Has it been doing the slow down before the tank was filled, or did it just start? Either way it sounds like a fuel delivery issue and I am just trying to eliminate the possibility of a tank of bad fuel.

If you have sludge or algae in the fuel tank, all you are going to do by changing filters is quickly clog new ones and still have the same problem. IMHO what you need to do is this....drain the tank. I know it is full but either drive it as much as you safely can to burn off the fuel, or get some buckets, 5 gallon cans, whatever and empty the tank. Once the tank is empty, clean the tank strainer, and while you are at it replace the short length of rubber hose going from the tank outlet to the metal fuel line. Then, replace both the spin on and the in-line fuel filters (remember to fill the spin on filter with fresh fuel before installing as it will save a lot of priming with the hand primer). Then put a few gallons of fresh fuel in the tank and a decent dose of your favorite biocide (Biobar, Stanadyne, etc). That should take care of the issue. You may have to change the filters again after treating with biocide so have a couple spares handy. Lastly, while it is running, check the clear fuel delivery line for excessive air bubbles, and then check the rubber fuel hoses for deterioration and replace as necessary, as well as tightening all fuel hose connections.

Good luck!

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