7 Definite Symptoms of Bad Spark Plugs

Spark Plug

Bad spark plugs are injurious for the engine’s health. Spark plugs play a vital role in engine performance by igniting the fuel mixture. If they are not doing their job right, there will be lots of engine shortcomings in your motorcycle.

To address the issue, here are the seven definite symptoms of bad spark plugs that you should be aware of:

  1. Hard to start
  2. Engine misfiring
  3. Low fuel economy
  4. Rattling engine noise
  5. Lack of power
  6. Reduced acceleration
  7. Rough idling

Let’s dig into each of these symptoms first and then look into the inspection and solution.

#1: Hard to start

motorcycle hard to start

One of the first symptoms of a bad spark plug is when it gets hard to start a motorcycle. The bike does not start easily and you need to try several times to ignite the engine.

As for why this happens, a bad spark plug will not be able to spark ignite the fuel mixture in the engine easily. As a result, rarely do you see the spark plug igniting the engine on the first try.

In the end, you might have to attempt multiple times to start the motorcycle.

#2: Engine misfiring

Motorcycle engine

Engine misfire occurs when the engine fails to generate sufficient power.

Whenever the spark plug is fouled or has gone bad in the engine, there will always be problems in complete fuel combustion. The spark ignition issues make the engine incapable to generate the required power consistently.

As a result, you will start noticing engine misfires more often than not in your motorcycle. And the primary cause of engine misfire is a bad spark plug. So the next time you notice too many engine misfires, check the spark plugs first.

#3: Low fuel economy

Motorcycle reading

Poor gas mileage is another symptom. A bad spark plug in the engine will often lead to incomplete fuel combustion. The spark plug will not be able to ignite the fuel mixture properly and hence, lots of fuel go unburnt.

Due to incomplete combustion and inconsistent ignition, your motorcycle will not utilize the fuel at its maximum efficiency. Lots of fuel exit the engine without burning completely to generate maximum power.

What ends up happening is a noticeable drop in gas mileage and fuel economy. All because of the damned spark plug.

#4: Rattling engine noise

Rattline noise arises because the engine is vibrating and making uneven sounds. A bad spark plug that fails to detonate the fuel consistently in each combustion cycle, is one of the causes of such rattling noise in the engine.

Inconsistent fuel burning, to be precise, is the main cause of such rattling engine noises. And since bad spark plugs cause inconsistent and uneven fuel burning, they lead to rattling noises.

And this rattling noise from the engine can be anytime. It can be while starting the motorcycle. It can be while idling. Or it can be during the ride. Rattling noises, in short, are also an after-effect of bad spark plugs.

#5: Lack of power

One more major symptom of bad spark plugs is the lack of power from the engine.

Since the spark plug is not igniting the air-fuel mixture properly, the uneven combustion of the fuel leads to reduced power generation in the engine.

So, if you start noticing a lack of power or reduced power in your motorcycle, it can be a symptom of a malfunctioning spark plug.

#6: Reduced acceleration

Motorcycle in speed

Problems during acceleration are one other symptom. You roll the throttle up to speed up your motorcycle. But, the acceleration is just underwhelming.

This again can be because the spark plug has gone bad and is not in good working condition. When you accelerate, you need the engine to burn more fuel and help you to speed up faster.

However, a bad spark plug in the engine will hinder the fuel combustion process by not igniting the air-fuel mixture properly. Lo and behold, just when you want to overtake slow grandpa on the road, you can’t!

#7: Rough idling

Even when you are idling your motorcycle, you will start noticing it to be rough. Yup. A bad spark plug will show its signs even when idling.

In fact, rough idling is one of the checks you can adopt to take a relook at what is wrong. Run the engine in the neutral gear and that’s all you need. Hear the idling noise next. If the idling is anything but smooth, there is something wrong that needs to be looked into.

Although there can be several causes for rough idling, bad spark plugs are definitely up there as one of the main reasons.

How to Know for Sure?

Now, all these symptoms are fine and good. But there is no guarantee that the spark plug has gone bad until and unless you confirm it firsthand.

Sure, these symptoms help. Especially the engine misfires and the hard-to-start parts. But they are still not a sure shot.

What these symptoms do provide is an indication of where to check and what to check.

With these above-mentioned symptoms in mind, you can zero in on the spark plugs to examine and confirm for sure that the spark plug has gone bad.

To take out the spark plug from the engine, using a spark plug wrench is recommended.

Once you have removes the spark plug, inspect the plug condition. A major indication that spark plugs have gone bad is it is oily and has black deposits. Rather than light brown residue on the electrodes. If the plug is worn out, blackened, has too much carbon deposit, or is in bad working condition, then the spark plug for sure has gone and needs to be replaced.

Solution: Replacing the Spark Plugs

Bad spark plugs need to be replaced with new ones. There is no other way to do it. You just have to remove the bad spark plug and change it with a new and good one.

As for how to go about replacing the spark plug, you can go to a service shop or you can do it yourself. Changing spark plugs is not a big deal and can be done by yourself at home.

The required tools include:

  • Spark plug wrench
  • A flat head screwdriver
  • Feeler gauge or spark plug gap opener
  • Small wire brush
  • Sandpaper
  • Spark plug boot removing pliers (optional)

Before starting anything, make sure you have switched off the ignition, parked the motorcycle on the center stand, and shift the bike into neutral gear.

Once you have taken out the spark plug, ensure it is indeed has gone bad. An oily, black residue on the electrode means the spark plugs are no more of good use.

Discard the old spark plug and place the new one. Make sure you are fitting the new spark plug properly in the cylinder head. Use the wrench to fit it nice and smooth. An under-torque spark plug will not come in contact with the air-fuel mixture properly. So, it is essential that you tighten the spark plug good