Organbuilder,
Do they make seasonal changes in your fuel in Sweden (most likely, yes)?
This might be clutching at straws but there is an article by Bob Adler in the most recent Vintage Truck magazine (Sept-Oct 2010) regarding the differences in volatility (vapor pressure) in gas formulations throughout the year (at different times and in different formulations in different geographic areas). Hard starting and poor running (and vapor lock at ambient temperatures - deadly, not only in fuel lines but also in fuel pumps and in carburetors) are some of the symptoms of using some formulations in the wrong season (symptoms that unfortunately could also be attributed to other factors). The author recommended not leaving full fuel-tanks in vehicles that will not be using that fuel in a 3-month period.
An extreme repercussion of wrong formulation for the season would be phase separation of alcohol and water from the gasoline, resulting in corrosion, diminished anti-knock properties, hard-starting, and poor combustion.
All of the above information might be meaningless in your situation; however, draining the gas and adding new gas is not a very difficult or expensive undertaking. Maybe even flush the fuel pump and empty the carburetor?
(As usual, Bob Adler has written an interesting and informative article in this issues' "Notes From The Corrosion Lab").
