Merc 2002 25 HP 25ELH Rough Idle & Missing

[mobax]

I have a 2002 25HP Mercury 2 stroke tiller outboard, electric start, model 25ELH. It idles roughly and misses at all speeds. Spark plugs always look wet. I have a second identical motor that runs perfectly and have swapped parts from this motor to the problem motor to affect a repair. The problem motor has very little hours on it, looks like new. Here is what has been done so far with exchanged parts from the second motor.

- Cleaned grounds.
- Checked water impeller and verified flow.
- Compression check (cold) 125 psi on both cylinders
- Set up throttle / shift linkage per manual.
- Changed to fresh gas mixture
- Gas tank vented.
- Checked idle timing and on spec. of 6 degrees BTDC.
- Disconnected kill switch and push button shutdown switch.
- Disconnected charging coil and rectifier.
- Exchanged spark plugs.
- Exchanged plug wires.
- Exchanged stator coil.
- Exchanged switchbox
- Exchanged trigger coil
- Exchanged flywheel.
- Exchanged reed block.
- Checked bleed check valves, bleed lines for leakage and verified lines and fittings are unplugged.
- Exchanged carburetor.
- Inspected running engine in the dark and can see corona between the spark plug boots and the block.

Have now run out of ideas. Any help in determining the cause for misfire would be appreciated.

 

[Really Old Guy]

Re: Merc 2002 25 HP 25ELH Rough Idle & Missing

small air leak or hole in fuel pump diaphram?

 

[mobax]

Re: Merc 2002 25 HP 25ELH Rough Idle & Missing

Thanks but I thought of that and checked the primer system and fuel pump. Then I replaced the entire carb with the carb from my good outboard. No joy.

 

[mobax]

Re: Merc 2002 25 HP 25ELH Rough Idle & Missing

Bump. Three years later and I have finally got around to looking at this motor again. Pulled apart the engine, checked all internals and within spec. Installed new rings and seals. Still runs poorly and I'm still stumped.