Soot covered spark plugs

[Jperkins]

I have a recently purchased 2001 Mercury 115 efi that is turning my two bottom plugs black(3 and 4 cylinder). Boat wouldn't plane out when I first got it, only sputter with more throttle and eventually die. Ran a compression check, cleaned the vst, new plugs, and changed injectors. Ran the boat for a solid hour around 5200 rpm. Low rpm acceleration is rough but smooths out at mid rpm on up. Cranks right up and idles, but I'm guessing the two rich cylinders are causing the low rpm missing. Can you point me in the right direction? If I need to provide more details just let me know, trying to keep first post as short as possible.

 

[Blutwtr]

Just a thought, but are you sure the two plugs are firing properly? May not be enough spark or at all causing it to miss. Bad coil/power pack or plug wires?

Just other things to consider.

 

[Jperkins]

Checked all four cylinders with a spark tester and they appear to be firing good. Motor is a little rough running at idle and just above. My two top plugs are a whitish/tan color and the two bottom plugs are black from the tips all the way down the white insulators.

 

[Jperkins]

Checked all four cylinders with a spark tester and they appear to be firing good. Motor is a little rough running at idle and just above. My two top plugs are a whitish/tan color and the two bottom plugs are black from the tips all the way down the white insulators. This is from an hour of continuous running with brand new plugs.

 

[Jperkins]

Sorry about that last post. Was trying to edit.

 

[Silvertip]

Is this a four stroke engine? If so, pull the dip stick and smell it. If it smells like gasoline or the oil level is high, the injectors are over fueling the engine. If it is an Optimax engine then injectors could be the issue there as well.

 

[Blutwtr]

The whiteish color on the plugs says too high a temp to me. Not sure beyond that. Sorry.

 

[QBhoy]

How is the oil injection system ?

 

[Jperkins]

It's a four stroke fuel injected. The oil level is even with the top of the crosshatch and doesn't smell like gasoline.I haven't changed the oil since I bought it. This one has me puzzled. The two plugs that are black are being fired by different packs.

 

[Jperkins]

I have a receipt of work done by a marine shop last year. Head gasket, intake valve, exhaust valve, and shim. This was done by previous owner who inherited from deceased father in law. Don't really have any other history on the boat and motor.

 

[dingbat]

What is the compression on the two “bad” cylinders?

 

[QBhoy]

Apologies. I presumed 2 stroke at the age.

 

[Jperkins]

OK, just took another compression reading. Using a Lang compression kit I got 175/175/175/160.The lower reading cylinder is Not one turning the plugs black. Is it time to do a leak down test? I'm going to change the injectors around and see if the problem changes cylinders.

 

[Jperkins]

I'm hoping to have found the low rpm roughness. The top pair of throttle bodies rest against an arm of the two lower and control the opening of all four butterflies. From a diagram it looks to be a spring there that's missing, allowing the top two to open a good inch before the bottom two cylinders. I've ordered the spring, but in the meantime I've rigged a temporary spacer to make all four open at the same time. Haven't taken it to the lake yet but it sure sounds much better now and throttle comes on instantly. Never owned a four stroke before, always older omc two strokes so this is a learning curve for me. I did go ahead and buy the oem service manual for future reference. Hope this cures the problem, fingers crossed!
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