The outside of a motorbike engine seen up close near the exhaust

Home Ren's Biking Blog

CBF125 Dies On Tickover

Blog Date 16 March 2020

The bike started perfectly as ever. I had covered over 10 miles without missing a beat. I had just powered out of a 30 zone uphill to 50mph, levelled off then cruised downhill to a roundabout. When I stopped I noticed the engine was no longer running. Natch.

This has happened before, not often at all though. Every few months or so the motor will just stop as I approach a junction, usually after a hill or some other need  for sustained full throttle. 

Most times it's a case of jab the starter, the motor will spin for 5, 10, maybe as much as 15 seconds before the occasional firing of the motor becomes more regular and then finally runs. "tch, tch, tch, tch, ka-tch, tch, tch, ka-tch ka-tch, tch tch tch tch, ka-tch ka-tch tch tch ka-tch brum brum brum...." At which point the engine runs absolutely fine as though nothing has ever happened and nothing will ever go wrong again.

On Friday the 13th of March the motor did not restart and the bike came home to base in the back of a recovery van. Kudos to Nationwide Building Society's recovery and SOS Motorcycle Recovery for a perfectly adequate and satisfactory service. 

Ren's CBF125 on a car park awaiting rescue
At least it's not tipping down.

Later in the day the bike still would not start. Using jump leads saw it start but very unwilling to tick over. I charged the battery overnight and recorded a healthy 13.5v after resting off charge for an hour (don't check battery voltage immediately after charging). The bike started well and ran fine but would not tick over.

I replaced the IACV (Idle Air Control Valve). Donkey's year back I purchased a spare CBF125 throttle body out of curiosity and it came with an IACV. The bike started and ticked over just fine as though nothing ever happened and nothing will ever go wrong again.

A tiny stepper motor and a plunger forms the idle air control valve
The Idle Air Control Valve.
A simple diagram to show what the control valve does,
The IACV varies the air flow through a narrow channel that bypasses the main throttle. 

So - IACV intermittently erroring causing incorrect tickover approaching to the junction. Motor stops, perhaps flooding the cylinder with excess fuel. Ren can't get it going. By the time the flooding has cleared the battery is too low to start the bike. Yes, sorted. Excellent. But...

The old IACV seems to be operating as specified. No errors in the computer, it spins the same as the replacement on power-on. Swapping it back in the bike starts just fine. Perhaps removing it has jiggled and jarred the problem away? Such are the complexities of diagnosis.

What else could it be? Here's a few thoughts and I'm sure you can come up with your own.

Bad fuel. Previously the bad fuel passed through with a little cranking. This time the "lump" of bad fuel drained the battery before it passed through. 

Fuel Pump. Intermittent error on the fuel pump or possibly overheating when working hard - this problem often arises after a hill or damn good frapping. I daresay the bike oft times takes far more punishment than this last hill climb though. Cold weather and cold fuel should keep temperatures down. This doesn't happen any more often in the summer as the winter.

The CBF125 fuel pump
Is the fuel pump causing trouble?

Idle Air Control Valve intermittently sticking. 

Intermittent sensor issue. One of the various sensors sometimes plays up. This feeds the computer with the wrong information causing the computer to send the wrong amount of fuel and/or the wrong IACV setting and/or the wrong spark timing. This floods the motor.

Overheating. I'm dismissing this as there are plenty of times the engine has been good-n-proper given the beans in warm weather with no sign of hiccup or issue. 

And so my list goes on. In the meantime I am using the bike yet I continue to ponder this peculiar situation.

If you'd like to sponsor Bikes And Travels contact

Reader's Comments

Upt'North said :-
The IACV seems favourite and I personally would run it with the replacement valve until it does/doesn't do it again. I wouldn't go any deeper until the problem persists.
19/03/2020 05:51:25 UTC
CrazyFrog said :-
I know nowt about FI systems, but Upt's suggestion seems logical to me!
19/03/2020 07:05:01 UTC
Bogger said :-
The YBR125 suffers from fuel pump problems. Mine used to start ok but if you switched it off when the engine was hot, more often than not it would refuse to start until everything had cooled down a bit.

Maybe 10 mins wait. I put up with this for about once.

Then I replaced the pump. Problem solved.

I'm not saying this is your problem, but if you can get one cheap, I'd go for it personally.

After all your bike is not, er, how shall we put it? In the first flush of youth?

19/03/2020 08:12:01 UTC
Ren - The Ed¹ said :-
CrazyFrog - would it be worthwhile me writing up a simple explanation?
20/03/2020 04:07:10 UTC
Bill said :-
Ren is there no error code?
Have you checked the tank breather?
Tried a new plug incase it's weak spark at low rpm/idle maybe why low voltage is worse
20/03/2020 06:17:52 UTC
CrazyFrog said :-
Hi Ren, yes, a simple explanation for the mechanically retarded like me would be a great idea. It might help decrease my fear of modern bikes if I understood how the damn things work! Cheers, Pete.
21/03/2020 07:36:29 UTC
Ren - The Ed¹ said :-
No error code Bill. Tank Breather? Good thinking - tank partially drains causing vacuum "drying" the piston. Thing is I get no problems on a long motorway run so I'm putting the breather to the back of the que. I have since replaced the plug and I must admit it is/was an aged plug. Thing is this has happened previously ages back but not as severely, when the plug was still fairly new.

The intermittent nature is making diagnosis very difficult. In the meantime I shall just be mostly riding it. Pffffft.
21/03/2020 08:03:41 UTC
Ren - The Ed¹ said :-
Oh and CrazyFrog - I shall create an FI basics just for you me dear.
21/03/2020 08:04:33 UTC
nab301 said :-
Any easy way of checking fuel pressure under normal operating conditions? I know there are kits available , not sure what might suit, or whether the fuel injector failed to operate? Never used them but a noid light to confirm the injector is receiving a pulse? And as above I guess check plug for spark if it fails , plug wet or dry ?
21/03/2020 09:07:01 UTC
Bill said :-
Check the plug when its stalled is it wet or black, maybe the FI is overfueling at idle bit like leaving the choke on with a carb type. Is the tps calibrated correctly at idle or what about the o2 sensor in exhaust faulty telling ECU it too lean and so causing overfuel, so on closed throttle it overich causing the stalling.
Just some thoughts
22/03/2020 04:48:57 UTC
Ren - The Ed¹ said :-
nab301 - The fuel flow can be checked quite easily. When the bike is switched on the fuel pump runs for 2 seconds if I recall. There's a figure in the manual, say 100ml over 10 seconds. By catching the fuel pump outlet into a jug you just switch the bike on and off 5 times. Pumps only provide FLOW, pressure comes from constriction in that flow.

Bill and nab301 - The plug wasn't removed till I'd been cranking for a while so as an indicator it's a but invalid. Otherwise the colour is just fine. I probably won't remember but if it does it again I'll try to pull the plug sooner.

Bill - there's no calibration in the TPS. Unlike the Fazer 600 the TPS is within a block of sensors solidly mounted to the throttle body. As for both the TPS and Labda o2 sensors it seems 99.9999% of the time they work then just rarely *something* goes amiss. This is the problem with intermittent trouble.

It might not happen again... it might happen in 6 months... it might die on my next ride. Just gotta work through things as/when/if they happen.
23/03/2020 10:58:00 UTC
nab301 said :-
Ren , but there should be around 40 odd psi of pressure . I can't figure out though on the CBF / CB whether the pressure regulator is incorporated in the pump? I can remember a guy on a Bmw forum had a non starter and it turned out the mechanical pressure regulator was stuck open ,ie fuel flow back to tank , no pressure.
24/03/2020 10:33:25 UTC
Ren - The Ed¹ said :-
I certainly don't have the tools to check the pressures! I would be 99% sure the pressure regulator would be in the fuel pump on the CBF. Why? There is a return pipe to the tank and no return pipe anywhere else. Logically this can be the only option unless there's something I've missed. I would be relatively sure it's the same for my CB500X and Sharon's Z250SL. Out of the tank the high pressure pipe goes straight to the injectors, there's no return from the injectors. On these bikes like many others the fuel pump is inside of the tank.
25/03/2020 07:33:48 UTC
Bill said :-
Your pressure control is within the pump with spill off back up tank. Your injector is high resistance type not PWM so it literally open and close only on ECU command with only the time open being controlled. You also probably have ambient air pressure sensor probably in air box and a tilt switch to cut off if it falls over.As you have high oil use it could be contaminating tha lambda sensor which is giving post combustion reading to the ECU for correct fuel burn. As you are not getting any blink code with the stalling it may be the oil burning is contaminating your plug and giving weak spark at idle. As you have already checked/replaced the IACV.
Next time it does it you could try disconnecting the battery cables and touching them together AWAY FROM BATTERY for 5 seconds this will do a reset of the ecu.refit pos 1st as always.
Just some ideas to try
26/03/2020 09:56:57 UTC
Upt'North said :-
Bill like the oil theory, especially if the oil is going through the valve guides on overrun.
26/03/2020 12:56:59 UTC
Ren - The Ed¹ said :-
To sum it all up - next time it happens don't try to start the bike because any fuel will clean the plug giving a false reading. If I pull the plug and it's oily I can push my thinking towards oil consumption being the issue. Yes. I could see coming from high revs, coasting down to low revs and oil fouling the plugs. This is particularly pertinent IF IF the CBF125 is smart enough to actually cut the fuel when not required. Not all EFI systems do this but the clever ones do.
27/03/2020 02:59:54 UTC
nab301 said :-
Ren , What spark plug are you using? If you're using the ***8EA Ngk for high speed running , maybe try the ***7EA slightly hotter running plug ? I did fit iridium plugs to a few of my bikes and felt they ran better , including my Enfield which would traditionally be an oil burner which doesn't utilise valve stem seals and if you stray away from completely standard (ie lean) requires quite a rich running fuel set up for optimum performance.

28/03/2020 01:37:16 UTC
Ren - The Ed¹ said :-
Oh there's nothing worse than someone who actually knows what they're talking about! Bear with me while I go and fish out the plug I have removed and check which one I've put in.
28/03/2020 07:42:15 UTC
nab301 said :-
I don't know much , I just tend to read the handbook/ manual.....
28/03/2020 10:33:16 UTC
Pocketpete said :-
Could it be rust in the system after all I've seen the state of your 125.

29/03/2020 07:35:35 UTC
Ren - The Ed¹ said :-
Right then nab301. I have realised the plug I removed went in the bin and the bin has been emptied since. BUT - I do seem to recall it was one of the plugs removed from Pocketpete's CB500X when we serviced it. That would make it a NGK CPR8EA-9. The standard plug for the CBF125 is a NGK CPR7EA-9 but for extended high speed running the CPR8EA-9 is recommended.

Now - and correct me if I am wrong - the "7" and "8" parts are the CPR #??# EA-9 of the plug that is different and this will be the "hot or cold" of the plug. If the 8 is for high speed this would suggest this plug likes to loose heat faster than the 7.

I have been using a plug that likes to get rid of it's heat well, a "cold" plug. The idea then is my plug is running a bit cold. After a short blast in otherwise gentle riding the excess oil in the very aged motor fouls the already cold plug. I can go along with this idea.

Bearing in mind the plug is only a little colder than the recommended plug this could explain the intermittent nature of the problem. The plug is close enough to be fine in 99.99% of situations but every now and then the stars align all wrong and I'm stopped at the roadside. Most times with fuel washing away the fouling it will restart eventually, but not this time.

I'm not 100% SURE this is the case but as cases go it has an awful lot of merit to it. Well done squire.

Pocketpete - pfffft! Remarkably the inside of the tank looks as new, it's just the outside that's skanky.
30/03/2020 08:42:36 UTC
Bill said :-
If not the plug then possibly oil is getting into the IACV from the engine side causing it to stick intermittently. Iridium plug will allow you to use a bigger gap so fatter spark.
I still wonder as the burnt oil is going down the exhaust, is the lambda sensor being contaminated thinking it running rich the ECU will lean the mixture by injecting less fuel. You have already been on closed throttle when slowing, so no injection as rpm drops to idle with closed throttle and at normal running temperature the ecu should select idle map for fuel level (duration injector open) and ignition timing to match the air flow via the IACV the lambda sensor then verifies the air fuel ratio is correct,lean or rich. I also think your model has a cat within the silencer the oil could be contaminating the cat which can also effect the lambda sensor readings.It is like a perfect storm you could have oil contamination that effects the sensors and at a particular point the injection is being switched off. One other thought have you checked the tilt sensor is securely mounted and its plug is clean? Intermittent faults are always the hardest to diagnose.
30/03/2020 10:25:34 UTC
nab301 said :-
As Bill says and I think I mentioned in the rebuild thread there is so much oil going through that engine that even using the correct low ash catalytic friendly oil it is more than likely causing problems .
As you say The higher numbers in NGK are the cooler running / harder plugs so you want the lower number hotter / softer plug . Back in the day it was all a bit confusing with the old Champion plug numbers , A 7 or 6 was a very hard plug or even something like an N3C needed for Triumphs and Velocettes and a 10 or 12 was a very soft or hot running plug favoured by old Triumph cars and the like.
30/03/2020 03:00:51 UTC
nab301 said :-
I should have said afaik the -9 at the end of your plug number is the preset gap , that is .9mm
30/03/2020 03:05:28 UTC
Ren - The Ed¹ said :-
Bill - the IACV is way upstream of the engine so I see no way oil could get in to foul the IACV. If it were sticking I would be looking more at the air filter letting muck through.

I can see the logic of excess oil causing the lambda sensor to send the wrong readings. I'll take a look at the tilt sensor too but all I can do is test the wiring and check with a multimeter what happens when I tilt it.

nab301 - hot and cold plugs should be a simple concept. But then, erm, a cold plug is better for a hot engine so does that not make it a hot plug cos it works better in a hot engine? No Ren no, stick with cold plug is a plug that runs cold, hot plug is a plug that runs hot. Sheez.
01/04/2020 09:52:44 UTC
John Almond said :-
Having speed read all the suggestions I'm not sure, but has anyone mentioned the HT lead.
Many moons ago when I ran vespas, (sorry guys) I came across a problem where it would conk out after not only a long run but also a short run and after pulling the thing apart checking all sorts it ended up being the HT lead breaking down then kinda fixing itself (don't ask me how it did that)
For a couple of quid it might be worth replacing it.

15/07/2020 10:19:28 UTC
Ren - The Ed¹ said :-
Cheers John. On another post with another CBF125 issue someone pointed out the coils could be breaking down. It seems like there's a reasonable argument to replace the coils and lead. Looking on Ebay I can get a used unit for next to nowt. Used, about £5 to £10, there's "pattern" new ones for just over a tenner. Straight from Honda it's £57 which ain't cheap, but well, you know.
15/07/2020 10:28:40 UTC
Upt'North said :-
Apology accepted John.
15/07/2020 10:52:31 UTC
John Almond said :-
Thanks Uptnorth.
So is it not just a cable that screws kinds of into the suppressor cap on one end and into a CDI at the other end?
On the vespas it's unscrew fromt cap, unscrew fromt CDI box and chuck, then reverse procedure with fresh cable, Bish bash bosh
15/07/2020 12:10:01 UTC
Ren - The Ed¹ said :-
Not sure about that John! I know the supressor cap is a standard screw on thing but beyond that I've not checked the coil end. At these prices though it's barely worth the effort of removing the tank.
15/07/2020 12:40:12 UTC
Tim Lewis said :-
Take off the rocker cover and adjust the exhaust valve clearance. Over time it tightens up, so after you have gone a few miles, the engine warms up and so the valve doesn't close properly. You will have to do this a couple of times, at about 8k and then at about 14k, then it will settle down.
24/08/2020 02:16:28 UTC
Ren - The Ed¹ said :-
Cheers Tim! Thing is the bike has done at that point 88,000 miles and since this event it's achieved the 90,000 mark. I am familiar with tightening valves myself having fought with them on this bike and a CBF250 as well.

But it still remains very sound advice - if you have an issue with tickover take a look at the valves!!
24/08/2020 06:05:45 UTC

Post Your Comment Posts/Links Rules



Add a RELEVANT link (not required)

Upload an image (not required)

No uploaded image
Real Person Number
Please enter the above number below

Home Ren's Biking Blog

Admin -- -- Service Records