Mercruiser 3.0 "Mystery" - should I be worried ???

[imjb]

OK - here it goes... 1998 Mercruiser 3.0, Mercarb and Delco ignition. Starts great cold or hot. Idles good. Reaches WOT on the water without backfiring or hesitation ( about 4600 RPM). Overall, runs GREAT (knocking on wood). Carb rebuilt (soak overnight, every orifice gone through). Did the WD40 around intake / manifold - no intake leaks ( that I can find). Engine temp was consistent, and never exceeded 160 during prolonged cruising yesterday ( couple of hours at 3400 RPM).

Timing was set following the instructions to the tooth couple of months back - and I checked / rechecked base timing, advanced, and max advance, and all were good.

Took her to the local islands yesterday - about 2 hours of cruising at 3400 RPM. Flushed her afterwards, and decided to pull the plugs and check.... all 4 spark plugs were CLEAN AS A WHISTLE. ZERO deposits of any kind...I mean PURE WHITE insulator, and the ground strap was clean an shiny.... no signs of water intrusion in the oil or plugs ( that I can tell).....

Spark plugs would indicate ( at least "traditionally") a L E A N condition... but as I said no hesitation, reaches WOT without an issue, etc, etc....So I am puzzled... I would have expected to see 'tan' on the plugs.

It is worthy of mention, that I have put some Marvel Mystery Oil ( mainly to combat ethanol/corrosion) about 4 oz to 10 gallons ( likely 6 or 7 oz per 10 gallons - little more than instructions indicated)... could it be that it is the MMO that is cleaning ALL deposits, etc ????

I am baffled... as I would have expected some popping, hard starting, hesitation, backfiring, etc....

Should I be worried that the spark plugs are so "white and clean" ??? or does the traditional spark plug reading somehow no longer apply?

 

[Bt Doctur]

MMO does help to keep things clean but whay type of plug is in there ,too hot a heat range will do the same thing

 

[Rick Stephens]

Pictures of your plugs would be nice. And I agree with you, worrying about it now beats finding out later. Good catch.

 

[imjb]

The plugs are AC/Delco R43T gapped to .045

Will try and take / post some pics sometime today.

 

[imjb]

Here are some pictures.... ALL of them are of the SAME plug ( the other ones look the same). Lighting is funny... the 'golden hue' is the early morning sun.

 

[Lou C]

They look brand new and suggest a lean condition, usually you want to see light brown deposits. No pinging/knocking?

 

[imjb]

One more piece of info.... accel pump is on the 'middle hole'.

 

[imjb]

No pinging / knocking or hesitation. Reach full WOT ( and pretty quickly) under load. Meaning boat accelarates to full speed without an issue, on the water.

 

[TurtleTamer]

Is this with regular pump gas at a gas station or 100% non-ethanol?

 

[Scott06]

Might recheck that float level isn't a little low. My previous boat had a 3.0/mercarb, it had a habit of the end cylinders being sooty and rich looking while innner ones were just a bout perfect. I intentially set the float a little low and this cleaned up the end plugs, but I had a bog on acceleration. Thinking you may have a lower fuel bowl level and may run a touch lean.

 

[imjb]

It is gas-station gas... likely ( but not sure) with some ethanol... I will re-check/re-set the float level to be a tad on the high side.

If it runs good as it does, should I just leave it alone for now? or will it likely to cause damage if I just leave it be ?

 

[imjb]

R E S O L V E D !!! ( or so it appears for now... )

Scott06 nailed it !!!

I had gone through the carb carefully, and rebuilt it... and set the float level / drop... Scott06 suggestion that the float may be off had me thinking.... so I took it apart this evening - and sure enough !!! Not horribly off, but it WAS off quite a bit. I must have 'bent it' or distorted it, when I was assembling the carb.

Spent 30 minutes removing the float assembly, making small adjustments, installing, and measuring.... repeating, until I got it PERFECT - the level and the drop.

I have not taken it out on the water yet (hopefully this coming weekend) but in the driveway, ran it for 20 minutes or so - at idle, and at approx 2500 RPM ( did not want to go any higher) and shut her down.... Spark plugs have a nice TAN coloring, like I am used to seeing

Thanks to everyone who chimed in and helped !!! Float level was not on my radar, since I had taken the time setting it before...

 

[Scott06]

Like a broken clock I'm right twice a day...so I only have one left. Glad this appeared to help as it's a pain to take the carb apart. Other thought was fuel filter... happy boating