Why is My Bike Only Firing On One Cylinder?

 A bike firing on only one cylinder or afterfiring from one cylinder can mean a few things. 

  • Fouled plug. This could be from excessive carbon build up from being way overdue for a plug change, running too rich, to being oil fouled the oil fouling would be due to either bad rings allowing too much oil into the combustion chamber, or bad valve guide seals, allowing oil past them from the top end, into the combustion chamber. 
  • Blocked or poorly vented fuel tank. Gas in the tank needs to be replaced by air as it is used, otherwise it creates a vacuum that stops the flow of gas from the tank to the injectors or carbs
  • Bad lead connection. The resistor caps or boots on the end of the ignition wires could have a slightly loose fit and vibrate off or they could have a bad contact from debris, build up, grime, hatred etc.
  • Bad lead wire. Check ohm resistance with a multimeter.
  • Blocked injector(s) this is only applicable if you have a fuel injected bike, however this is a rarity. 
  • Sticking float valve. (single carb) A carbureted bike is prone to issues if not stored properly or used with bad gas that leads to a dirty carb. A float valve tends to get stuck easily if a carb has enough build up. With today's fuels, leaving a bikes carb full of fuel for just a few weeks can do some horrible things to the jets and the float. 
  • Blocked jets. There could be a blockage in one of the jets causing the bike not to fire at idle, half throttle, full throttle, or the jets have been partially clogged this only allowing a reduced amount of fuel into the combustion chamber. 

    Rule Out Fueling Issues
    When you first notice your bike is only running on one cylinder or afterfiring badly from one cylinder , you will want to make sure that the fuel petcock is turned on AND that you have fuel in the tank. Preferably fresh gas. If you can hear gas sloshing around, but it seems very low, you might have to turn the petcock to RESERVE, which will draw gas from a lower point in the tank.
    One simple way of telling whether your bike is getting fuel to the cylinder is the apparent smell of unburned gas that will still be forced through the exhaust.
    Ensuring that your bike has gas, and that it is flowing to the carburetor will rule out a dry or evaporated bowl where the jets of the carb draw fuel before mixing with air and flowing into the cylinder. If your bike was not low on fuel, the gas has been flowing, and the carb(s) or injectors are getting fuel then you are most likely having a spark issue, though a single bad injector is a possibility. Bikes with multiple carbs make this a bit harder, but if your carbureted motorcycle has 1 carb per cylinder this makes it easier to narrow down fueling issues.
  • Ignition or Spark Issue
    Your bike has fuel, and that fuel is being mixed with air, or injected into the cylinder, but not igniting? Check the spark wires for damage like tears, cracking, melting etc. Look in the obvious places first where it is prone to damage. Note the condition as any damages should be repaired to limit the chance of future issues. 
    A simple thing to check is the spark plug boot. 
    If the boot that connects the wire to the spark plug comes off relatively easy, that might be your only issue and it probably just needs to be seated correctly on the plug. The resistor caps have a tendency to vibrate with the engine and can loosen themselves after thousands of miles of riding. If this is the case you might want to consider new wires and resistor caps.
    There is still a chance that the plug has fouled from carbon build up or excessive oil in the cylinder. Pulling the plugs out and inspecting for spark gap and build up would be the next step in the process. Replace the plugs with new ones or after cleaning and torque them to the appropriate amount before attempting to fire the bike up.