200 HP Merc Rough Idle

[JohnnySCV]

Hello all and thank you in advance for your time.
1992 Mercury 200 H.P. 2.5 litre Carbs not EFI has a rough idle.
I have decarbed the engine with power tune, run two cans of sea foam, changed the plugs, had the timing checked. Compression is good 114 highest cylinder 110 lowest. Also had trouble starting at times, which has improved, but possibly due to warmer weather. Idle is about 800 RPM and sometimes stalls out when in reverse. Runs great at WOT and really anythimg much above idle.


Any thoughts on what mught help smooth things out?

J.V.

 

[Faztbullet]

Re: 200 HP Merc Rough Idle

Check the thermal air valve on starboard head as if its defective it will cause engine to run rich at idle, also ditch the idle stablizer. Also suggest you check the recirulating valve as if stopped up it will also load up at idle.

 

[JohnnySCV]

Re: 200 HP Merc Rough Idle

also ditch the idle stablizer

So just remove the idle stabilizer? Can this cause problems? Does this thing have a purpose? Is it like a spleen or something?

 

[Faztbullet]

Re: 200 HP Merc Rough Idle

So just remove the idle stabilizer?

Yep.. You can leave it on engine just disconnet the wire to switchboxes

Can this cause problems?

Yep...

Does this thing have a purpose?

Yep...if engine rpm falls below a preset level it will advance timing to try and increase rpms.

Is it like a spleen or something?

You can live without your spleen

 

[JohnnySCV]

Re: 200 HP Merc Rough Idle

Ok, I looked in my repair manual for the Idle Stabilizer and could not find a reference. What wires would I need to disconnect?

Can removing, or in this case disconnecting the Idle Stabilizer cause serious engine damage?

I have located the thermal air valve, is there a proper way to check if it is working? It seems to be a like a thermostat, could I remove it, place it water heat the water and see if it opens? According to my manual 100 degrees is the magic temperature..

Finally, where is the reciculating valve?

 

[j_martin]

Re: 200 HP Merc Rough Idle

I'll bat

Ok, I looked in my repair manual for the Idle Stabilizer and could not find a reference. What wires would I need to disconnect?

Any wire on the bias terminals except the jumper between the switchboxes goes to an idle stabilizer or advance module.

Can removing, or in this case disconnecting the Idle Stabilizer cause serious engine damage?

NO. If it happens to be an advance module, it might slow down WOT slightly until you retime WOT.

Finally, where is the reciculating valve?

Don't know, but this might be refering to the fuel recirculating system. Small lines go from behind the reeds to the opposite cylinder, through check valves. Bad lines or check valves can really screw up idle.

hope it helps
John

 

[JohnnySCV]

Re: 200 HP Merc Rough Idle

I'll bat



Any wire on the bias terminals except the jumper between the switchboxes goes to an idle stabilizer or advance module.



NO. If it happens to be an advance module, it might slow down WOT slightly until you retime WOT.



Don't know, but this might be refering to the fuel recirculating system. Small lines go from behind the reeds to the opposite cylinder, through check valves. Bad lines or check valves can really screw up idle.

hope it helps
John

Ok.... Now I'm confused..... Advance Module?

Any idea on what color wire I need to disconnect?

 

[j_martin]

Re: 200 HP Merc Rough Idle

Ok.... Now I'm confused..... Advance Module?

Any idea on what color wire I need to disconnect?

There is only one terminal on the switchboxes that is connected together. I believe the color code for that circuit is white with a black stripe. That is the bias circuit. Any other wires connected to those terminals go to something that modifies the bias, either advancing, or retarding timing. They go by different names depending on function, and some have both functions in one module. It varies from engine to engine.

I don't know how to make it any plainer.

John

 

[CDI Tech Support]

Re: 200 HP Merc Rough Idle

Can this cause problems?

Yep...

If you properly reset the timing (typically between 23-25 degrees WOT) after removing the Idle Stabilizer, you should have no problems.

The Idle Stabilizer was a poor design. No benefit comes from it. When it fails, it can cause problems. It's not needed.

Reference:

Mercury 6 Cylinder

WILL NOT ACCELERATE BEYOND 3000-4000 RPM:

2. Disconnect the idle stabilizer (advance module) and reset the timing between 23-25 degrees Wide Open Throttle. If the problem clears, discard the idle stabilizer as it is not needed.

MISS AT ANY RPM:

2. Disconnect the idle stabilizer (advance module) and reset the timing between 23-25 degrees Wide Open Throttle. If the problem clears, discard the idle stabilizer as it is not needed.

 

[CDI Tech Support]

Re: 200 HP Merc Rough Idle

Ok, I looked in my repair manual for the Idle Stabilizer and could not find a reference. What wires would I need to disconnect?

The Idle Stabilizer has three wires:

1. Black
2. White/Black
3. Red/White

The White/Black is always connected to White/Black on one switch box.

 

[JohnnySCV]

Re: 200 HP Merc Rough Idle

Ok I disconnected the white/black wire from the white/black wire. Took the boat to the lake, but the lifeguards closed the lake for launching due to high winds. Took the boat down to the lagoon which is shielded from the wind but only electric motor can be used execpt for trailering. While trailering it seemed to idle much smoother, it did stall once but the true test will be when I can actually run the boat.