Mercury 175hp won't rev over 3000 rpm

Clint56pu

Recruit
Joined
Aug 17, 2017
Messages
4
Please help. I am about to loose all my hair. I did some work to make it better, now this. I have a 24' pontoon with a 1979 merc 175hp 2 stroke. Under a load my outboard will rev up to 3400 rpm and drop back to 3000 rpm instantly. Last year it ran 4500 rpm under a load all day. No load it will rev 6000. I wanted to replace some bad wiring and now this. Here is what I did.

Replace fuel filter and add small inline filter.
Replace the stator, twice.
Replace the trigger.
Replace the rectifier.
Clean and rebuild the carburetors, twice.
Fuel pump output is plenty.
Compression 125 across all cylinders.
Good spark and plugs.
Ohm checked plug wires.
Bench checked coils.
Removed the low idle module, wiring was rotten.

I don't know what to do. Any input is greatly appreciated.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,036
Gee. remove the small inline filter and try it again. Fuel restriction can cause what you see. Also check fuel tank vent and antisiphon valve, for restriction.

Are you using a different prop, than last year?
 

Clint56pu

Recruit
Joined
Aug 17, 2017
Messages
4
I just put in a second stator. The last new one bench tested good. I did not check this one. I will do that. It has an electric fuel pump putting out 3 gpm at 3 psi past the fuel regulator. Vent is open.
 

Dukedog

Captain
Joined
Oct 6, 2009
Messages
3,235
stator can "bench test" fine but in reality be dead meat when put in operation............ needs ta be tested with dva.

​also when everything is right 4500 is way to low for wot for that motor... needs ta see 5500 min at wot..
 
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