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Author Topic:   2004 Mercury 125-HP 2+2 Engine Speed Surges
michaelinbrisbane posted 05-01-2010 06:16 AM ET (US)   Profile for michaelinbrisbane   Send Email to michaelinbrisbane  
I have a 2004 Mercury 125-HP outboard that has started to surge when it warms up--a pain in the neck. When we pump the fuel primer ball, the [Mercury 125-HP 2+2 engine] picks up speed and RPM again. We have changed the filters and the plugs. We run [the Mercury 125-HP 2+2 engine] on 97 octane fuel. [Seeks] any suggestions [on how to prevent the Mercury 125-HP 2+2 engine from surging in engine speed as it warms up].
jimh posted 05-01-2010 10:46 AM ET (US)     Profile for jimh  Send Email to jimh     
[This article has been separated from another thread and moved to REPAIRS/MODS.]
jimh posted 05-01-2010 10:52 AM ET (US)     Profile for jimh  Send Email to jimh     
The most likely cause of the change in engine speed in a Mercury 125-HP 2+2 engine is the transition between running on two cylinder and running on four cylinders. This transition is controlled by the combustibility of the fuel-air mixture in the two cylinders that are turned on and off. There is no control mechanism associated with this transition, other than the carburetor design and fuel orifice sizing. That is to say, there is not a mechanical linkage or some electronic control system which might be adjusted or calibrated. The extra two cylinders apparently begin to contribute power when the fuel-air mixture in them becomes combustible, and when this happens the engine doubles in power.

In my direct conversation with several owners of this engine or similar Mercury engines which use this technique, all have reported that the transition from two-cylinder operation to four-cylinder operation is abrupt, and that precise control over the timing of the transition versus engine speed was difficult to maintain.

JMARTIN posted 05-01-2010 12:21 PM ET (US)     Profile for JMARTIN  Send Email to JMARTIN     
97 octane fuel, where do you get that? Read your owners manual. I'll bet they recommend 87 octane. I did not know they still made the 2+2 in 2004.


sosmerc posted 05-01-2010 11:55 PM ET (US)     Profile for sosmerc  Send Email to sosmerc     
That engine is designed to idle on just the top 2 cylinders.
When you pump on the primer bulb you are most likely momentarily raising the fuel pressure to a point where some fuel is being fed into cylinders 3 and 4 through their acceleration circuit check valves. Also, when you push the key in to activate the primer you also momentarily supply fuel to the lower cylinders and this will increase the rpm's.
If you are running the engine on a flush device there is very little back-pressure in the exhaust system and this makes it very easy for cylinders 3 and 4 to begin to run with just the slightest amount of fuel entering through the bleed system and check valves.
Surging is a fairly common complaint on these "2+4" engines.
It is very important for the carb linkage to be adjusted properly. It is important that the bleed system check valves be functioning properly. It is important the the accelerator pump and circuit be functioning properly. The fuel pump gaskets, diaphrams and check valves must be in good shape.
The carb needle and seats and floats must be working properly.
That engine does not require premium fuel. Mid-grade is what I recommend. But perhaps the requirements are different outside of the maybe you should consult your owners manual just to be sure.
Surging can also be caused by a bad exhaust cover gasket, but that is not a common problem and I would cover all the other bases first.
I have never been a particularly big fan of this model, yet I do have a number of customers that really like their 125's and they have been strong and reliable with regular preventative maintenance.

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