Spark plug quiz

[beckoning]

OK, here's the quiz, and I don't know the answer. My 2009 Mercury 9.9, 2 cylinder, 4-stroke, single carb engine is used exclusively for trolling at 900-1,200 rpm. Before I had the prop pitch changed to allow the engine to run faster for 1.8-2.5mph, it would foul plugs. Now, it does not foul plugs, unless I troll for an extended time (1 hr) below 1,000 rpm. QUIZ: periodically, I remove the plugs to sand blast them clean. Every time, the top plug is totally black, dry, sooty. The bottom plug is white to the point of looking as though it is burning very hot. What causes this? Why would one be dry sooty black and the other white (not ideal tan colored)?

 

[alldodge]

Hey dingbat your presents is requested

 

[dingbat]

You have an combustion imbalance with a possible rpm influence.

First thing to do is rule out the obvious...make a close inspection to look for a vacuum hose that fell off and loose fasteners.

From there I’d run a compression check and a spark test. See if the fouling cylinder is dropping spark.

If nothing turns up there, try spraying starting fluid around the base of the intake manifold while running to check for a leak.

When was the last time you changed the water pump and thermostats?

 

[Chris1956]

White spark plugs are a sign of overheat. Real clean spark plugs (but not white) is a sign of water infiltration.

Ideally the spark plugs would be light brown to dark brown.

 

[Texasmark]

White spark plugs are a sign of overheat. Real clean spark plugs (but not white) is a sign of water infiltration.

Ideally the spark plugs would be light brown to dark brown.

My thoughts exactly. Water in the combustion chamber....in controlled amounts, sanitizes (for the lack of a better word) the combustion chamber making it look like new. The next reason I suspect that is the the abstract difference between the two plugs. I had a 6 cylinder Chevy auto and the head cracked over a cylinder....one of Chevy's goofs at a design that incorporated the intake/exhaust manifold in the head casting. Pulling the head and observing the condition of the cylinders, 5 were dirty with combustion carbon residue and the one under the crack was squeaky clean.

On running at slow speeds, if you don't get the tan insulators your plug is running too cold and can't burn off contaminants....per spark plug supplier usage data. Only consideration there is (opinion) if you run full throttle for extended periods, the plug "could" erode excessively requiring more frequent changes, or possibly pre-ignition problems with the glowing embers of carbon being burned off if using lower octane fuel.

I have a 2 stroker that had OEM plugs and ran greyish insulators and always had an oily film when checking.....no doubt I didn't run high speeds long enough to satisfy the requirements for the temp. rating of the plugs. I replaced with a different electrode style and in the process upped the temp range 2 numbers. I now have a smoother running engine, tan insulators and dry, clean plugs when I pull them out to check progress on the change.

 

[beckoning]

You have an combustion imbalance with a possible rpm influence.

First thing to do is rule out the obvious...make a close inspection to look for a vacuum hose that fell off and loose fasteners.

From there I’d run a compression check and a spark test. See if the fouling cylinder is dropping spark.

If nothing turns up there, try spraying starting fluid around the base of the intake manifold while running to check for a leak.

When was the last time you changed the water pump and thermostats?

Dingbat & Texasmark - Thanks. I'll look for a loose intake manifold, and do the spray test. I don't think there is a vacuum hose to check, but I'll look. Regarding water pump, when running, the water squirts strongly out the back. Compression is good, and spark is strong.

Texasmark: Good suggestion about water in the combustion chamber. 9.9 is OHV. Could be a bad head gasket; a winter project.