I'm no mechanic but the more I read about this ethanol the more I worry about it. I just read that boats prior to 84 need to change their fiberglass fuel tanks and hoses. Whether this information is factual I don't know but it sure has me concerned.http://boatingsailing.suite101.com/article.cfm/ethanol_fuel_problems_for_boaters The ethanol is supposed to be kept at or below 10% but I don't think they're adhering to it myself. This probably has nothing to do with your present problem but if you have a fiberglass tank it might be worth looking in to.
