Fuel cap problem

Hi guys,

The CB500 cuts out when you go over say 50 mph and won't start again until you open the filler cap, at which point you hear a loud hiss and then everything is fine until repeating the process.

I've asked my mechanic and he says its the filler cap that is the problem...

This makes sense as the bike runs fine with the cap left open whilst riding even at top speed (not recommended yes, but I needed to test).

Apparently there is a small ball bearing in there (in the cap) somewhere that could be stuck in position.

Does anyone here have any experience about maintenance on the filler cap?

Thanks.

Please help...The Mrs was so happy to have her bike back on the road again after a year of it sat in the garage not working...Only for us to go out at the weekend and it died again.

I didn't need to do the clearances after all (I thought it was the valves tightening that was causing probs although they were tight), but at least I now know the culprit.

I dunno if you can open it up, but it is completely removable. You could immerse the whole thing in your favourite solvent/brake cleaner/diesel/simmer on the hob and it should be okay.

Sorry no idea about the cap but there is a vacuum tube from the petcock and a breather tube on the other side, are they clear and connected?

Sounds like a blocked or trapped breather to me too.

Remove the tank and check the breathers are all clear and not pinched.

Undo the bolt at rear of the tank and lift it slightly (a few inches will do). On the right hand side (other side from the fuel tap) you'll see two pipes.  Make sure they're not kinked, pinched or blocked up.

If the aforementioned breather pipes are unkinked and you're sure the blockage is at the cap itself, you can dismantle it. It's fiddly, but possible. 

Open it, then remove the three screws that hold the assembly to the tank (approx 12, 4 and 8 o'clock). Take the cap assembly to the bench (or the kitchen table), so there's no danger of dropping anything in the tank. 

If you then carefully prise the rubber seal away to get the steel ring that holds it in place out, then you'll see the three small screws that hold the assembly together. Once these are removed you'll be able to access the convoluted seal and clear it of any blockage. Reassembly is a reverse of the above. Getting the seal and retaining ring back in place requires patience. I did mine the other week to sort the lock, sorry I didn't take any pictures. 

Good luck.

GlenAnderson wrote:If the aforementioned breather pipes are unkinked and you're sure the blockage is at the cap itself, you can dismantle it. It's fiddly, but possible...

This is exactly the pointer I was looking for.

Thanks very much.

Will tackle it in a few days when I'm off shift.

Cheers 

Getting the seal and retaining ring back in place requires patience. I did mine the other week to sort the lock, sorry I didn't take any pictures. wrote:
Good luck.

Hi, I did as you said, took cap off and took it apart by prising thick rubber seal to reveal metal ring.

Removed that to reveal screws which I undid to dismantle cap.

Saw another seal that has an edge which exposes a small hole. Cut this seal down a little as thought it was obstructing that hole a little, and did some general cleaning of the cap innards.

Put everything back together, removed breather hoses on underside of tank just in case.

Ran the bike...and still the same..cuts out after reaching about 80 now so something has changed.

Again, the bike rides fine with petrol cap open.

One question...Are you supposed to have screws on all the caps retaining screw holes.

I notice that one of them feels like there is no thread.

Please help guys.. Need to sort this out as she needs the bike for work starting in a couple of weeks.

Should I just buy another cap?

Cheers.

It might not be the cap, in fact, I don't know if the CB500 petrol tank is even supposed to breath through the cap - it has a breather and overflow pipes (the ones you removed) so shouldn't need to take air in through the cap.

As I recall (from someone elses' post) the breather pipe (the metal bit that goes to the top of the tank) has a ball-bearing valve in it, this, I suspect, can get stuck and block the breather. If you take the tank off (preferable when you don't have much fuel in it) and shake it around you should be able to hear the ball bearing rattling around.

If you **really** need the bike to work then take the fuel cap off and drill small (half millimeter?) hole in the top - this will solve the immediate problem and won't let too much petrol out. You can seal the hole up later with 'plastic metal' or similar epoxy putty/glue when you've fixed the problem.

Thanks...So this metal breather pipe which I can see just past the cap opening...

Which pipe on the underside of the tank does this link to?

If I empty the tank first, would compressed air blown through this pipe help or how would you suggest I clear this pipe?

Thanks for your help so far.

Important: Also should every screw hole in the cap retainer have a screw in it

@theincredibleaf wrote:
Important: Also should every screw hole in the cap retainer have a screw in it

Mine has. Three of them hold the cap assembly to the thank, the rest are dummies.

The breather and overflow drain hoses run down the bike from under the right hand side of the tank.  I think the breather is the thinner one.

Check the breather is not blocked up at the bottom, has no kinks or pinch points, and blow some air down it to make sure it is clear.  If you need to disconnect the hose you may need to unbolt and lift up the rear of the tank. You probably don't need to drain it, but it helps if there is less fuel in.

Some different bikes have vented fuel caps and no breather.