I think your problem may well be 'stale' petrol.
The Old 4 Star fuel did not surffer this problem, but the new Unleaded contains high amounts of water, and, if left for any period, 'goes off'.
I discovered this to my cost with a lawnmower a couple of years ago, and the local dealer merely emptied the fuel, replaced it and all was well. I felt a bit of an idiot.
I have an old Military Landrover that is used only rarely, and I have found an additive that removes the water and therefore prevents this from happening.
You may find you will need to get this additive - available from any garage, or your local marina, and run it through. If you can get rid of most of your old fuel that may also help.