Mercury 20HP problems - help

I have a 20HP 4 stroke Mercury on my wife’s little pontoon boat. Motor is only 2 years old. It does stay on the water for probably 9 months out of the year. This last weekend the motor would start just fine but when you gave it gas the motor would choke down. It would not die, but it would not power up like it should. The motor is only 2 years old so I am surprised I am having an issue like this.

Could it be an issue with the Fuel Demand Valve or the fuel line? If there is a small leak in the fuel line or primer bulb letting air in, would it cause this type of problem?
Any suggestions would be appreciated.

Thanks

check the fuel line first. if you have ethanol fuel and the wrong line, it's likely the inner liner has deteriorated.

I bought the motor at Bass Pro and they gave me the fuel tank and the fuel line, so it is probably not a Mercury OEM fuel line, probably an Attwood brand.

Just went through this on our 30 (same series 4 stroke Merc.) The high speed fuel pump was bad. Our own fault, we generally just put-put around and you need to run them (rpm) up frequently or they'll seize.

Well that’s not what I wanted to hear, but thanks for the information.

Did you change it yourself or have someone do it?

How much did it cost to fix?

My 115 did that in the Spring and it was the plugs.

I had the fuel lines changed, siphoned all the gas out and put fresh gas in, and had the oil changed etc. but they didn't change the plugs because Yamaha didn't recommend it at the hours on the engine.

I told my wife I was going to try new plugs and when i did it took off like it was supposed to!

Didn't do it myself. The hardest part was the part itself, had to wait over a month just to get it in. I had replaced the plugs, fuel filter, fuel lines, on and on (I didn't realize these things had separate pumps for hi and low speed). The pump is on the left side of the engine looking forward, above and forward of the fuel filter, under its own cowling. $300 bucks and change was the cost (part included).

It was a very frustrating problem since we couldn't figure it out. Would run fine at any slow speed, never skip a beat. Give it the gas and it would sputter and eventually die.

under 30hp 4S engines are made by tohatsu. if you need parts, i'd look into the tohatsu brand parts. easier and faster and probably cheaper to buy.
i'm a tohatsu and mercury dealer. if you need it diagnosed bring it out it to Quitman. i'll be happy to check it out for you.

Yes - ours is Tohatsu - so you're the guy that could get the part and have the diagnostic computer to rig it. Good to know. Thanks.

yes.. i have the diagnostic hardware for both engines

Thanks for the input. I think I will start with checking the fuel line and fuel filters and move to the spark plugs before I tackle the fuel pump.