2001 Mercury Mariner 225 Hp Outboard

[Matthew]

Hello, I purchased a 2001 Nitro 288 F&S, with 225 hp Mercury Mariner outboard engine 2 yrs. ago. Took it to Table Rock lake last weekend and she wouldn't idle without putting it in neutral and giving it some throttle. When I bought the boat, I took it to our local Mercury dealer and they had it running great! It only has around 120 hrs. on the motor. Any ideas on what the problem could be. I did get her up to around 20 mph, with full throttle. Could it be a fuel problem? Any advice would be most helpful!
Thank you,
Matthew

 

[cq]

When did you change the fuel lines last? Ethanol destroys the inner lining of fuel lines over time.