Hello Gizmo,
I am sorry to hear your problems with your
Numpty.
Yes, there is a fuel tank breather valve it situated under the pillion seat and is white/off white in colour, round and the external diameter is 75mm or nearly 3 inches in old money. Internal diameter is 48mm x 48mm in depth.
Sorry, I'm unable to be precise with measurements as I don't have any callipers or vernier gauge etc.
It is held in place by 6 Allen screws,
C shaped metal
Rollover Plate (A type of washer to spread the load) then the breather valve and lastly a rubber O-ring type gasket.
The 3 items cost about £80 - £90 the plastic breather being the most expensive. The "Rollover Plate and the rubber O-ring type gasket being about £3 - £4 each.
There is a moving /lifting float within the fuel tank breather valve. I suppose this could stick or jam. I'm certain that the breather valve lets air and petrol pass in to the black plastic
Fluid Recovery Reservoir located nearby and connected by hoses.
I'm fully aware of the costs of these parts because when I had my bike re-sprayed the idiot who sprayed it installed the breather valve incorrectly despite me giving him a photocopy of the original workshop manual. I have witnesses, my wife, Paul (Phsyclist), Loz, who both saw petrol pouring out the back of my bike after a fuel stop, a mate, and a Ducati dealership yet he still denied all responsibility when I approached him regarding reimbursement costs.
If you need any info on the multi just inform me and I'll have a go at scanning the manual and sending it to you as an attachment.
Good luck, I hope you manage to sort the problem.