look up the spark plug part number to figure out what the OEM gap is, then measure the gap. The gap can be measured by a shim set linked to here. The bigger the spark plug gap the more voltage required to create the spark.

How Long do Motorcycle Spark Plugs Last?

Motorcycle spark plugs don’t last as long as car spark plugs because motorcycle engines operate at higher rpms. The higher rpms cause the spark plug to work harder for the same amount of time and mileage.

Spark plug life can vary drastically by the material used. In a car, copper plugs will be good for 10,000 to 20,000 miles, while platinum or iridium plugs can be good for 60,000 miles or more. In motorcycles, plugs typically only last 15,000 miles, while car and truck spark plugs can last from 30,000 miles to even 100,000 miles.

One of the best places to shop for motorcycle spark plugs is at Revzilla. Revzilla is a great place to shop for any motorcycle parts especially spark plugs. Each part that they sell has a gear representative that you can contact if you have any further questions, and that is one of the best reasons to shop at Revzilla.