As the title says - what causes the fuel pump damper to rot?

Is it ethanol or is it stale fuel or both?

I use 98 octane fuel which I'd assume to be ethanol free yet the replacement damper rubber I installed (used) is now very soft and makes your fingers black when you touch it.  Admittedly the fuel would have been stale as health issues meant the bike was not used for almost a year.

Some parts sources specifically say the dampers they are selling are good for ethanol fuel.  Munich motorcycles in Perth, Australia have two types with a premium price for the ethanol safe one.

you have to remember that ethanol and lots of additives now used in fuel ...consist of smaller particles and permeates the old style rubber so any eaten away areas will be eaten away more

the new style of rubbers have a different index of permeability ...ie letting in-through  small particles within the fuel

maybe you should read up on the different types of fuel hose available as well , it may explain lots of things

currently the old sae rated fuel lines 30 r6 and 30 r7 will weep ethanol type fuels at quite a rate
whereas the later 30r8 and 30 r9  weep less as you go up the numbers , quite considerably less in fact

there are octane improvers , not ethanol, added to the 98 grades of fuel regardless of brand
so just be aware

hope that helps

A mates 3 Bricks of which all had sat at some stage and been run on ethanol suffered damper disintegration. My 1992 which I've had for 21 years, has never been left sitting and has never had ethanol fuel and has never suffered damper problems. I did replace it as a new one fell into my lap extremely cheaply. However the OEM is still perfectly fine. I was once told buy a mate in the oil industry 20 years plus ago not to run a certain brand 98 octane, as it was corrosive to some older fuel systems. I was not sure whether this is true or not, but I do avoid running that brands 98 octane. I generally stick to 95 octane and only occasionally use 98 octane when they don't have 95 octane. 
Regards Martin.

My theory is that it is either one or both of stale fuel and/or the water that ethanol sucks in from the atmosphere cause it. A bike that is run often, even on 10% ethanol, does not suffer from fuel pump mounting deterioration so it can't be the presence of ethanol alone.

Also, BMW Technical Service Bulletin SB-00-021-07 (NHTSA 10030825) states that gasoline with up to 10% ethanol is OK to use in a K bike.

My 84 RT does not like 15% ethanol fuel. 10% seems ok.

Thank you all for your views and information.

So the answer is - ride it and it wont be a problem!