Black Oil...Black Spark Plugs

6lucky13

6lucky13

Senior Member
I was having problems with an engine miss and stumble that is noticeable at idle and very pronounced on acceleration in 1st or 2nd gear. This is a 37 engine in a 36 Cabriolet. I’m using a 37-41 distributor with an FA49 condenser. I recently installed a rebuilt coil, new condenser, and new terminal caps. I also replaced the under dash ignition resistor. Still misses and hesitates.

Pondering my next steps, I recently checked the oil and the dipstick was BLACK! The oil has only 142 miles after a recent change. I removed all plugs and they too were sooty black.
What are the possible causes for the black oil and fouled plugs? I’m guessing a carb issue with a massive leak causing over rich condition but the carb was recently rebuilt and has <300 miles run-time. Am I correct about a carb issue or are there other causes which would give the conditions I described? What are my next steps?
I need help…thanks…
 

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Also, check to see if the oil smells like gasoline and is uncharacteristically "runny". Sure seems like the mix is heavy on the fuel. And the plugs explain the stumbling.


Mike
 
"Black, dry soot on the electrodes and insulator tip indicates a carbon-fouled plug. This can be caused by a dirty air filter, excessive driving at low speeds, too rich of a fuel/air mixture or idling your vehicle for too long. You may want to consider switching to a “hotter” spark plug (the higher the spark plug number, the hotter the plug)."
 
Could be jets are too large or power valve passing too much fuel, or float set too high.. Let it idle for several minutes. If it gradually loads up, blows black smoke and dies, you have an internal leak. If idle stays the same, there's likely not a leak. Look into the carb as it's idling. If you see fuel dripping, the float is likely too high. The engine should get all its fuel through the idle circuit when idling. Try turning the idle mixture screws in while idling. The engine should lean out and die before the screws are seated. If it keeps running, the float is too high or possible internal leak.
What is your fuel mileage? And what kind of carb? That will help diagnose.
 
Stick a vacuum gauge on it . A steady 17-19 should be observed . If it has a beat , meaning a timed low needle or bouncing needle look for a weak or dead cyl . If just low ,look for a vacuum leak , blocked exhaust , rich or lean , but your black so sounds like rich . If the compression is good , which I would check after the vacuum check , I would look towards a power valve . Then possible timing issues which would also show on the vacuum gauge .
Electrical , bad spark output . It should jump a gap and be blue .
A old Motors manual or chiton manual would better help . Buy one they don't cost much .
 
Black oil , how long has it been since the motor was rebuilt ? If you don't know ,then new oil will change to black in a very short time IF the motor is full of sludge .
 
Hotter plugs won't cure a mixture problem--may mask it for a while
running too rich will make the oil black quick---partially burned fuel make carbon, excess fuel can wash the oil off the cylinder walls and increase blowby which will also dirty the oil

I have run my old cars on the emission machine to know whats happening----when emissions were tested for inspection I ran my model A, wasn't in the database though , used a honda civic because the similarity in weight (a load was added, it made the A pull pretty hard in 2nd)----I kept upping the year till it would fail, got to 2005, it wasn't driveable(way too lean), but it would pass, set the way I would drive it would pass as mid 80s car, gets good fuel mileage (low 20s)

What fuel mileage have you been getting?

The plug in the socket looks to have spark ---I can see where the spark is burning off the carbon, that cylinder is firing--at least most of the time, how do the other plugs look there?
 
I'd be very concerned about what Kurt mentioned:

"running too rich will make the oil black quick---partially burned fuel make carbon, excess fuel can wash the oil off the cylinder walls and increase blowby which will also dirty the oil"
 
No mention has been made as to how hot/cold the engine operating temp is?
Does the engine have thermostats, and if so what temp?
The recommended temp fora pre-emission's engine is 180 degrees..
 
I’ve encountered the same issue recently. Here was my problem and what I found is surprising.

The Problem:
My oil went from clear to dark black very quickly. I had a burnt smell of gas during idle. Gets all over your clothes. My intake manifold pressure fluctuated between 15-17inHg and felt like the engine was stumbling but not exactly dieing. I had absolutely no control of the idle adjustment screws basically I could take them out and the car would die after maybe 20 seconds. My spark plugs looked fouled out (tons of carbon build up). So I knew something was wrong with either insufficient vacuum pressure or misfiring or something distributor related maybe. So I wanted the tackle the vacuum pressure first (always easiest first lol).

For safety and since I did not know when the previous owner replaced spark plugs I went ahead and replaced them all for 20 dollars (I suggest doing the same based on how carbon built up it is). But that carbon build up means it’s burning rich. So I took carb cleaner (or take a propane bottle) to spray around the intake manifold gasket area and then around the base connection of the carb. Nothing revved up the rpm indicating I had a leak in those areas so I plugged my windshield wiper port on the intake manifold. No luck either. I was pondering and pondering if my carb is at fault. Whether the float is bad and dumping gas (rarely happens). To test that have someone drive behind you and hit the brakes at the stop sign and see if the tail pipe blows black smoke. Mine was invisible could hardly tell if it was dumping fuel. So I thought maybe my power valve is bad or my idle chambers were plugged. If you take one of the idle screws out and it’s sucking air that’s a good indicator of no plugging (you can blow it out by using a bike pump). It seemed clear. To test the power valve, fill the fuel body with half fuel and let it sit and see if over a day the level drops rapidly then you know it’s bad and floods your engine after operation.

On a side note, the way a carb works is simply put the idle chambers are to be used ONLY while the car is idling. No other fuel is flowing from anything else unless you press the accelerator. So when you have a small vacuum leak the pressure drops a few inHG and the main jets or power valve will creep open and compensate for the loss in vacuum. That normally happens when the car is under load during driving conditions not at idle because the throttle wafers are more open so vacuum decreases. Even more so when you try to pass a car at full throttle your vacuum decreases to almost nothing and power valve opens dumping fuel in the case to pass a car. When your throttle wafers are in the close position (vacuum increases sucking under the throttle wafer which allows the idle circuit to be used and the power valve is closed due to vacuum suction keeping the valve closed).

So to make a long story short my uncle had over tightened my carb on the intake manifold a while back and a crack slowly developed which allowed a small vacuum leak to occur on the base of the Holley 94 below the throttle shaft. I had a Holley 94. Now I have a healthy 21inHg at the intake manifold with a new base. Sorry for the long explanation but I like explaining. Someone else can correct me if I’m wrong in anything I said, I’m only 26.


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Anytime. Always happy to help someone else out especially if it’s a topic where it’s happened to me before.

6lucky13, let me know what you discover after reading my long post. Love to hear what is causing your issue. Start with hooking up a vacuum gauge to intake manifold. Then look into idle screws. Then see if there are vacuum leaks causing an excessive dump at idle. Then test power valve. It’s easier to start on the fuel system before jumping to conclusions on condensers and so forth. My two cents.


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How's your compression? Oil blacking maybe a condition of blowby/bad rings. Maybe a light spark, maybe a plug change. Run a compression check.

If you pull a plug and it smells gassy, it's running rich.


Vacuum reading at idle are good for bad valve and idle settings. I run one full time in my car.
 

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Just a side note if you run a vacuum gauge on it and rev the motor and the needle goes to 0. your rings/compression are bad. Thinking you are dumping in to much fuel. Compression check is really the first thing I do to an engine.
 
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