Bought a “new to me” pontoon boat 2 weeks ago. Motor is a 2011 Mercury 115 EFI 4 stroke. Ran great all day first day. Boat has 2 separate fuel tanks. Ran almost all of the 24 gallons out of tank 1 on first day. Second day I launch boat and go half mile and swap to tank 2…or so I thought. Boat started sputtering and cutting out. Swapped to tank 1 and ran great. Did this a few times back and forth and same result. Rode a short distance on tank 1 and went back home. Assumed tank 2 had bad gas…called seller and he confirmed they always used the other tank as it was on same side as his truck gas tank and they seldom used the boat. Drained gas, dried out tank, thoroughly cleaned it, and put in fresh fuel in both tanks. Launch boat, tank 1 runs great , tank 2 cuts out and sputters, go back to tank 1 and it runs rough and sputters. Ends up that it wont run on EITHER tank…had to be towed in…I’m hating my boat! Bring it home and replace the low pressure and high pressure filters under the cowling and put a new water separating fuel filter on the external bracket. Also took apart the brass fuel tank petcock and realized the internal piece was backwards to what the seller told me so when I went to tank 2 I was actually in a “neutral” position and not pulling from the tank. This explains the ORIGINAL issue. I cranked the boat and it ran great on both tanks ( on muffs). I disconnected all 4 injectors one at time and had noticeable drop off at idle so I feel like they are ok. Put it in the water and cranked right up, idled great, went a few hundred yards and opened it up. Within a minute it was sputtering and cutting off…on both tanks. Getting towed back again! Tomorrow I’m going to pull all the fuel lines which are only 2 years old, blow them out, and check for blockages. I’m also wondering if the fuel select petcock could be letting air in the fuel line…is this as detrimental on a 4 stroke as it is a 2 stroke? Maybe when I initially switched to tank 2 and it was not pulling from the tank it sucked some trash from line? I’m at a loss…any advice greatly appreciated.