115HP Inline Six Misfire

[ejthomp]

I'm looking at a 1984 Mercury 115HP Inline Six for a friend. The motor starts but is misfiring. I pulled the plugs and>>>

#1...Nicely dirty....some carbon build up ....oil residue
#2...Nicely dirty...some carbon build up....oil residue
#3...Much cleaner...almost too clean
#4...Some oil residue, not much carbon build up.
#5...Clean as a whistle....looks like a fresh plug never fired
#6...Almost as clean as #5...maybe fired a few times..

Bottom line is the 3, 5, & 6 are definitely not firing as required.

Isn't this an obvious switch pack problem?

Thanks for your help!

 

[njrextreme]

Re: 115HP Inline Six Misfire

It could be a water jacket issue, or gasket. They might be getting steam washed. Check for spark on all 6. Do a compression test and report back.

 

[ejthomp]

Re: 115HP Inline Six Misfire

Okay...compression is 120ish across the board and I have spark at all plugs...

I disconnected the rectifier and it seems to run much better. Close inspection of the yellow wires coming from the flywheel to the rectifier shows cracking insulation and possibly shorting across those wires.

Obviously need a new rectifier....and need to replace those bad wires....wonder if that will be all that is needed...

 

[njrextreme]

Re: 115HP Inline Six Misfire

I doubt its the rectifire. Try taping the wires first. You really need to do some tests on the stator, trigger, and switch boxes if you have verified its not a fuel problem. A real clean spark plug usually means water is getting in the cyl.. That would cause a mis fire.

 

[ejthomp]

Re: 115HP Inline Six Misfire

I reconfirmed good compression on all six cylinders...122-128... I cleaned carbs and confirmed fuel to all cylinders.... I confirmed spark to all cylinders though #'s 3, 5&6 are questionable.

Disconnecting the rectifier allows it to idle much better, but it isn't smooth or responsive like I would expect.

I tested the stator and the readings are:

blue to red 6.27 (acceptable 4.5-6.2) FAIL
blue/white to red/white 6.36 (acceptable 5.4-6.2) FAIL
red to ground 146 (acceptable 125-175) PASS
red/white to ground 143 (acceptable 125-175) PASS

My plan right now is to replace stator and rectifier and re-evaluate from there.

Curious what you guys think in regards to the stator results...out of spec is out of spec, right? Close isn't good enough, right?

 

[njrextreme]

Re: 115HP Inline Six Misfire

The switch boxes control 1, 3, and 5, and 2 ,4, and 6. They are spaced every other cyl.. Im not sure on the stator specs. A good manual would help you out. The rectifire only converts power to 12 volt to charge the batt.. Is the batt. going dead? That motor should rev like a indy car not a slow vibrating rumble. What rpm will it rev up to?

 

[ejthomp]

Re: 115HP Inline Six Misfire

The switch boxes control 1, 3, and 5, and 2 ,4, and 6. They are spaced every other cyl.. Im not sure on the stator specs. A good manual would help you out. The rectifire only converts power to 12 volt to charge the batt.. Is the batt. going dead? That motor should rev like a indy car not a slow vibrating rumble. What rpm will it rev up to?

The stator tests out of range and the manual is very direct in saying that it MUST be replaced so we're not wasting money changing it.

I have a 90hp I-6 that runs great so I have that as a reference. There is no sense in trying to rev this 115hp up high as it doesn't sound or respond to the throttle near as good as my 90hp. I'm looking at this 115 for a friend and "Yes" the old battery was going dead. We just installed a new one and want to put a new rectifier in. The voltage reading coming off the old rectifier flies all over. Something definitely wrong there, I guess that may be due to the stator too, but since a rectifier is only $35 bucks....

After we get the new stator and rectifier installed, I will let you know if we still have problems. Hopefully the switch boxes are still okay....