You would think that the simple task of running the engine at a steady low idle was an easy one. But things are unfortunately not so simple for our motorcycle engine, and there are a lot of causes for the common problems with weak and rough idle.
Why the motorcycle engine often have idle problems.
A lot of motorcycles have inherent idle problems where the idle is rough and the engine speed pulses slightly, or the engine feels weak - like it is about to stall all the time. But idle have never been an easy condition for the modern motorcycle engine.
The first problem is that the motorcycle engine have virtually no flywheel to help the engine run at a steady speed at low RPM. If you look at the huge and heavy flywheel on the family car you will see why the car engine runs so smooth and stable at an even lower idle speed.
So on the motorcycle engine, the ECU is working overtime to keep the idle steady, and it's not an easy task.
Next problem is the extremely lean air/fuel ratio from the modern fuel injection that will drain power from the engine in the situation where it needs it the most.
There is obviously nothing to do about the flywheel weight, and we do not really want to turn the crisp motorcycle engine into a heavy car engine, but we can improve the air/fuel ratio to ensure that we do release the full potential in the engine.
Other possible causes for rough, pulsating and/or weak idle.
It's hard enough for a fully functional and correctly adjusted motorcycle engine to maintain a stable idle, but if the bike have one of the following problems, the idle will be very weak, and you will probably see the engine stall quite often.
- Valve clearances too tight.
- Unlucky combination of aftermarket exhaust and/or air filters creating unwanted pulses in the air flow through the engine (This will make the idle rough, but unfortunately these resonance pulses are not easy to get rid of as they are not ignition or air/fuel ratio related)
- Throttle valves not correctly synced
- Throttle position sensor (TPS) not correctly adjusted
- A bad ignition coil, HT lead, or spark plug - especially on dual spark cylinder heads
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