If the motorcycle is in working order, the idle level does usually not change from the standard setting, so an unusually high or low idle is a warning that something may be wrong.
Setting the idle level.
On the older motorcycles where a carburettor is mixing air and fuel, the idle level is set by a simple stop screv that prevents the throttle valves from closing completely, and you can simply turn this screw in or out to adjust the idle level. The same system is used on the first generations of fuel injected motorcycle engines.
On the newer fuel injection systems, the throttle valve is controlled but a small stepper engine, and it is the fuel injection ECU (a computer) that is controlling the idle level. And you will usually not be able to adjust the idle level in any way.
Usually we see the idle speed being between 1000 and 1300 RPM (engine revolutions per minute) when the engine is at operating temperature, and often as bit higher during warm up. But the correct idle speed depends on the type of engine and the air/fuel ratio, so first step is to consult the user manual.
Our firm recommandation is to keep the idle at the factory level, and if you must change it, you should aim for slightly higher idle level. The low oil pressure issue at a very low idle level is a genuine risk.
The rest of the text below contains lists of the possible causes if the idle level on your bike is too low or too high. Please note that we are focusing on fuel injected motorcycles here, so the list of root causes does not necessarily apply to the older carburettor bikes.
Idle level too low
If the idle level on your (warmed up) engine is lower than the level decided by the manufactorer, you will usually have two problems:
- The engine will easily stall - especially on take off
- The oil pressure inside the engine will be very low, and you may see the oil pressure lamp flicker.
Here is a list of possible root causes - not in any particulary order_
- Idle stop screw or eletronic idle level setting not adjusted correctly (This is the obvious one that everyone thinks of first)
- Too lean Air/fuel ratio (the mixture is already borderline lean on the stock bike, so if you installed an aftermarket exhausts and/or air filters that flows more air, you will be running even leaner than stock)
- Throttle valves not correctly synced
- Throttle position sensor (TPS) not correctly adjusted
- Valve clearances too tight
- Charcoal cannister in the tank ventilation line clogged by fuel or dirt.
Idle level too high
A high idle level is not dangerous for your motorcycle, but it have a few disadvantages and it's still a sign that something is not right.
If the idle is too high, the main problems are these:
- Increased noise.
- You may hear a rather loud "Clunk" noise when first gear is engaged.
Here is a list of possible root causes - still not in any particulary order_
- Idle stop screw or eletronic idle level setting not adjusted correctly (probably not a big surprise....)
- Throttle cable jammed so the throttle valves can not close.
- Cable to cold start lever jammed so the throttle valves can not close.
- Throttle valves not correctly synced
- Throttle position sensor (TPS) not correctly adjusted
- Leaking intake rubber hoses between throttle valve housing and cylinder head
- Faulty oil temperature sensor or water temperature sensor
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