A motorcycle can misfire or jerk when accelerating for several reasons, the first thing we have to take into account are the factors that influence combustion, which is the mixture of air and gasoline together with the spark.

To determine the cause of the problem we will have to follow some steps or guidelines, in this way we will analyze the necessary components to determine if the problem may be electrical or, on the contrary, it is the air/gasoline mixture.

One of the components that provide us with more information about what is happening in the combustion process is the spark plug, therefore, being one of the components that can cause jerks if it is in poor condition, we will disassemble it for analysis.

What does the state of the spark plug tell us?

The spark plug, depending on its condition, can give us valuable information. Next, we will see some images and we will explain what they tell us.

Spark plug with carbon


Normally a spark plug with black carbon indicates that the air/gasoline mixture is too rich (excess gasoline or lack of air) The motorcycle will react slowly when accelerating and when it has enough carbon we will not have an insufficient spark for correct combustion, it can cause jerks when accelerating.

It can be caused by a dirty air filter or bad carburetion, we will check the filter and replace it if necessary. If the filter is in good condition we will have to check our carburetion or injection body.

Spark plug with burnt electrodes or white debris

They are signs of an excessively dry mixture (lack of gasoline or excess air.) A very dry mixture can also cause jerks in acceleration.

Possible causes of a lean mix are:

  • -A dirty carburetor or injector.
  • -Petrol pump with the insufficient flow, clogged fuel filter (dirty)
  • -An air filter that is too open or an air intake on the nozzles (cracks, poorly adjusted, etc.)

Spark plug worn or burned electrodes

The wear of the electrodes increases the distance between them, making it difficult for the spark to jump. When accelerating we need a more powerful spark and the spark plug will not be able to supply it continuously, this would produce those jerks or misfire as the mixture does not burn well.

Once the spark plugs have been checked, if they are in good condition, we can rule out faults in the mixture.

Check the rest of the electrical components

First of all, we will begin by checking the spark plug pipet visually that it does not have cracks, good contact with the spark plug and the cable, that the resistance is within the values ​​specified by the manufacturer, etc ...

Pipette

We will check the cables that are not cracked, peeled, burned or sectioned and that they make good contact at their ends.

Any leakage of current to ground through the cables or the pipettes would cause the spark to be insufficient, if both the pipette and the cable are verified and are in good condition we will inspect the next component.

Ignition coil

The first thing that we will check as in all electrical components will be their connections with ground and with the cables, to rule out possible failures that could be caused by a bad connection.

We will check the ignition coils by measuring the resistance of the primary coil (the 12v current input) output to ground and the secondary coil (the current output to the pipe wire) with ground output, comparing the data with the manual from the workshop we will know if it is in good condition or on the contrary we have to replace it.

In motorcycles of medium or high displacement we can find that the pipette and the coil are the same component, the verification is very similar to the previous ignition coil, we will follow the manufacturer's instructions for correct verification and we will compare the data with the workshop manual.

Regulator / rectifier



On some occasions, if all the previous components are in good condition, we will have to check the regular rectifier since it can suffer overheating and damage, although it apparently works, it may be having lower performance than necessary when we have more current demand.

There are very few cases in which these circumstances occur, but we must be aware that it is a possibility and once the rest are ruled out, it is best to check it or if we have the possibility, replace it with another to test.

Once these possible causes have been reviewed and ruled out, if the problem persists, we would tend to review the engine to rule out mechanical failures.