Really high fuel consumption - 38MPG

Hey guys 

6 months ago, I got my first bike, a 1999 CB500.
She has been treated really badly by the former owner though.. here are some of her issues, just so you can have an idea:
- it it was running on one cilinder (fixed)
- the spark plugs kept failing (getting wet, so the bike wouldn't start, fixed)
- no lock under the seat
- the exhaust pipe (it that what you call it?) was cut to fit a CBF500 exhaust (so big it scrapes the ground sometimes)
- no fuel reserve (the Res switch position works as the On)
- lots of aftermarket parts, including the paint itself (he said he fell on the bike but tried to get it as original-looking as possible)


What worries me the most, though, is the fuel consumption. It got a little better after the cilinder issue was taken care of, but it's still doing ~7.5L/100km (38 MPG), way more than the 5.5L I thought it would do.. Also, I think it spends more if I put more than 7L/7.5L (1.8/2 gallons) at a time, at first I thought it was dripping from somewhere, but nope.^
Also, it doesn't seem to go faster than 160km/h (100mph), and runs badly on low rpm (i'm going to test drive a friend's cb500 to see how it rides compared to mine).

Any ideas of what could be causing this? 

By the way, sorry for any grammatical errors, english is not my main language 

Welcome to the forum. Well done on all of the work you've done so far.

They all run 'lumpy', at low revs - your friends will be the same.

Have a read of this regarding fuel consumption:

https://cb500.forumotion.com/t2206-fuel-consumption?highlight=Fuel

Hello!

If your bike gets 38 mpg on a longer run (not in town) then there is almost certainly something wrong, unless you are riding like a lunatic!

For example, my bike gets around 45 mpg when I am running around town and can get better than 60 mpg when on a long run, if I don't go too fast.

You could have a fuel leak even if you can't see it dripping: petrol evaporates quite quickly, especially in a hotter climate.  Is there any smell of petrol around the bike?  If there is then some is getting out somewhere.

A few things which will harm your fuel economy:

* Dirty air filter.  Have you changed/cleaned the air filter?

* Sticking brakes.  Can you turn the wheel fairly freely by hand?

* Broken hoses.  It is possible for the fuel hose can have hidden cracks in it - you will only see them if you look at it really closely and give it a squeeze.
the fuel tank off.

There are probably lots of other things that will harm your fuel economy, but those were the ones I thought off the top of my head.

Good luck, your bike looks good in the photos!

Welcome, that's a great looking CB! (Black is the best colour, not that I'm biased )
Not much more I can add that hasn't been mentioned above, let us know how you get on!

Sound like the previous owner may have fitted bigger fuel jets in the carburettor. This may explain the high fuel consumption and poor performance. Only way to find out though is strip them down and take a look.

Thank you guys for the warm welcome 
And thanks Sullivj, I was already reading that topic.

eternally_troubled wrote:
You could have a fuel leak even if you can't see it dripping: petrol evaporates quite quickly, especially in a hotter climate.  Is there any smell of petrol around the bike?  If there is then some is getting out somewhere.

A few things which will harm your fuel economy:

* Dirty air filter.  Have you changed/cleaned the air filter?

* Sticking brakes.  Can you turn the wheel fairly freely by hand?

* Broken hoses.  It is possible for the fuel hose can have hidden cracks in it - you will only see them if you look at it really closely and give it a squeeze.
the fuel tank off.

It used to smell a bit of petrol, mainly inside my garage, when she used to run on once cilinder. Since it got fixed, I didn't notice it anymore (nor do my neighbours), so I just assumed it was gone. Maybe we just got used to it.

I've already changed the air filter, and the brakes aren't sticking either.
I'll have a careful look at the fuel hose though, thanks (:

teamster1975 Thanks! Being black was one of the main requirements when I was looking for a CB ahah, it gives her such a meaner look.

mcalvert39, checking the fuel jets really isn't something i'm comfortable doing, but i'll keep that option in mind, thanks (:

mcalvert39 wrote:Sound like the previous owner may have fitted bigger fuel jets in the carburettor. This may explain the high fuel consumption and poor performance. Only way to find out though is strip them down and take a look.

I was going to suggest this also. Mine had them left over from a Honda 33bhp/25Kw restriction kit, a previous owner had taken out the restriction washers but hadn't known about the larger jets. It was thirsty on fuel and totally gutless at low revs. Also it crackled and popped a lot through the exhaust on deceleration and turning off the choke was like hitting the brake.

That seems to be an absurd fuel consumption. I'm running on 4 to 5 L/100 on both bikes, depending on the use. Unless your driving is ridiculous, it can be, in my mind, one of two things :

- Leaks. There's an overflow pipe, I think, that runs down the bike, and is stuffed with the others where the centre stand sits. I would check there (possibly block it just for test purposes, a little while?). You need to check all of your fuel pipes and seals (I would remove the tank and give it a close inspection, look for leaks at the tap, check the joints etc. If your reserve no longer works, clearly it has been messed about with.

- Carbs. Running much too rich (this would also explain poor performance - what do you spark plugs look like? They are a good tell tale sign of how the engine is running), or leaky carbs. Sorry to say that, but I fear you'll have to remove your carbs, open them up and take it from there. It's a fiddly job, but if you take pictures along the way and ask for advice on the forum, you should be able to determine if something is up with them. Cleaning them and restoring them to "new" settings won't do any harm if it's done properly (probably a word the previous owner has never heard).

Good luck...

The fuel tap would be my first port of call.  Whatever's causing it not to work properly could also be the cause of the high fuel consumption. 

Worst case it'll turn out to be something else but at least you'd run less risk of running dry miles from a fuel pump.

fuel tap issue could be the diaphram just did mine.

dirty fuel filter? mine was minging, carbs need balancing/cleaning? have you done an oil and filter change and air fliter change along with sub air filter?