2002 25 HP ELH Misses and Rough Idle

[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.
- Checked primer circuit
- 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.

[boats.net]

I didn't see ignition coils on your list of exchanged parts but I'm sure you changed them as well.
About the only thing left is a crankcase air leak, in crankcase halves, intake or crank seals.

[mobax]

Coils

Yes, replaced the coils.

Leaky seals or crankcase might be the answer but I don't have a compressed air source to check them for leakage. I was also thinking that the exhaust ports might be carboned up or somehow exhaust gas is getting back into the cylinder, which would lean the mixture. I'll pull the exhaust cover and take a look, inspect the decompression ports then too.