There is nothing worse than starting your bike and finding out it doesn’t run the way it did the last time you took it out. One fairly common issue for V twin engine bikes, and bikes with more than 2 cylinders is when the bike runs - but only on 1 cylinder.
What does this mean exactly?
When a motorcycle is running only on one cylinder, it is oftentimes one of two issues: fuel is not being delivered into the combustion chamber, or there is no spark to ignite fuel due to a handful of causes.
If you think your bike is running on one cylinder you can quickly tell by feeling for heat near the exhaust pipes, or by listening to each cylinder.
Modern electronically fuel injected (EFI) bikes don’t have quite the same issues as carbureted bikes, but the advantage of a carbed bike is that it is typically much easier to fix, even for those with limited mechanical ability. A fueling issue with an EFI bike could mean a much more extensive repair than potentially just cleaning the carb and jets.
Possible causes for 1 cylinder not firing:
- Bad or fouled plug. This could be from carbon build up from being way overdue for a plug change, to running too rich, to being oil fouled.
- 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 boot could have a slightly loose fit and vibrated off or is not making good contact from debris, build up, grime etc.
- Bad lead wire. Check ohm resistance with a multimeter.
- Blocked injector(s)
- Sticking float bowl. (single carb) A carbureted bike is prone to issues if not stored properly or bad gas that lead to a dirty carb. A float tends to get stuck easily if a carb has enough build up.
- Blocked jets. There could be a blockage in one of the jets causing the bike not to fire at idle, half throttle, full throttle.
Internal diagram from a Yamaha Virago
Rule Out Fueling Issues
When you first notice your bike is only running on 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 premium grade 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 a lot carbureted motorcycles will have 1 carb per cylinder and tends to make 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. The up side is that if 1 cylinder is firing, it would be quite uncommon for there to be an ignition coil issue as there is already a functioning lead going to 1 or more spark plugs.
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. These boots have a tendency to vibrate with the engine and disconnect themselves after thousands of miles of riding. If this is the case you might want to consider new wires to prevent this from happening again in the near future.
You should test the plug and wire with an inline tester, which can be bought for under $12. Connect the tester to the boot and to the plug and fire up the bike. The tester will flash a light indicating that the plug is getting electricity and potentially creating spark as it should.
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.