[utopia]

Slob beat me to it but I'll post this anyway.


Firstly, I don't think this is a "general thing with old monsters".
My own yr2000 M750 does not suffer from the same problem as far as I'm aware (although in fairness it doesn't see rain very often these days).
But its not an issue which is generally reported on this forum .. in fact I can't recall a single previous instance.
Anyway, before tearing into the filler cap to replace seals suspected of leaking I would do two things ...

Firstly I would make certain that the drain hole (and its associated hose) from the filler recess is not blocked.
If you open the filler cap you will see this drain hole on the lefthand side of the filler recess. Being on the lefthand side, it will be at the lowest point when the bike is on its sidestand and so should drain away any water which gets in the filler recess before it has chance to build up to a level at which it can pass the filler neck seal. I would be inclined to blow through the drain hole using a suitably sized short length of flexible tubing. I guess you could even pour a little petrol down the hole to see if it exits via the drain hose. Its also worth checking that the hose is not kinked or trapped somewhere.
Naturally, while your looking there you will be able to see if the rubber seal on the filler neck looks ok and also if the seal area of neck itself is smooth, but I suspect that these will be fine and in any case, the drain hole should get rid of any water before it can threaten the main seal.

Secondly I would make sure that the water you're getting in the tank/carbs is not a result of ethanol in the fuel which is hygroscopically absorbing atmospheric water. I would do this by using only ethanol free fuel. The only source that I know of for this is Esso's premium grade fuel. This is described as E5 and although this grade is generally permitted to contain UP TO 5% ethanol, Esso's best (red nozzle) still contains zero ethanol in almost all areas of the UK. I never use anything else myself. It will also help somewhat in avoiding gummy residue build-up in your carbs and may also prolong the life of some rubber carb components, eg o-rings, needle seats etc. Another benefit may be a reduced tendency for the tank to rust internally (assuming your tank is steel and not plastic).