In the engine bilge of my VP 2030 I have just found a small, 15mm long, black plastic/rubber cap about the size of a tyre schraeder valve, it looks like it was painted silver originally. Can anyone tell me where it was likely to come from off the engine - coincidently I have a developed a fault whereby the revs occationaly drop off then pick up again. I have changed both fuel filters recently so assuming I either have some air in the system or sucking air, but would the small black rubbery cap have anything to do with it?