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

Home Repair And Restoration

Kangarooing CBF125

Bodge Date - Monday, July 6, 2020

By Ren Withnell

The CBF125 has a well known problem. Kangarooing. Or, in other words, going forward then jolting as though to stop, then going again for a second, maybe less, then jolting as though to stop again. A bit like a learner who's not got the hang of the clutch yet.

The rumour is that Honda had a bad batch of fuel pumps in 2011 and 2012. Apparently, allegedly, Honda replaced a load of them under warranty but this is hearsay. As my model is a 2009 version with almost 90,000 miles on the clock and this problem has not arisen I have always assumed this to be correct. 

Then I took the bike out for a ride one hot midsummer's eve (a rarity I know). Regulars will know I like to potter and explore, and while pottering and exploring some back lanes and fascinating housing estates just east of Preston my bike started to buck and splutter, much akin to riding an Aussie "roo" rather than my sweet genteel smooth 125.

I'll explain in more detail.

I'm riding at 15, 20 mph along side streets and housing estates, nosing into people's gardens and trying to work out if that lane will lead to those buildings. The bike starts to feel "woolly", the throttle feels a tad unresponsive. It is hot and I can feel the heat on my inner thighs from the motor.

The bike starts to falter, to hop. Oh dear. Out onto a main road and I need 30mph to keep with the flow, the bike struggles to pull me up to 20, hopping and bouncing and spluttering. Plug? Injector? Out of fuel?

I stop to check inside the tank, no there's plenty there. I restart the bike and all is well. Meh, carry on. The bike is OK for a mile then back in the side streets the issue returns. Hmmm. Blocked breather in the filler cap causing a partial vacuum? I stop and open the tank, there's no "pssshhhh" to suggest a vacuum but that's a dangerous assumption.

Same again, OK for a short while then I'm spluttering again. Deffo the tank, but to be sure I stop, turn off the ignition but don't open the tank, wait 20 seconds and restart. It's OK again but soon starts to be troublesome once more.

I repeat. Stop the bike, kill the ignition, wait 20 seconds, start up again, fine again, later it kangaroos again. How to test for blocked breather? Take it on the open road. Once I reset the bike I head out of town. The bike still feels somewhat "woolly" but I can achieve 50mph for some distance, not a blocked breather then.

If you wish stop for a moment and have a think, what could it be?

The plug is newish and correct. It could be compression at low revs being so low that bypass is sufficient to stop combustion but then why would stopping for 20 seconds fix that? Overheating motor - same. No, I have an idea forming in my feeble brain cavity.

Vapour lock. 

It's fair hot and while I don't yet fully understand vapour lock I know it's to do with hot fuel. In my mind I can see the fuel causing bubbles and flow issues in the fuel pump, thus leading to incorrect and lean fuelling. As soon as I stop the bike the bubbles rise back into the tank and I can continue. If I'm moving at pace the heat is blown away from the tank and the fuel pump, it's as soon as I'm pottering slowly the heat builds up. 

The fuel pump off the CBF125
Fuel pump, or fuel kettle?

I successfully keep to the main roads and get home. The bike still feels a bit woolly but rideable. I start my research. I have my suspicions not properly confirmed with solid science, but enough anecdotal evidence to steer me to think there's some merit in my thinking. 

There's 3 schools of thought.

1. The pump is knackered. A genuine Honda pump is (gulp) £205. Wemoto offer one for £90 (better but...) and Ebay has some random brands for, ready for this £27 (excellent)!!! 

2. Tank lining. There's a recurring tale that Honda lined the tank for some unknown reason with some unknown gunk. When the tank gets hot this gunk comes off into the petrol. This then blocks the filter, which is in the fuel pump, hence the need for a new fuel pump. The pump is "sealed" so you can't get in there and replace or clean the filter.

3. Vapour lock. While the shape of the low pressure fuel line to the pump (pretty much straight down) would make a vapour lock difficult I can imagine within the hot pump the hot fuel might vaporise causing bubbles and a poor feed to the injector. This is a vicious circle, lean motor equals hot motor equals more heat equals more vapours. 

The solution? Well if we're going with vapour lock it's because Honda didn't insulate the bottom of the tank like most manufacturers do. Wait... WHAT?!?! I ain't never ever seen insulation on the bottom of any fuel tanks. I'm not convinced by this... but... I do have a roll of aluminium tape... Hmmm. Might as well do something so I feel like I've done something.

The underside of the tank is covered in a layer of aluminium foil sticky tape
Shiny aluminium tape. Yeah science huh.

Another suggestion is to fit a fuel filter to the low pressure side to catch any of this gunk. Again if there was gunk in there after nearly 90,000 miles I'm sure some of it would have come off by now. Meh, it won't do any harm to fit a filter.

A cheap plastic fuel filter fitted to the low pressure fuel feed to the pump
There, fixed it innit. 

So basically I've done nothing, nothing of genuine scientific value at least. I've fitted shiny stuff to the underside of the tank to reflect a bit of heat and fitted a filter. I suspect I won't know if any of this makes the slightest bit of difference until the next really hot day. If the problem returns I'll fit a new cheap fuel pump.

In the meantime with the weather back to traditional British Standards (cold and wet) the bike is fine. I say fine. It's got almost 90k on the clock, the rattles are getting louder, I think another horse or two has left the engine in disgust and the oil addiction is not improving. If there were genuine horses in the engine they would have shot the remaining few out of pity by now.

If you desire mockery and dissection of your futile non-scientific bodges then click here.

Reader's Comments

nab301 said :-
Ren , the only 2 questions I have are 1 , why didn't vapour lock occur before? I seem to remember 2018 being hotter than this year? and 2 why didn't you make a tin foil hat while you were at it ....
On a serious note , can you not direct some cool water onto the pump body (externally) when the problem occurs and see if that helps?
06/07/2020 03:14:32 UTC
ROD said :-
Just ride the bike and forget the problem. The cool weather will probably out last the bike.
06/07/2020 03:46:34 UTC
Upt'North said :-
Put some corks on ya helmet Cobber.
06/07/2020 04:45:05 UTC
Ren - The Ed¹ said :-
nab301 -
1 - Damn good question. As like as not I haven't pottered so slowly in such heat. Maybe on other hot days I was "givin it large". Maybe something is failing...
2 - I already have my tin foil hat, my underground bunker, 45 million round of ammo (but no gun) and 3,000 tins of baked beans (gunna be smelly but safe).

ROD - I suspect you're absolutely right.

Upt' - Good idea! Keep the flies outta me face. Sharon suffers with dead flies on her visor, I'm so slow I get dead flies on the back of my helmet.
06/07/2020 05:22:08 UTC
Bogger said :-
The YBR125 used to suffer from fuel pump woes. When I first got mine, the mate who sold it to me said that occasionally, when hot, the bike would fail to start.

If you left it to cool down for ten minutes it would start and run fine and keep running until it was turned off again.

Before I even took it on the road I did a web search reference the problem. I bit the bullet straight away and bought a non genuine fuel pump off Ebay. Problem solved.

Not exactly the same issue as what you are experiencing. But I just wonder if your pump is getting hot and semi seizing, then freeing off a bit. When it's running 'tight' so to speak, there could be a loss of fuel pressure giving the problems you are having?

06/07/2020 06:07:26 UTC
Ren - The Ed¹ said :-
It's a perfectly valid option Bogger. It's also one of them difficult things to check. I must admit seeing them there cheap pumps on Fleabay makes me wonder if it's worth shelling out for one.
07/07/2020 08:03:59 UTC
CrazyFrog said :-
I'd be looking at the coil - we all know that 90% of fuel problems are electrical. Poor running when hot is a symptom of a coil failing....
07/07/2020 09:12:40 UTC
Bogger said :-
For £27.00 I'd just get one. If not, you'll be in the Trough of Bowland one day, obviously when it's raining and you'll breakdown without a phone signal. You know how it goes?

07/07/2020 11:35:17 UTC
Upt'North said :-
Wash your mouth out with soap and water, £27.00, it doesn't grow on trees ya know.
It always amuses me on the BeaST site when some Newby requests advice on what to check before riding his new to him ST. Now my response would be oil, coolant, brake and clutch fluids, tyre pressures and ride. A lot.
However the advice that is given would include a complete strip and rebuild where it is almost guaranteed the bike will never run again.
£27.00.......wild talk.
07/07/2020 01:16:55 UTC
Ren - The Ed¹ said :-
£27!!! Good heavens man, next you'll be suggesting I put that expensive fluid in, oh, wassit called, erm, petrol, that's right, petrol in it. I tried that once, rode around for a few miles and it all disappeared!

Curious on the coil CrazyFrog, I'd never expect that to be an issue when hot. It does live right above the motor though and right where it will get hot when riding slowly. What's the thinking regarding the coil and heat? Old coils breaking down, particularly when hot I'm guessing, but why?
07/07/2020 01:30:27 UTC
Upt'North said :-
Using old plugs beyond there useful life can cause coils to run hot and the subsequent degradation of the insulation between primary and secondary windings. The degradation will then lead to further heat build up and the overheating of ignition coils can hinder their ability to conduct electricity.
Or something like that.
07/07/2020 01:46:06 UTC
Upt'North said :-
Although, and I do mean this in the nicest way, please cover the bikes ears.
The little duck is plain tired, gets to us all. I would think everything must be on the verge of expiring, I'm nearly there myself.
I know whenever I check the BeaST I find something that isn't too great anymore but what is one to do.
A full nut and bolt rebuild, new engine internals, new paint......or just accept the inevitable. I feel like I'm talking someone through having to have their dog put down.
07/07/2020 01:51:17 UTC
Ren - The Ed¹ said :-
..sniffle...sob sob... oh my... sob... but... booo hoo!!

Yarp it's an owd dawg now and I fear it ain't gotten many more mile to ride on this earthly realm. I would like to have seen the magical 100,000 miles but maybe it ain't to be. We've had 7 good years and over 84,000 miles together but one day even this too shall pass.

So, anyhow, what next, as and when it does finally expire, I reckon there's more life in the owd dawg yet. With me working from home is it worth me getting another 125 for commuting? Maybe an off road 125 or 250 (if I can find one at a reasonable price). Maybe a 250? Maybe another CBF125 as I know the bike well, parts are still plentiful and I can break down the current one for spares? Maybe, possibly an electric bike for my local riding? Prices are still high presently though. Maybe something totally left field like a Harley or a GSXRR-RRRRR-RR 1000?
07/07/2020 01:59:51 UTC
nab301 said :-
I did have a coil break down on my Em Zed 301 a few years back . Broke down in the middle of nowhere, leave it for a while and it would fire up and stop almost immediately , ended up being recovered on that day.
For years before that , occasionally after the first start up of the day, it would stop at the same junction after leaving home and not start until I replaced the wet spark plug , i'd just put it down to character , now i'm wondering if that could be described as the longest drawn out coil failure in history...

07/07/2020 03:26:18 UTC
Bogger said :-
How about a PCX 125. Feet forwards is the war forwards. Comfy, quickish, extremely frugal, good underseat storage. You may even look young and hip on it.

07/07/2020 04:00:54 UTC
Bogger said :-
Two things, way forward, not war. I also think I said young and hip. Sorry scrub that as well.

07/07/2020 04:02:20 UTC
Bill said :-
Ren look a Yamaha Bws 125, well made, simple home servicing, air cooled, washable oil filter, screw tappets etc, very economical, easy round town where twist and go is a bonus and underseat storage and bag hook at front, even has some off road ability. Best of all cheap to buy and I have one ;-)
Posted Image
07/07/2020 07:36:21 UTC
Ren - The Ed¹ said :-
Oh lordy, I gunna end up with an adventure scoot ain't I.
07/07/2020 07:52:34 UTC
Snod said :-
Look at all this loser talk about the CBF being past it's prime, this is just where ownership starts to get interesting! Get wrapping some of that there shiny tape round the coil and report back.

The frame hasn't even broken in half, loads more life in it yet!
08/07/2020 01:51:00 UTC
Bogger said :-
I've got the answer for you Ed. Just take Sharons 125. She won't mind. Sorted

10/07/2020 07:04:01 UTC

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