High fuel consumption

I've been reading through the forums and it seems people are getting between 170 and 200 miles from a tank.  I've only had my CB5 a month or so but it's doing just 140 to a tank and I don't drive it hard, just a 15-mile commute part motorway part country roads.  It's a '96 with 24,000 on the clock.

I overhauled the front brake which was definitely sticking and that's now running smoothly.  The back wheel seems to spin easily but I haven't opened it up (it's a drum brake) yet - could that be it?

Oil use seems to be a bit high - I'm having to top up after 1000 miles. Is there a clue there?

Any tips please?

Oooh, you might want to do some basic diagnostics.

If the back brake is sticking "on" you'll notice that when maneuvering the bike, the drum brakes can do that, especially if not oiled in vile winter conditions.

What is the idle speed? Check action of choke, if the needle valves aren't returning fully when you switch it off, it's spraying extra fuel in.

What's the compression? Basic tester is a cheap and very useful tool, if low consider checking valve clearance (or just do it anyway, satisfying job). Also, if it's low, throw a splosh of oil in the cylinder and re test. If the piston rings are responsible for low compression, the oil will (temporarily) increase compression, helping your diagnosis.

What does the spark plug look like? Oil fouling if there's lots of oil leaking in, sooty if it's running too rich. 

Have you cleaned / replaced the air filter lately? If it's full of gunge and dead spiders, it's effectively a choke, so it will run rich. Also look in the airbox for oil, it accumulates there when the bike is all getting a bit old and leaky.

When was last oil change? Old oil goes thin and gets eaten.

Finally, are you heading out of the driveway on cold mornings and hooning the tits off it all the way to work? 15 miles isn't far for the engine to warm up. Cold engines use oil, wear, and use more fuel, especially if ridden hard. Best be gentle on it until it's nice and warm, before you play with the 8,000 rev treat...

Does it have the standard gearing?
A lot of these have been raced, if you have a rear sprocket that is more suited for acceleration then your fuel economy will suffer badly.

Thanks for these.  AndyC, how can I tell if it's a standard rear sprocket?  And I'll run those checks Basil, see if anything crops up.  Idle speed is just under 2k but it was a bit all over the place when I first got the bike and I had to play with the idle screw to bring it down, so I'll look closely at that choke.

Another thing I wonder about is the petrol tank inlet (ie where you put the fuel in!) has been damaged in the past so I'm not sure it forms a 100% airtight seal when the lid's closed.  Could evaporation play a part?

How much fuel are you getting in on an average fill-up? It's an 18 litre tank, to brim it you'd have to stand the bike upright while you fill.

teamster1975 wrote:How much fuel are you getting in on an average fill-up? It's an 18 litre tank, to brim it you'd have to stand the bike upright while you fill.

When I fill up (do tell me to piss off if you know this already) I fill the bike on the centre stand and fill it to the bottom of the filler ring, in this way I know I have filled it the same as the previous time hence any mpg calculation that I want to do (from the trip meter, which I also reset at every fill up) will be accurate.

For the record, using the above method, I get 160 - 170 miles between full and reserve on my short, through town, commute.
My bike normally doesn't use any oil between the 4000(ish) mile oil changes.

To add to all the previous (good) suggestions - once you have ruled out the easy things (choke, air filter, rubbing brakes, fuel leak, changing oil, praying, goat sacrifice etc) then it might be worth doing a carb sync and while you are there checking the mixture screws (in case they have been pissed about with in the past).

eternally_troubled wrote:

teamster1975 wrote:How much fuel are you getting in on an average fill-up? It's an 18 litre tank, to brim it you'd have to stand the bike upright while you fill.

When I fill up (do tell me to piss off if you know this already) I fill the bike on the centre stand and fill it to the bottom of the filler ring, in this way I know I have filled it the same as the previous time hence any mpg calculation that I want to do (from the trip meter, which I also reset at every fill up) will be accurate.

For the record, using the above method, I get 160 - 170 miles between full and reserve on my short, through town, commute.

No I didn't know that was how you filled your bike ET

I've never got off the bike to fill it, always straddle it and stand it upright (ooer!). As you say fill to the neck, I usually flick to reserve about 210-220 depending on riding style.

Andy has a point regarding sprockets...
Measuring petrol economy the easy way may be done eg this way. or if you're so inclined: SpritMonitor.

The latter also offers insight into economy of other riders.

To my knowledge, carburettor mixture screws do not affect anything but near-idle-running.

* Mixture-screw: Idle mix.
* Needle: 1/4 to 2/3 throttle mix.
* Main jet: 2/3 to fully open throttle mix.

This graph depicting component influence tells a story...

http://www.gadgetjq.com/keihin_tuningparts.jpg

By all means, correct me if I'm wrong.

This CB500 encyclopediaforum is such amazing at times....

Thanks everyone, I've got some good leads to follow up and I'll report back once I've bottomed it out!

teamster1975 wrote:
No I didn't know that was how you filled your bike ET

I've never got off the bike to fill it, always straddle it and stand it upright (ooer!). As you say fill to the neck, I usually flick to reserve about 210-220 depending on riding style.

I sometimes do this (it can be quicker, especially when combined with pay at pump), however I should pass on some words of warning I heard from a petrol station attendant:

'You shouldn't sit on it while you fill up 'cos you won't be able to get away if it catches fire'

which is not quite as stupid as it seems, given the ease with which you can cover your hot engine and exhaust pipes with petrol if (say) the nozzle slips out of the tank or it overflows etc.

--

On a different note: Yes the pilot screw/air screw/mixture screw (whatever it is called) does only effect the mixture when at idle and small throttle openings, *but*

1. making sure it is set right won't do any harm

and

2. if you are really concerned it is worth taking it out and looking the o-ring behind it - it might be knackered (our bikes are getting to this kind of age now, I reckon). The air leak won't help the bike running well.

3. if you travel a lot in town you might well use the bike quite a lot between 0 and 1/4 throttle... if you like going slowly

You are probably right: it isn't likely to be that. I reckon the petrol tank has a hole in it...

eternally_troubled wrote:I sometimes do this (it can be quicker, especially when combined with pay at pump), however I should pass on some words of warning I heard from a petrol station attendant:

'You shouldn't sit on it while you fill up 'cos you won't be able to get away if it catches fire'

which is not quite as stupid as it seems, given the ease with which you can cover your hot engine and exhaust pipes with petrol if (say) the nozzle slips out of the tank or it overflows etc.

Wise words.

Hmmm, just opened up the air filter only to find that there isn't one!  OK, will fit one tomorrow but wondering why previous owner left it out?  And what will that have been doing to performance and fuel consumption?  NB the airbox is clean so no oil leaking in.

dominicacland@gmail.com wrote:Hmmm, just opened up the air filter only to find that there isn't one!  OK, will fit one tomorrow but wondering why previous owner left it out?  And what will that have been doing to performance and fuel consumption?  NB the airbox is clean so no oil leaking in.

Hmmm...
Running a bike without a filter might give a few more horses - at the start anyway, But if the filter is out of there for long you risk dust & other crap getting sucked in there which could damage valves, rings & cylinders. This could explain oil usage and high petrol consumption. I do hope it's not that serious but....
Like Basil said, do a compression test and take it from there.

What are the plugs like? Any oil?

That's interesting... I wonder why a bike with only 24k miles would have no air filter. Given it only costs 14 quid for an air filter (from wemoto) it seems dumb not to get one. In fact, even if you just shake the spiders out and give it a vacuum (rather than replacing it) at least it won't break your bike.

Anyway, let's hope your bike is OK!

As mentioned above, a compression test will tell if there has been any permanent damage.

To answer your question from right at the top - if the rear wheel is spinning freely then the drum brake isn't binding.

Thanks again everyone ... next step (as well as the filter) is the compression test.  And if that shows a loss of compression I guess I'll be digging deep into the Haynes and getting to know this bike a lot better!!

Once a year or so I brim the tank and add the full contents of one of these 

Redex

then take the bike out for a long, hard run, at least 100 miles,  making the engine work. I've found this can make about 5mpg difference after a year of town riding.

No air filter would result in running lean. I really want to know what your plugs look like now. If pale and pasty I'll be surprised - this sounds more like a cowboy attempt to get it running faster, so you may find the jetting has been played with to rich it up to match the increased airflow. Sooty looking plugs would be consistent with this. Is it really fast?

I wouldn't worry too much about damage to the engine - none of them used to have filters, I suspect that any spiders and fluff that gets sucked in just adds calories... If you ride in the sand a lot might be a different story,

OK, air filter in... and checked spark plugs which look normal - not oily or black... and yes Basil, it is really fast - at least to me it feels fast but this is my first big bike so I'm probably easily impressed!  But certainly it starts easily, there's no smoke, runs well (but see below...)  I looked at the choke needle and that seems to slip back in smoothly.

I'll see how it runs tomorrow with the air filter but I'm wondering if I have a carburation balance problem.  There's another issue I haven't mentioned on top of the fuel consumption, and that's a kind of judder when changing up as the throttle is turned and the revs kick in, the engine seems to labour to get power before everything gets smooth again.  Looking at Haynes I wonder if the carbs are out of sync - could this also explain the high fuel consumption?  Plus there seems a fair bit of vibration, I get numb hands sometimes... So if it is a sync problem, Haynes says this is a job for a specialist - any thoughts?  

As ever, thanks for the help!

A job for an expert. Or... 



I haven't tried this myself yet but I'm looking forward to having a go. There are a few different videos on there but the idea is broadly similar.