Motor surges with top end.

[Okstater2008]

I have a 2005 90hp 2 stroke Mercury, and after running wide open for 5-10 mins the motor will lose RPMs and start surging like it's not getting gas or getting it in surges too. But if I back it down to 1/2 throttle it runs great and will do all day.
History on boat and motor. The boat came with 60 hp big foot Mercury and the dealer switched it out for the 90 Hp to give it more power. However I have been having this motor issue since I have got the boat home. So far I have switched out all the external fuel lines, new gas cap with vent, new primer bulb, new fuel filter, sea foam to the gas, checked the intake hose and replaced the fuel pump. And nothing has helped so far any other suggestions?
PS it only does this when in the water off the trailer. At home with the muffs or on the trailer backed into the water it runs great with all the top end you want. But when I got to unload it and go across the lake it's a total different story. Thanks.

 

[ondarvr]

When it surges does it instantly lose RPMs like someone flipped a switch, or does it do it slowly?

If it's actually a fuel related issue don't run it that way or you will destroy the motor.

 

[Okstater2008]

It does it just like a switch. If you pump the bulb it runs good again for about 2-3 mins and does the same thing. RPMs normally falls from 4500 to 3500. It sucks cause take it in to the mechanic and he saids it's fine and runs great. Cause it doesn't do it on the muffs only if it has the load of the boat on it.

 

[ondarvr]

If squeezing the bulb helps, and since you replaced the fuel pump, it sounds like you still have an air leak. Most of the time a sudden change in RPMs is from an electrical issue though.

 

[Okstater2008]

I have checked for air leaks but haven't noticed any but what's the best way to tell if you have any for sure?

 

[Gibsonman2005]

I have checked for air leaks but haven't noticed any but what's the best way to tell if you have any for sure?

Cigar smoke. Cigarettes don't put out enough smoke.
Or soapy water, and look for bubbles.
I think most auto parts stores also carry a "leak detecting fluid" don't know the name brand. Rub around suspected areas and look for bubbles.

 

[ondarvr]

Soapy water and smoke are not going to find a leak in the fuel system. Squeeze the primer bulb until it's hard, then look for fuel leaking someplace between the bulb and carbs. Between the bulb and tank is a little harder, but you can put a section of clear fuel line in near the carb and look for air bubbles flowing through.