1995 Mercury 135 V6 down on power

[RoyE]

Ok.. here comes a long story (again). Having trouble with my 1995 Mercury V6 and I hope someone can point me in the right direction.

History:
2015 new impeller, thermostats, poppet valve, rebuild fuel pump.
2015 new OEM switchboxes (replaced both at the same time)
2015 new OEM stator
2015 carbs ultrasoon cleaned and reassembled with new gaskets
2016 new plug wires
2016 new plugs
2016 rebuild fuel pump
End of 2016 complete new impeller housing, impeller and gaskets

Was doing a trip with some friends. Stopped for a drink for 10 minutes and after that it was down on power. Where I was running 6100 RPM @ WOT before, it now only got to about 5200. Seems like it is running on 5 cylinders, but not all the time. Sometimes when I idle around it is smooth again. When hitting the throttle it is incredibly slow out of the hole, when out of the hole it slowly reaches only 5200 rpm.

When I got home I tested compression: 110 on all six cylinders. I continued to check spark with a Quicksilver spark tester, set gap to 1/2" on all and got a strong blue spark at all 6.

I then did ohm tests and DVA test:
Stator passed both ohm and DVA tests.
Trigger passed ohm tests. DVA test barely got to 4V in one wire combination (around 3.5V)

I tested output at the coils. It was around 100 on the right bank and around 85-90 on the left bank. Later on I realised that I grounded the DVA meter on the block while I should have grounded the DVA meter to the negative terminal on the power coil itself. I have to repeat this measurement, just to be sure. Merc manual calls for 90+V @ 400 RPM. All power coils passed the ohm test.

Just to be sure I changed all 6 plugs, simple to rule the plugs out that way. I also checked al coil wires for damage or abnormal condition, all was fine. After this I went out again to see if the problem was still there, so I could investigate a little more. As expected, same problem was still there.

First at the ramp, we let the engine warm up and then throttled it to about 3000-4000 RPM in gear while stil on the trailer. We looked at the spray through the carb throats and all looked steady and identical. Then we unstrapped the boat from the trailer and went on the river.

One thing I then noticed was that between 3000-4000 rpm the engine surges in a very steady rythm, constantly dropping 500 rpm and going back up 500 rpm, dropping 500 rpm and going back up 500 rpm. When running between 1500-3000 rpm, the surging is barely noticable, if at all. When running WOT, there is no surging at all (I'm just about 800 rpm short). Again, idling around up to about 1500 rpm is sometimes smooth, and sometimes I have to play with the throttle to keep it alive. As the CDI manual suggests, I disconnnected the yellow wires to the regulators and retest. Exact same problem was still there.

When we got back in the marina, my friend suggested to look at the plugs. This was after idling for about 3 minutes to the dock. I know that the surface gap plugs can't be read (I've been told), but all were blackish wet (I guess from idling around) except for #4, which looked really white, but wet. It didn't look like water but I'm not sure. We cleaned the plugs and made another run, after that checked plugs again, and #4 again looked like before. At this point the plugs in the heads gave up, so I couldn't continue.

I'm leaning towards an ignition problem, because of the rythmatic running between 3000-4000 rpm. When I bought the engine 2 years ago I had a similar problem with the engine running on 5 cylinders, and it turned out to be the stator. I just want to be sure before I throw money at it again for ignition parts. Could a bad power coil cause this? I also thought about bad grounds, still have to check that, but I would think something like that wouldn't come up that suddenly. Also not sure about the white plug, haven't thought about it that much as the surface gap plugs usually don't tell much?

Thanks in advance for any advice.

[XstreamVking]

Wet with water on the white plug...Oil gas and water get milky when mixed.

[DanUmbarger]

I would put it back in the water and run it in gear again this time put a timing light on all 6 cylinders and see if your loosing spark on #4...if not it's either water or fuel...then I would drain the carbs and retry...could have a piece of crap in the high speed jet. just my thoughts.

[Capt.Insane-o]

Switch the coil for 2 and 4 around and try that.

[RoyE]

Wet with water on the white plug...Oil gas and water get milky when mixed.

I've read back my post and see that I said white, what I meant was clean.

I would put it back in the water and run it in gear again this time put a timing light on all 6 cylinders and see if your loosing spark on #4...if not it's either water or fuel...then I would drain the carbs and retry...could have a piece of crap in the high speed jet. just my thoughts.

Switch the coil for 2 and 4 around and try that.

Thanks for the suggestions, it's much appreciated. Would a plug not firing eventually show up clean and wet?
I'm going to repair the heads next weekend and try your suggestions.

[DanUmbarger]

In my experiences a plug not firing because of no spark or no fuel will be clean...dry with no fuel and wet with no spark.

[RoyE]

I repaired the heads today with inserts. I checked the middle carb and it was very clean, just like I assembled it two years ago after they were ultrasoon cleaned. When I removed the cylinder head to repair the threads, I noticed that all three cylinders were evenly wet/oiled so to speak, so I assume it's no fuel delivery problem. I will get a new head gasket and test next weekend, and look for ignition problems.

Cylinder 1


Cylinder 2


Cylinder 3


Is the amount of carbon on the pistons normal? Stock motor

[Steve200]

I had a similar issue for what its worth. I was getting 5900 at WOT and then only 5100. Sometimes it would fire for a few second and then stop again at lower RPM (surging).

I took a video of the carby stack while going WOT ... I could not see fuel in the top right barrel on the video at WOT, there is fuel lower in the RPM range though.

Turned out to be a stuck needle causing the float to get stuck down and hence not enough fuel available for WOT. Enough though so it would idle normally.

The plug looked fine, I think because it was an intermittent issue and also because i didn't run it long after it lost power .. but it didn't show up on the plug.

PS I see you have put a carb kit through it recently, but you never know the needle might have jammed from some rubbish.

[outboards4life]

stuck float

[Steve200]

Here is the video of my carbs at WOT. I couldn't edit the video due to the format. I go WOT at 1:05 and then at 1:12 the RPMs die back and you can see there is no fuel splashing from the right hand top barrel (looking from the camera). I then dropped the RPM to different levels .. at 3000 RPM it will surge as spurts of fuel go in .. at lower RPM it runs normally.

As for the sound, its like music when its at full song.

https://www.youtube.com/watch?v=rNzyq2huHYs

[Steve200]

I see now that the issue I had with my float getting stuck was the seat had unwound itself about 1 turn and that lifted the needle. Simple fix, tighten the seat up.

[RoyE]

Today I went at it again, with spare power coils. First tested the motor as is with timing light, but no apparant miss could be detected on any cylinder. I also looked at the carbs as suggested above but no strange things there either. I decided to switch all 6 power coils for 6 spare ones anyway, and while disassembling the power coils, my buddy found one coil to be completely cracked under the rubber isolation.


We put in the 6 spare coils and the engine ran like a scalded dog again!! Technically we don't know which coil caused the problem, but I think that is pretty clear

I was not the power coil of the plug that looked 'clean' earlier. It was also strange that bad spark did not show up with a timing light at the problem area of 3000+ rpm. I must say that we did the testing with the timing light with the trailer backed into the water with the boat strapped to the trailer, running in gear. Nevertheless, problem solved.