High idle CBR | GTAMotorcycle.com

High idle CBR

Newblet

Well-known member
2008 CBR 125r

35,000km

Fine at start up but after a few KM of riding probably once or twice per week [usually on the hotter days] the idle will sit at about 3500-4000 while stopped at a red light. A few blips usually sorts it out or if I put it in first and let the clutch grab.

I looked at the throttle cable but don't actually know what I am looking for. The only thing I can think of is the valve clearance. I am due for another check [which I will be doing in the next 1500KM or so] but unsure if tight valves can cause this to happen sporadically.

I checked the other high idle threads with the 'check if already posted button' but they were for carbed bikes with chokes etc. Mine is FI.

Input?
 
Tight exhaust valves could cause the problem you're experiencing. As the engine warms, the heat closes the gap even more, preventing the exhaust valves from closing completely. Long term it may result in burnt exhaust valves. Perform the valve clearance check soon.

Did this problem suddenly appear after service being performed on the bike? Or has this problem crept up slowly over time?

Some other possibilities are the throttle cable binding. Ensure the fast idle cam is not interfering with the operation of the throttle plates in the throttle bodies. The fast idle cam pushes on the throttle butterflies and causes the bike to fast idle. If it is maladjusted, the TB butterfly valves will not fully close. Another hangup point could be the throttle position sensor. Another possibility is that you're getting unmetered air into the intake through a leak somewhere (most likely based on your symptoms).

So,
- Do the easy job first. check for binding in the throttle. this could indicate a binding throttle cable caused by improper routing, binding inside of the cable housing, and/or the throttle hanging up on the handlebar because of dirt or old grease. Removing the connection to the throttle cable at the throttle will help you determine where any binding may be occurring.
- check that the fast idle cam is not interfering.
- Leaks in the intake system sounds like a likely cause. Check the TB bushings (i.e. the rubber bits joining the TBs to the block) for leaks by spraying carb cleaner or quick-start in and around the bushings when the bike is idling properly. If the idle momentarily increases, there is a leak. Another area could be where the crankcase vents into the air box. Cracks in the rubber bits will allow unmetered air into the system. Old rubber tends to crack right at the hose clamps.
- The TPS is maladjusted is likely a problem. The TPS tells the ECU what is closed throttle and any maladjustment can throw off the fueling. Use the manufacturer's process for setting the TPS.
 
Last edited:
Crept up on me this season. Never happened in passed season.

Thanks for the detailed response. I'll have a go at it this afternoon.
 
This was a common issue on this particular model and year. Do check the valve clearance and make sure the cable isn't binding, but the real issue is likely a sticking IAC (idle air control) valve. It's built into the throttle body. If you look at the inlet and outlet of the throttle body you can see where the IAC valve draws air from before the throttle and lets it back in after the throttle and if you follow those passages, you can identify the valve (which also has some wires leading into the solenoid that operates it). I don't remember if the valve itself is possible to remove from the throttle body. About all you can do is clean it with parts cleaner.
 

Back
Top Bottom