Oil consumption issues

Hello all. Hope you're enjoying the marginally warmer weather- I know I am!

I posted a while ago regarding the oil consumption of my CB. It's a high-miler - 128000 to be precise - but I've had it since 24000 and looked after it well. Sooner or later I intend to replace the engine as it's using quite a lot of oil now - on a 200-mile journey of spirited riding with a fair amount of overtaking up to about 9.5k revs the oil level will drop from full to about halfway down the dipstick's hatched area. It's not outrageous but enough to be a bit annoying. Sometimes after a short burst of acceleration into high revs at low speeds I can smell burning oil, but there's no blue smoke out of the exhaust except a puff if I redline it in neutral (not something I'm in the habit of doing mind you!).

The consumption is really minimal when the oil's new, but after about 4000 miles as it starts to get old it increases until I change it again. So...I'm thinking of trying a slightly heavier oil, namely a 15W/50 instead of the normal 10W/40, to see if this helps a bit. I'm guessing it's the piston rings as there's never smoke on startup or at idle, and it certainly goes down quicker on longer journeys where I'm using more revs than when I'm pootling (ish) around town.

Anyone have any thoughts on the wisdom or otherwise of trying the heavier oil? Anyone else with high miles have these issues?

If it gets much worse I may as well get a 2-stroke : )

AFAIK there isn't any reason you couldn't try something like 15W50 - especially now the warmer weather has come along (it can be a bit thick at sub 5 degree temperatures).

It *might* not be the piston rings, it could be the valve guides that are leaking.

If you have a compression tester (or know somone who has one) it might be worth doing a compression test...

Oh yes actually I should have mentioned, I did a compression test about a month ago. Both cylinders were down from the specified 199psi, at 180 and 175. How low is this in the scheme of things, considering the high mileage? Both went up to 199 with oil in so I assume that narrows it down to the rings. Well I've ordered some 15w/50 (Motul). I will be changing it next week so will report back after a week or two with that.

I don't know how much the compression figures vary by - maybe I will do mine at some point (mine doesn't seem to use any oil at all).

It is encouraging that they are the same for both cylinders - at least there isn't anything catastrophically wrong with one cylinder.

I wander if a previous owner was a little lax at changing/topping up the oil, hence causing the problem?

Anyway, good luck with the heavier oil...

One thing often overlooked when people do a compression check is that the throttle should be fully open or the readings will be lower than they should be. Just a point to note before you get the urge to tear your engine to bits for no reason.
Honda specify 4000 miles as the service interval for these bikes, if yours works fine up to that point then maybe they have already taken note of it and decided on the mileage accordingly. This I have to say was a real pain when as a courier my bike had to be serviced every 2-3 weeks.
I believe that an engine lives or dies on it's oil, so use a good quality one. It doesn't have to be the hyper expensive stuff.
If you switch the viscosity to the 15-40 to stop any loss, you will be risking damage with the thicker oil when it is cold as the 15-40 will be just as thin as the 10-40 when it gets warmed up. Bear in mind that most engine wear happens in the first few seconds of running, presumably until the oil gets everywhere it should.

My bike has 30,000 miles now, 22 when I got it. Compression test at 24K was about 180 on each cylinder, apparently not dramatically low. Mine eats a bit of oil, I seem to top up every 500 miles or so (ish).

Impressed with the mileage on your bike! And agree maybe worth your looking elsewhere for the oil loss, if it's just a seal somewhere you can replace that's a better solution.

The first time I did a compression test I forgot about opening the throttle and so was horrified with the results, but then I realised and did it again, arriving at the figures I quoted above.

Actually, the oil I've got is not 15w/40 but 15w/50! I know warm-up is the time when most engine wear occurs and am very fussy about not ragging it until fully warmed up.

I do all the servicing on my CB myself and I'm sure there isn't a leak anywhere. The only seal I've ever changed was the gearchange lever seal. The other thing is that, as I said, the oil consumption gets significantly worse once the oil needs changing (when it's thinner as well?) which makes me think it's blow-by and/or failing oil control rings. I empty the drain tube from the airbox quite frequently but the amount collecting in there hasn't really changed while I've had the bike. Last time I checked there wasn't an excessive amount in the airbox either. But are there any other breather tubes I should check?

The oil burning is definitely related to high revs but as I say I never notice exhaust smoke. It's rather puzzling as you see the odd car or bike with tons of blue smoke coming out, which makes me think they must have to top up absolutely all the time if I do after 400 or so miles!

Valve stem seals?

If your gearbox has no issues i wouldn't bother with a replacement engine.


Seems to me that with that kind of mileage the bike deserves a serious working over, so i would look at pulling the top end apart, changing the valve stem seals, re-seating the valves, new piston rings and hone the cylinder. Would change the camchain while i was in there too.


That will probably not cost too much more than a new-old engine that could have all sorts of other issues anyway.


I believe the CB bottom end was built to last 300k miles with proper care.

Hario and others, thanks for the advice. The gearbox is fine. I had considered doing as you suggest, and I'm sure it would be more satisfying than getting a new-old engine which, as you say, could have its own issues.

When you say you think it would cost around the same to do those things, do you mean to pay a mechanic to do it or just the cost of the parts to do it myself? I'm a little wary of doing those kind of things to the engine as I've never worked on the top end or cylinders of any engine before (other than changing valve shims), but perhaps I should go for it when I have a spare week or so. I've always done all work on the bike myself (except tyres) and have obviously learnt a lot on the way, and so far haven't f*cked anything up. How difficult a job would you say those things are compared to, say, changing wheel bearings or changing valve shims? I'm just nervous about what seems like really taking the engine apart. There's good advice in the Haynes manual though, so maybe I will give it a go... In the meantime I'll see what difference the slightly heavier oil makes.

Engines are quite simple to work on if you know one end of a spanner from the other. Just be methodical & take plenty of pictures as you strip so you know where (& in which order) everything goes back together again.

Did you sort it out ? Mine smoke on the exhaust at high rev, but it doesnt seem to consume much oil. I use 15W-50 but here is really hot.