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94 Mercury 115 rough idle

G

GC Dave

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I have a 94 Mercury 115 4 cylinder have changed plugs, water seperator, new fuel filters last year. But it has like a small miss around 1800 to 1900 RPMS idle great open throttle great. Not for sure if its the 2+2 the coils have also been checked and are good is this just the nature of the beast if its the 2+2.
 
Not sure where the trigger wires are. But I just noticed the plunger on the accelerator pump is gray when push it down after wiping it off.
 
I used to own a 125 like this and it was a feature I disliked, with fine tuning you can minimise the feel of the transition from 2 to 4 cyl but not get rid of it.
 
So should it miss during this transition? Yes or no?......or maybe? Perhaps Dave is only noticing an inherent trait of this design. I had a 1981 Fleetwood Cad with the 8-6-4 (modular displacement) V8. During transition between cylinders there was no stumbling or miss. The deactivated cylinders maintained air to cushion the effects of their own shutdown. Of course this was accomplished through ECM (electronic control module) controlled solenoids which modified valve function. Primary inputs were vacuum, throttle position, rpm, egr activity, and exhaust 02. Cruising down the highway at 60 mph it was running on 4 cylinders and producing nearly 30 mpg.......out of a 368 cubic inch engine, a downsized 425/500 of previous years. When it worked, it worked well, when it began to act up, many shops started to just cut wires and deactivate the system. Some level of ignorance was involved, giving the system a bad reputation, much like OMC VRO's. In the case of VRO's, however, ethanol played a key role.
 
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If you try and hold them around 1800 rpm the added cylinders do pop in and out, the bottom carbs don't have idle circuits , so the transition is not smooth. The thing I disliked was when accelerating hard from idle it would be sluggish up to 1800 and then take off like a power band when the other 2 cylinders come in.
 
Okay, I stand corrected. Thanks. So its not really a "miss" but a surge or flat spot. Probably wouldn't be hard to stay out of that RPM range. I would surely throttle through that spot and not encourage the motor to run there.
 
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timguy said:
Okay, I stand corrected. Thanks. So its not really a "miss" but a surge or flat spot. Probably wouldn't be hard to stay out of that RPM range. I would surely throttle through that spot and not encourage the motor to run there.
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If I was to push down the accelerator plunger should the enging rev or stay the same rpms. What makes the bottom to carbs kick in. Guys thanks for all the replays I really appreciate.
 
carburetors respond to " airflow"----So when the motor reaches 1800 RPM it is pulling enuff air for the high speed circuit to start adding fuel to the air.
 
The technology is older than 25 years.-----Fact is that older technology is repairable by the common man that wants to learn.------New technology is NOT repairable.-----You can repair a carburetor in a small town.----Doubt that any one can fabricate a new computer / sensor / fuel pump / injector in a backyard shop.
 
Vacuum then dictates the need for the other 2 cylinders? Makes sense. You can't develop enough vacuum to trigger unless engine has load.
 
In theory yes.-----It has been tried bo doubt.------Have not seen any posts saying that this results in a smooth running motor at idle !!
 
I looked into that and there is a whole lot more to it such as exhaust and port design, simply put, you would not be able to get the idle speed low enough.
 
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