Spark Plug Analysis

[Loren Warner]

Removed plugs on 1997 mercruiser 7.4L, carburator, generation VI. Engine has not been run since fogging for layup last year. At that time the engine ran good. The insulator tip on every other plug in its firing order has black dry carbon. They don't look bad but maybe a little rich. The insulator tips on all other plugs is nearly like new, only slightly brown. What could possibly be going on with the ignition system that would cause every other plug to differ equally? Firing order is 1-8-4-3-6-5-7-2

8 light 7 dark
6 dark 5 light
4 dark 3 light
2 light 1 dark
Fwd

 

[Bondo]

Re: Spark Plug Analysis

Ayuh,.......

I believe you have a dual plenum manifold,.....
Maybe 1 Side of your carb is Leaking,..??

 

[Coors]

Re: Spark Plug Analysis

I think it is your carb; one bank seems to be running rich.
But a plug change and re-reading after a full run may help.\
Then read again, if good, then run them on low speed/ idle for a while, then check again. If one bank is black, look to your carb.

 

[RCSConstruction]

Re: Spark Plug Analysis

Ayuh,.......

I believe you have a dual plenum manifold,.....
Maybe 1 Side of your carb is Leaking,..??

Dang he's good, eh??

 

[Coors]

Re: Spark Plug Analysis

His one finger typing, beat my two finger typing.

 

[Bt Doctur]

Re: Spark Plug Analysis

I concur too. Carb is running lean on that side. air leak or jet is different or is restricted.

 

[Lakester]

Re: Spark Plug Analysis

hello LW~

i dont think u have provided us enuff info. how many hrs has these plugs been run since last changes? how are u running the engine? ie: using it?

are they fresh since last fogging? maybe fogging oil?

given a divided plenum chamber and the firing order, it does not mean one side is feeding same. you have to chase down where each runner comes from and where it goes...

if it was mine i wouldnt sweat it. suggests normal to me. maybe the guides leaked over the winter season. i would run it as is. maybe check fasteners on intake and carb if concerned. if it runs smooth, is powerful and throttle response is good, i wouldnt worry about it.

personally, if it was mine, i'd wire in a marine MSD...

can u provide more details on use, time on plugs, new, used, changes etc?

good luck

lakester

 

[Bigdaveaz]

Re: Spark Plug Analysis

Dual PLANE manifold!

 

[Lakester]

Re: Spark Plug Analysis

Dual PLANE manifold!

dual plenum; dual plane manifold... same thing.

regards
lakester

 

[Coors]

Re: Spark Plug Analysis

Dual PLANE manifold!

You are correct, got to wondering, in my shop are 3 duals; one side of carb feeds 2 cylinders on each bank.

Now, why one bank on his sooty plugs.

 

[JustJason]

Re: Spark Plug Analysis

i'd be wonderin if there were 2 different size jets in the carb.... ya never know whose had the thing apart in the last 11 years.....

 

[Bondo]

Re: Spark Plug Analysis

You are correct, got to wondering, in my shop are 3 duals; one side of carb feeds 2 cylinders on each bank.

Now, why one bank on his sooty plugs.

Ayuh,..... That equals 4 cylinders,.....Where's the Other 4,..??

 

[Coors]

Re: Spark Plug Analysis

1 side, 2 cylinders,2 banks. Other side same, =8

 

[Loren Warner]

Re: Spark Plug Analysis

Thank you all for your responses. The engine has a total of 892 hrs. At 858 hrs, changed all plugs, rotor, distributor cap, and plug wires. Replaced replaced the carburator (weber) with new one last spring at 861 hrs as the throttle linkage in the carbs base was worn and causing engine not to consistently return to proper idle. The carb was changed following the service manual with new quad rings for the spacer. At that time the engine was tuned per the service manual and idle mixture screws set using a vacuum gauge. By the way, the vacuum is a steady 15 inch at idle. I change spark plugs, distributer cap and rotor every spring and have never noted the plugs being of different color. Timing was dead on at 10 degrees and the advance was good at 2000 rpm. I'm not sure about the intake manifolds design but to me seems too coincidental that that the plugs color differs following the firing order. It is always possible that one mixture screw is set too lean but seems unlikely this would cause different fuel burning as I've described. I've also rechecked the distributor and rotor and they look fine. I tried to download pictures but file size too large. Will look at getting another picture.

 

[Coors]

Re: Spark Plug Analysis

I agree, this is strange. with a single plane manifold, 1/2 of carb odd could affect one bank. dual plane, no.
Other than the small chance that one bank of spark plug wires weak, it makes no sense.
Has on side of engine gotten hotter(bad plug side)?

Do compression test, if that's ok, then with engine running, squirt wd40 or something on the rich side of intake, were it bolts to the head.
If engine changes rpm's, you have a intake leak.

 

[Coors]

Re: Spark Plug Analysis

Squirt it on both sides of intake, and around carb base.

 

[Loren Warner]

Re: Spark Plug Analysis

Unfortunatly I'm not in a position to test run the engine yet. Other projects going on such as removed outdrive (do this every spring to service bearings and verify alignment). Besides, since the engine ran WOT prior to layup it's unlikely there's any mechanical problems. Also, why would every other cylinder in the firing order be effected? The last hour the engine was run was for winterization. At this time the engine was light loaded at slip to warm up for oil change. The manuvered to ramp for trailer and then run on the traielr at idle to flush sea water from system and RV antifreeze into system. Stopped engine while fogging.

 

[Coors]

Re: Spark Plug Analysis

It's a minor problem. Plugs dirty from fogging, carb idle system clogged, etc
when you can run it under load, it may clean up.
meanwhile, don't worry, it is simple to correct.

 

[RCSConstruction]

Re: Spark Plug Analysis

Still debating I see???
Bond-o nailed it.
One side is too lean/rich.
Take a look at a manifold. Starboard side of plenum feeds 3-5-2-8
Port side feeds 1-7-4-6.

 

[Coors]

Re: Spark Plug Analysis

Still debating I see???
Bond-o nailed it.
One side is too lean/rich.
Take a look at a manifold. Starboard side of plenum feeds 3-5-2-8
Port side feeds 1-7-4-6.

1357; 2468. GM