Fuel starvation over 5000 revs

Just signed up and posting as a first timer.

I bought my CB new in 2003. I have done 36000 miles and don't rev/race the bike too often. I don't maintain it too well either!
Recently my bike appears to be starved of fuel if I open up and hit 5000 revs.
At low revs all seems normal.
Is there a common fault?
The carbs have not been tampered with since new.
My rudimentary mechanical knowledge suggests either
1. Sediment in the fuel tank.
2. Worn internals (jet?) in the carb.

Can anyone suggest help please

Are your fuel tank vents blocked? Check the hoses on the lower right corner of the tank.


You could also try a quick run with the fuel cap open.. be very careful of course!

Hi,
Sounds like a vacuum problem, i get slight vacuum symptoms when i fill my tank fully, goes away after a few miles ....

take the main fuel pipe off of the fuel tap off and leave the smaller vacuum pipe in place. turn the engine over quickly without letting it start (tap the starter button) and see if fuel shoots out freely from the tap (beware, if your fuel tap is fine then it can shoot out a fair amount so mind out for your eyes!). that will tell you if you have sufficient vacuum pressure and that your fuel tap is working.

may also be worth checking fuel filter and as other members suggest, breather pipes. If you have a compressor and an air duster then shoot some air through the breather to make sure its not blocked.

Hope this helps!

Had the same prob myself, would get so far down the road and the bike would stop due to no petrol, inspected the vacuum pipe and found it had perished near the petrol tap and was just sucking the air in.

I have to give credit where its due and my local bike shop (Burwins, Essex Rd, Islington - the guys there run CB500's) had it sorted in 5 mins. After all the head scratching it was a blocked air filter - Simples! I feel such a donut for checking this myself. Anyway back to better MPG and smooth running.

isnt that where kwackboy works. great ad for him and his mates. well done

Thanks lads for the excellent discussion.

Had the same symptoms myself today after replacing the radiator. Obviously the tank had been off to do that and I put it back together on saturday. Seemed to be running fine.

Today took it to work and after about 5 miles engine faded but restarted after a minute or  2. Then another 6 0r 7 miles the same thing . Managed to nurse it to work and worse problems coming home. Decided it was lack of air in the tank and took the fuel cap off every time it stopped to good effect.

Finally got to have a look a few minutes ago having read all the above. The breather tube with the right angle join in it had been trapped when I put the tank back on - I freed it with a gratifying in rush of air for about 20 seconds. I think I may have solved this one - will try in the morning. 

Maybe I wouldn't have found that for hours without the insight from above - cheers!

wheelywonderful wrote:Just signed up and posting as a first timer.

I bought my CB new in 2003. I have done 36000 miles and don't rev/race the bike too often. I don't maintain it too well either!

...
After all the head scratching it was a blocked air filter - Simples!

Was it still running the original filter?   ho ho ho

It is often best to start with the simple things first, but surprising how often they get overlooked.  In this case it is good idea to write (on the metal end of the filter) the date and mileage when you change it - at least then you know when you last changed it!

I think everyone makes these kind of mistakes at some time or other - I was messing around with my 'other' bike at the weekend (an ancient Ducati) and it was running for a bit, then stopping and being generally difficult to start (it has a kick start BTW).

I spent a while trying to rule things out - including getting the battery out of the car and hooking it up to that, just in case it was the problem.  I was considering doing all sorts of measurements on the ignition system etc

In reality I had just let the petrol level get too low!  I emptied some fuel out of my CB500, chucked it in and away I went... to the petrol station!

As I said, always check the obvious stuff...

lol you aint the only one E.T. when I had my CB I went thru everything as it had fired easily when I bought it. The guy said there was a couple of gallon in the tank and, stupid me, I beleived him. It was running on fumes and after leaving it for a week in the garage, the fumes had evaporated.
I was going thru everything I knew,checking this n that, then the mrs, bless her says, I take it it has got petrol in. I didnt look then, waited till she went in, and sure enough it was bone dry. I went to get a can of fuel from the local BP and put that in. it fired after a couple of turns to prime the carb.

what is it they say, theres no fool like an old fool, well here I am/ well was .

Yes indeed... I got my first bike (an old Honda H100) from a local garage who were selling it - I asked them if they could deliver it round to me, they said they could and they did.  So far so good.

In my exitement I went out for a ride on it.  I didn't check the fuel tap.  It was on 'reserve'...... need I say any more?

Suffice to say, that (thankfully) I didn't get very far, so I didn't have far to push it home.

Haha! When I got my bike the tap was on reserve...