My .02:

I wouldn't say it has anything to do with a "cracked pick up" - whether it is carbbed or FI the line going to the motor/carb would be in the bottom of the tank, so regarldless if it was full or half full it would act the same.

you say it works better after you fill it up - If the tank vent wasn't venting, this could Give you some of your problems:

- Hard to start (YES)
- Stalls when idling (YES)
- Backfiring (possibly)
- Poor acceleration & power (YES)
- Top speed ~30mph (should be 60+) (Sounds right)
- Engine sounds "slow and loud" (Not sure)

Maybe when you are filling the tank up you equalize the pressure inside the tank (instead of having negative pressure) and this is a temporary fix?? Sounds Plausable to me anyways.

 

Ah, Excellent bighweel366! Why I didn't think of that is beyond me. I often rode an old '86 Honda big red that had that same problem with the same effects. Easy test is pull the gas cap off when she's having trouble and see if it clears up.

 

I did some searching on the Internet about the effects of Ethanol on fuel systems and it seems to be a big problem in the boating industry since they used fiberglass tanks a lot. The stuff is also being sighted as the cause of metal tanks, both steel and aluminum, corroding prematurely since it sucks water out of the air in hot weather and separates from the gas collecting at the bottom of the tank in cold weather.

The recommendations I keep reading are to add a water separator type fuel filter (not practical for an ATV), keep your tank completely full or completely empty to reduce condensation if your machine is going to set for more than 90 days and use a non-emulsifying stabilizer. Sta-Bil says their Marine formula fights the affects of moisture from Ethanol. But if as my mech says the Ethanol is eating the plastic tank I don’t think stabilizer is going to help.

I have found a few tank coating products that say they are Ethanol friendly but none of them say they can be used on plastic tanks.

BTW there was some thought that the boat mechs are making a killing from having to clean fuel systems (sound $400 familiar).