Good question, Dave. The best practice would be to drain the old fuel and start fresh with 2 gallons of fresh non-oxy gas and a whole 20 oz can of Marine PRO.

If the fuel is only a year and a half old, though, it should still start and run fine. The goal would be to create a high cleaning concentration of Sea Foam to fuel so it can clean up any fuel gum or varnish that formed while the boat was sitting.

What I’d do: Add a full 20 oz can of Sea Foam Marine PRO to your half tank of fuel. Run it for 20 minutes to burn through the treated fuel. Then, top it off with fresh fuel. You can add another can of Marine PRO if you want to the fresh fuel. It’ll continue to clean as you use up the tank, as well as prevent fuel gum and varnish from forming.

Good question, Wayne. If the sputtering is caused by fuel gum/varnish, Sea Foam will help clean it up and help overcome the sputtering.

Here’s what I’d do: Wait until you have a low tank of fuel (1-2 gallons). Then, add a full can of Marine PRO to the fuel. Run your boat at least 20 minutes if you can (just don’t run out of gas!), and don’t be afraid to run it hard. After that, refill your fuel tank as normal.

Here’s a (rather lengthy) explanation of why a high concentration works best. For upper engine cleaning (fuel injectors/carburetor circuits & chamber deposits that can affect running performance), it works much better to add the full can to a low amount of fuel for two reasons, 1) the cleaning concentration is considerably greater and, 2) the cleaning effect will start much faster with greater effectiveness. If you were to add Marine PRO to a high volume of tank fuel, it might take several hours of engine running for a lower concentration of the treatment to work its way through the fuel lines and through the engine. Understanding the concentration approach will go a long way in terms of understanding how Sea Foam’s engine products work for overcoming fuel residue restrictions or a rough-running engine. Pay attention to starting, smooth idle, throttle response, hole shot, acceleration, and top speed. The #1 cause of each of these issues is heavier suspended fuel gum and petroleum varnish that restricts fuel passageways. The most important thing Marine PRO does is liquefy and clear away sticky gum and varnish restrictions so the engine can go back to starting fast and running strong.

Hope that helps!

Yes, you can add Sea Foam to your external oil reservoir. Add 1 ounce of Sea Foam per quart of oil. Either Sea Foam Motor Treatment or Marine PRO would work great in the oil.

Hi Tim,

Thanks for the question! You can use Sea Foam Motor Treatment in a marine engine. (High Mileage Motor Treatment is also effective in a marine engine).

That said, I’d also consider using Marine PRO. It’s a great cleaner made specifically for marine engines.

Either one is a good option!