Engine losing power at high rpm

Frenchy5

Recruit
Joined
Jul 23, 2011
Messages
1
Hi,

I have a 1997 40hp 4cyl mercury that is losing power/surging at high rpm/full throttle. If I pump the primer bulb it runs fine which leads me to the conclusion that it is not getting enough fuel. The problem is however, that I have replaced everything from the fuel tank to the carberators (external line with fittings and primer bulb, all internal lines clamped tight, fuel pump, and fuel filter) and cleaned and "rebuilt" the carberators (if you can actually call that a rebuild...). The only thing I can think of is that the floats are set incorrectly but I have tried the settings the manual gives and have even tried to set them by "feel", sadly neither has worked and I am stuck with the same results. Any help would be greatly appreciated.
 

99tracker90

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Joined
Jul 23, 2011
Messages
4
Re: Engine losing power at high rpm

I had the same issue today. I have 1999 Mercury 90hp 2 stroke 3 cylinder. We recently replaced the starter battery, spark plugs and #1 cdm (coil pack).

It runs fine at lower rpms but is flaky at higher rpms and does not feel like it quite makes it to WOT. We replaced the fuel line from tank to engine along with primer bulb and the fuel filter. The fuel line connector was not changed. Should this be replaced? We tested flow through fuel line connector by blowing through it and it seems fine.

There was also a small amount of fuel on the stern after starting. Any ideas on this? I primed it and then my friend primed it again prior to engine starting. Could this be the reason?
 

kschauss

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Joined
Apr 6, 2010
Messages
4
Re: Engine losing power at high rpm

I am having the same issue with my 85 50HP 4cyl. I have done full carb rebuild and gone through the entire fuel system. I am adjusting the timing today for the second time and then I am moving to the electrical system. I spoke with a Merc service place and they where confident that it was a either a carb issue or fuel line issue but either way something on the fuel side. He blamed it directly on the ethanol in the fuel and has said that he is rebuilding dozens of carbs each week.
 

CharlieB

Vice Admiral
Joined
Apr 10, 2007
Messages
5,617
Re: Engine losing power at high rpm

Did you rebuild the fuel pump or replace it?

A rebuild with an incorrectly installed gasket or valve will not pump full capacity and limit upper RPM and power just as you describe.
 

99tracker90

Recruit
Joined
Jul 23, 2011
Messages
4
Re: Engine losing power at high rpm

I replaced fuel diaphragm and it runs great. Cost about 30.
 
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