Camchain and tensioner seen up close in a cutaway bike engine

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Weird 125 Oil Usage

Blog Date - 09 November 2018

I wish my CBF125 would make its mind up.

It uses oil. This is to be expected with 77,000 miles on the clock. What is inconsistent is how much oil it uses. I typically check the oil after each fill-up with fuel, around the 250 to 300 mile mark. 

2 checks ago I had a shock! Nothing on the dipstick and the motor took best part of half a litre to replenish. Now if this were a car with a 4 litre oil capacity half a litre is not ideal but not a disaster. However the 125 only holds 1.3 litres so it was less that 2/3rds full. Yikes!

The hole where the oil is poured in on the CBF125
I put oil in - and then it's gone.

So the next time rather than waiting 300 miles I checked it at about 180 miles. The oil on the dipstick was exactly where it was when I'd topped it up. I checked it again at 300 miles and it had dropped but by there merest of margins, enough to suggest experimental error rather than burning oil.

This has happened several times before. I find it has drunk a half litre of oil, go into panic mode, check it more often, find it's barely using any, relax and return to less frequent checks for a while and then whoosh - it has another drinking session.

So what could it be? I have 2 hypothesis so far.

1 Motorways. A 125 engine is working rather hard on a motorway. As such especially with an air cooled engine the oil will be hot. Also the piston is moving very quickly giving the oil control rings less time to sweep away the oil. 

I have tested this hypothesis but not thoroughly. Ideally I need to take a 300 mile motorway excursion. I haven't done this but I have ensured I do more motorway miles than I normally would and to be honest the results have been inconclusive.

2 Piston Rings. Maybe - bear with my here - the piston ring gaps come into alignment which allows for big oil consumption then go out of alignment which reduces the burning to acceptable levels. 

The piston still within the engine of the CBF125
What is going on inside that little motor of mine huh?

My thinking is the must be quite worn by now rings are able to rotate in their slots and are doing so. Correctly installed rings ought to have their tiny gaps offset to the other rings' gaps to reduce blow-by and oil clearance. If my rings with now larger gaps come into alignment this would allow a lot of blow-by and oil consumption. When they then shake out of alignment the consumption returns to normal.

I am more than willing to listen to other suggestions. There are a lot of ways oil can be burnt or lost but before you offer your suggestion consider why the amount of consumption may change - that's the part that puzzles me.

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Reader's Comments

Ian Soady said :-
I've never been quite convinced by the "set ring gaps at 120 degrees" theory. If you think about it, the area exposed by a typical ring gap and piston clearance is tiny - say .1mm x .1mm ie .01 mm squared. How much oil will get through that gap?

Having said that, a very superficial bit of research came up with the link below.
9/11/2018 10:04:08 AM UTC
Ian Soady said :-
The link below is even more interesting especially where it says:

"Whoever heard of rings with 1/16" gaps - a ridiculous figure - but the interesting thing was that the increase in blow-by and oil consumption at 0.0625" was only marginally above the figures obtained with 0.015" gaps."

The conclusion was:

"All rings are free agents to rotate as they like, making staggering of gaps on installation a joke and ring gaps are not a problem provided that the gap is not the manifestation of reduced ring radial thickness caused by peripheral wear."

The whole article is well worth reading......
9/11/2018 10:08:50 AM UTC
Jim said :-
If the engine is really going from using none to using lots and back again, can you smell the burned oil in the exhaust when it's at its height?
9/11/2018 2:54:14 PM UTC
Ren - The Ed said :-
Fascinating Ian. It seems rings are free to wander around in their slots which suggests they can line up. Equally it seems the wear on the rings leading to a larger gap is not the issue, it's the loss of springiness off the rings due to their thinning that's the real culprit.

Buuuuuuut - if it is the loss of springiness of the rings is the problem this would be the case at all times. Which doesn't help explain the variable loss.

Jim - I haven't noticed but then I haven't been looking or smelling for it. I shall endeavour to be more observant in the future.
9/11/2018 3:45:24 PM UTC
Upt'North said :-
Firstly, respect to that little Honda, 77,000 miles, wow, wow and thrice wow.
Just curious, if that is your engine in the piccie why didn't you put new rings on it whilst you had it apart? Although I can see there's been little or no blow by suggesting the bore, piston and rings are OK. So I've probably answered my own question there. If not piston related it's got to be valve guide/seal rated. Does it smoke at start up occasionally, indicating oil has been sucked into the bore after switch off. Does it smoke on overrun. If it smokes only occasionally on start up this could still be valve guides.
But again wow! Honda's are great aren't they?

9/11/2018 4:47:22 PM UTC
Upt'North said :-
Re your spinning rings, ooohhh eeerrr matron. See a proctologist.
But no, no, behave yourselves..... Calm down missus.
The rings will spin unless pegged, more common in two strokes. So yes when they all align oil consumption will be greater and this rotation of the piston rings is controlled by load, rpm and wear. The rings could be turning by as much as 1 rpm but equally at certain loads, usually greater, will not turn at all. So this could explain some variation in oil consumption but it would probably equal out over the duration of a tank of fuel.

9/11/2018 4:58:18 PM UTC
Ren - The Ed said :-
The image is from the last time I had the CBF125 engine apart. That was way back at 20,000 miles when I was trying to work out what the knocking noise was. That turned out to be the clutch basket gear ring. Since then I've not been inside the motor save for the tappets.

It appears through various comments and Ian's research my hypothesis regarding the rings aligning (fnarr fnarr) would be incorrect. As for smoking on startup - no, nor at any other time.

It's a curious conundrum but don't go worrying yourselves, I shall just keep on pouring oil in as and when it requires it.
9/11/2018 5:37:43 PM UTC
Snod said :-
I always find my bikes stop drinking oil after a good top up, I have long suspected the fresh oil just makes the oil thicker in general and this stops it getting past the rings so much. Lots of motorway work can also cause a bike to become a heavy drinker, particularly if the oil is already worn/thinned as the bottom end in particular gets hotter than normal under these conditions - not so much your CBF but many water cooled bikes only have water around the head, the bottom end is still air cooled.

Any good bike uses a bit anyway, of course! :'D
9/11/2018 7:23:57 PM UTC
Ian Soady said :-
I suspect (as you hinted at originally) that it's more to do with the riding conditions than anything else.

Now's the time for an exhaustive test!

Check that the sump is full. Ride for 300 miles at a constant 50 mph - probably on motorway. Check sump level. Repeat at speeds of 55, 60, 65, 70.....

Then do the same but at varying speeds on give & take roads.

Make sure environmental conditions (temperature, air pressure, humidity) are the same for all tests.

Alternatively just accept it's a quirk and carry on riding.....

Or do some research (also know as googling). Throws up loads of academic papers etc eg the following:
10/11/2018 10:11:52 AM UTC
Ren - The Ed said :-
There is a veritable mine of information out there and to be honest it's bewildering and often contradictory.

Ian I shall see if I know anyone with an accurately environmentally controlled room with a suitable rolling road fitted with precise emissions and measuring equipment. I'm sure they won't mind me borrowing it for a week while I carry out my extensive research.

I plan to keep on using the bike for as long as it starts. I am academically curious to see how long the relatively untouched motor can last you see.

Snod - I can see your thinking but if you read Ian's last post this one suggests oil burn off with fresh oil is higher due to the lighter elements of the oil being burned off at first. I'm sure if we look hard enough we'd find a paper that supports your own experience that fresh oil burns slower.

Sharon has been enduring a horrendous few years with doctors trying to diagnose what is wrong with her daughter. While it seems some of their attitudes are appalling I have some sympathy when it comes to difficult diagnosis of complex issues.
10/11/2018 11:45:52 AM UTC
Upt'North said :-
Just like a fuel gauge but probably for different reasons the dip stick oil level doesn't drop uniformly to miles travelled etc. I would suspect that in the case of the dip stick or oil window this could be caused by sump capacity and shape. That is to go down the first half might mean 1 litre but the second half maybe only 0.5 litre, or pints if you prefer.
As for fuel gauges, that's probably witchcraft.
10/11/2018 1:41:51 PM UTC
bill said :-
Is the bike at the same level when checked , on these engines with small oil capacity even when on centre stand you get a relatively large variation with it tilted front wheel down or rear wheel down. Equally on the side stand is no good. Garage forecourts are sloped to deal with spillage so not level if your checking when you refuel. Try checking with both wheels on floor on level ground and bike upright every time. Also are you checking cold or hot on all occasions. If it is after a run it takes a while for the engine oil to all run back down to the sump. Is there any sign of oil in the airbox where your crankcase breather is connected to, If it burning oil it will not vary just get worse with time/miles
Or its just witchcraft :-)
10/11/2018 5:49:30 PM UTC
Snod said :-
In my experience the oil consumption practically stops when a decent amount of new oil is added or after a change, then 500-1000 miles later it starts to disappear again. If the oil is already well worn by the time it gets topped up it'll disappear sooner and more quickly. Super cheap (thin) oil also seems to disappear sooner and faster than gloopy/more expensive oil, it always seems the same. Well, except for the Z250SL which simply doesn't use any ever - that really is witchcraft!
10/11/2018 7:21:09 PM UTC
Ren - The Ed said :-
So let me get this straight. The sum collective intelligence of BAT readers is that, in the final analysis, witchcraft seems to be the problem. Now then, oh great diagnostic geniuses - do I need a witchfinder general, a Padre, a black cat or does Sharon need to join a coven?

Bill - my standard measuring practice is this. First thing in the morning before the bike is started it is places on the same part of the back yard. I clean the dipstick and while holding the bike on its balance point I dip it. I fully accept there are measuring inaccuracies but not to a large extent, particularly half a litre.

I haven't looked in the airbox for a while. Hmmmmm, might have a gander in there, it can't do any harm.

In this modern day of equality for everyone, would I be allowed to become a witch?

11/11/2018 7:06:23 AM UTC
Bill said :-
Quote from MCN, so maybe it's just the difference in use.

"Know your bike's oil consumption

Just because a bike uses oil doesn't necessarily mean that it has a problem. For a long time now, bike makers have designed engines with under piston oil feeds.

These are there to cool down the piston crown so that the engine can reliably produce more power. However, just like petrol, engine oil will evaporate at certain temperatures, and it gets pretty hot under pistons doing 14,000 revs a minute, so the consumption can mount up, especially if you use the bike hard.

A race bike at the TT can get through half its oil in six laps, so manufacturers quote maximum oil consumption figures of one litre per thousand miles.

As long as there is no evidence of oil burning at the exhaust end can, and you get into the habit of checking the oil level before you go out for your next ride, there shouldn't be a problem."

Not sure about you becoming a witch even in this day and age, maybe easier to become a wizard

Wizard Ren
11/11/2018 12:14:58 PM UTC
Ren - The Ed said :-
Cheers Bill. That kind of leads back to some of the technical papers Ian linked in to. Heat could be the key then?

Maybe some days on the motorway I may be cruising at my usual 50-55 mph but with a tail wind and little strain upon the motor, meaning the engine thence the oil is cool.

I may make the same journey at the same speeds but with a headwind putting the small motor under great load. While a headwind provides additional cooling across the air cooled fins it may be insufficient to compensate for a much greater load. The oil gets hot and merely evaporates off, not burns off.

I had a fair run last evening with a firm headwind, I shall dip the oil later with interest.
12/11/2018 8:34:10 AM UTC
Jammy said :-
Just my two pennies 1989 well worn Honda c90 cub (43000) never uses much oil on my daily commute but drinks it on a long run ie annual meeting in Wales 170 miles each way and mini holidays into Europe. Also did 1000 miles over four days to Southern Ireland and used a whole litre! Effectively giving itself an oil change as only holds .7 litre!
13/11/2018 1:29:44 PM UTC
Ren - The Ed said :-
I certainly believe my 125 drinks more oil on a long run than the local commute to work or my mothers. Since the 125's consumption has increased instead of changing the oil every 2,000 miles I now change it ever 3,000.

I like the idea of a self-servicing motorcycle! Never change the oil just keep it topped up eh? I reckon I'd still change it once in a while if for no other reason than to drain out the gunk that won't burn off.
14/11/2018 9:21:42 AM UTC
Ian Soady said :-
It's called total loss Ren - very popular in the 1920s. You set the pump so there was a drip every few seconds - the oil was just dumped in the crankcase where the flywheel flung it about and oiled everything, with the excess being burnt. Most had a pump so that the rider could add a bit more oil when working the engine hard.

Or even - though I hesitate to mention it - petroil in a 2 stroke.......
14/11/2018 10:21:22 AM UTC
Ren - The Ed said :-
Did you just utter a rude word at me Mr Soady? Go and wash your mouth out young man.

While it may have been popular in the 1920's I don't believe my 125 or Jammy's C90 were ever actually designed to be total loss. I guess in this day and age total loss is not really a good thing, environmental reasons and all that.
14/11/2018 8:18:02 PM UTC

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