Merc 5.0L Carbon Fouling
 

I just found this forum and there seem to be some pretty savvy folks on this forum so here is a good brainteaser I am experiencing.

Merc 5.0L SN 0L612807
Year: 2000
MCM 2bbl carburetor
Gen+ Thunderbolt Ignition

1. Replaced spark plugs end of first season, all plugs black and dry. Replace stock AC plugs with Champion RS12YC
2. Ignition coil failed 2nd season, arcing at tower, replaced.
3. Did not replace plugs again until this winter. Plugs for cylinders 1,2,3,8 black and dry, others normal.
4. Replaced plugs with NGK BPR6EFS. Ran boat one time and same plugs black again, others white/normal.

Tests:
1. Ohmed all wires, 17.6k to cylinders 1&2, resistance drops as you measure remaining cylinders. Cylinder 8, whic h carbons up has same length wire and cyl 7, both read the same resistance. Wiggle wires all around, no resistance changes.
2. Inspected distributor, no carbon tracks, no continuity between posts, even at 1000v with a megohm tester. Slight buildup on some of the contacts but nothing unusual
3. Timing is on the money at 10degrees btdc
4. Vacuum is 18 and steady
5. Sprayed carb cleaner all around carb gaskets and intake manifold, no change in rpm
6. Idle varies about 25rpm around 650
7. Idle mixture screw out 1 3/4, reset to 1 1/4
8. Ran engine in dark, no arcing
9. Engine runs great at any rpm
10, Pulled airhorn off carb and checked float adjustment before first run, it was fine, needle and seat fine
11. No observed fuel spattering on butterfly valves at idle
12. Switched plugs so that clean plugs were in cylinders 1,2,3,8. After doing the above checks, plugs fouled in same cylinders.
13. Flame arrestor clean as a whistle

On another forum, folks speculate distributor cap, wires. I have a hard time with that since the problem stays with the cylinders and the wires ohm out fine, even when comparing a bad and good cylinder and other tests done.

If it is the carburetor, then why only 4 cylinders.

The way the intake manifold is setup, Cylinders 1,4,6,7 are one barrel, 2,3,5,8 are the other. So 3 of the 4 carboned up cylinders are from one barrel, and one from the other.

I'm miffed.

Only other data point is that there is some tapping from valves or lifters. I read that sticky valves can cause carbon buildup.

Any ideas on how to find the problem?
What's the best way to confirm sticking valves?

hmm what about cylinder temps
 

I wonder if this could be a problem with the design of the water flow in the head area. It is interesting that 2 of the 3 cylinders that carbon up are at the front of the engine and 8 is in the starboard aft corner. Does the cool water hit the 4 corners of the engine first. I have no clue how the water is flowing inside the engine.

If this is the case, a different heat range of plug might resolve it.

Also, the thermostat is brand new and temperature is perfect. Measured with a digital temp probe. Maybe I'll measure the temperature around the area of each spark plug where it is screwed in and get a clue or 2 there.

All those newer 5.0L 2 brls. seem to do that.Seems like Merc runs those motors rich for some reason.They hunt at idle because that MercCarb sucks and really doesn't adjust well. Could be an intake design problem, but the 5.7L 2 brl doesn't do that.I wonder if they are using the same carb (metering and jets) on both the 5.0 and 5.7. That could be why the 5.0 runs rich.

Replace the intake manifold and Carb ASAP!!!!!!

Intake/Carb Replacement
 

what is the reasoning behind replacing the intake manifold and carburetor? This is quite expensive.

How about running a spark plug 1 heat range hotter.

Marinetrans, that is exactly what I am going to do next. I think it would help.
I wonder about the Splitfire plugs, any experiences with them?

I really don't know to much about them. I would use NGK plugs.With NGK as the number go down the plug is hotter, yours is a "6" drop to a "5".

Try this before swapping plugs
 

Merc runs a spring-loaded inlet needle in that carb from the factory-- The make a "fix-kit" for 2bbl motors that run with odd fuel problems-- it (kit) consists of a solid inlet needle and a top gasket. I've had different carb problems that the solid needle fixed (don't forget you have to re-adjust the float height). I don't have the part number on hand but there are service bulletins concerning the 2bbl problems and the solid needle kit. The kit, if I remember correctly is about $40 give-or-take!?!