I am always looking at maintenance and keeping a bike on the road. Some times the simple fix if left along can cause problems. On my Ducati I found that if the drain pipe around the petrol cap is blocked water then leaks into the tank causing misfire, running issues.
So when I checked the Moto Guzzi Bellagio petrol tank. I found when filling up. I could see the water was not draining away. So I managed to take the petrol tank apart today and unblock the breather tank. If you look at the outside of the cap there is water still holding there. It has not drained away. I used an air line on the service to blow back from the bottom of the tank but this has not worked. I connected a plastic pipe and blew from the bottom of the tank and could tell that the overflow was blocked.
I connected up a push bike pump with the narrow adaptor and tried at first to blow back. This did not work. I managed to get the air pressure up to over hundred pounds but it would not work. I then tried later to blow back down with the pump and managed to dislodge some dirt. I cleaned all the rust that had appeared on the parts and put some grease on the screws and put it all back together. I tested the overflow and sprayed a can of dwd40 from the top. It now drains away easily.
I hope that you do not leave it if yours has the same issue. If you have water around the cap. Either it could leak in while raining or when you open the cap to top up with petrol.
Keep your bike on the road.