Help with a Misfiring Merc 115 I6

[avtech23]

I hope someone will be able to help with this 1984 Merc 115 Inline 6.

I bought the boat and the engine was running fine. I managed to get a couple of decent runs out but shortly after it developed a misfire.

The engine starts very easily but when at idle on muffs, you can feel the engine is lumpy. Under way it is also very apparent there is a misfire.

The idle compensation box had been removed previously.
I have changed out the old coils, leads and plugs as a matter of course.
I put a new rectifier in as the rectifier was cooking the electronics with 17v+.
New batteries as the old ones were end of life.
New switchboxes.
The stator looks fairly new and is a 9amp from CDI. The trigger does not appear to be new
New fuel lines, water separating filter and tank. Fuel is fresh.

====================

Following page 83 of the CDI Mercury and Force CDM booklet, I've checked as follows:

Stator:

Blue - GND = 2240 ohms = 270v DVA
Blue/White - GND = 2220 Ohms = 255v DVA
Red to GND = 51 Ohms = 152v DVA
Red /White - GND = 51 Ohms = 149v DVA

Trigger:

Brown (BLK) - White (Yel) = 1248 Ohms = 18v DVA
White (BLK) - Purple (Yel) = 1239 Ohms = 18v DVA
Purple (Blk) - Brown (Yel) = 1257 Ohms = 15v DVA

Coils Primary: (New Switchboxes)

#1 = 51
#2 = 67
#3 = 62
#4 = 68
#5 = 70
#6 = 54

Coils Primary: (Old Switchboxes)

#1 = 54
#2 = 75
#3 = 68
#4 = 59
#5 = 74
#6 = 80

Stator DVA: (Old Switchboxes)

Blue = 240v
Blue/White = 255v
Red = 110v
Red/White = 110v

===================

The voltages on the coil primaries seem to be low. Plug #4 appears to be much cleaner than the others.

I don't have a spark gap tester. I have a digital oscilloscope with an inductive pickup and attenuator but the laptop craps out whenever I try to take a waveform.

Can anybody see an obvious culprit from the measurements?

I really appreciate any input.

Cheers.

 

[racerone]

`Mixing gas & oil at 50:1 ?-----Sounds like #4 has an issue.-----Do a compression test on this motor.-----Post the numbers.

 

[avtech23]

Yep, using Gulf Western TC-W3 oil at 50:1.

Compressions were all ~130 and pretty much equal on all cylinders.

 

[Chris1956]

If #4 spark plug is real clean, like new, you may be getting water into that cylinder. Common culprit is warped exhaust baffle or hole in one of the exhaust covers.

 

[avtech23]

Hi Chris,

What would be the best way to check for water ingress? I have a borescope if that is likely to show anything.

It's hard to tell exactly how clean the plug is because they are all new. However #4 still has it's shine when you wipe the oil residue off, whereas the rest seem to have a slightly grey mottle to them now.

I've been focusing on the electrical side because the CDI troubleshooting book says that for the ignition coil primary 'You should have a reading of at least 150v or more', but I'm only getting sub-70v on all primaries with normal stator readings.

 

[Chris1956]

Sometimes you will see water droplets on the spark plug.

However, with usage, the other plugs will get black to light brown. If you have water, the plug will always look clean, as the heat of the motor steam cleans the spark plug.

just check #4 for a good spark. If she has it, and she still misses on that cylinder, something is wrong internally......