[chris vickery]

Rob, the ethanol type gasoline is a major source of trouble for many reasons.
Lucky here in Ontario there are a couple places where the gasoline is ethanol free (Esso Premium and Shell V-Power)
When I bought a couple new pieces of lawn and garden equipment last year, both places told me under no circumstances to use ethanol blended fuels and to only run premium.
As you pointed out, needle valves with rubber tips- bad.
Ethanol is a solvent which destroys any and all rubber components. add to the fact that many of our small engines today are also made off shore from material of dubious quality and it furthers the problem. Old type rebuild kits are also trouble whereas modern kits for the most part have upgraded the materials to include diaphragms, gaskets etc which will stand up to ethanol.
There is also the other side of the coin; what many people do not understand is that ethanol is a CHEAP additive for gasoline. Petroleum companies have been adding it as a way of "cutting" the product to the consumer, for lack of better terms, watering it down...
The burn rate of ethanol is much quicker than gas as well, in essence, you burn way more quantity and get less MPG. I calculated it out comparing Regular to Premium on a cost per Kilometer and as it turns out, Premium is actually the better value. I know that my truck gets way more miles out of a tank of Premium than it does Regular.
Anyhow, back to your question. This has been discussed at length on the G503 Forum, specifically on the M274 Mule section. The common theme is that whenever guys mules quit running for some mysterious reason, it always comes down to a new fuel pump and carb. Co-incidence???
I run all my MVs now on Premium and will likely run my mule on 100LL Avgas once its finished. Problem solved.