Too rich -- too lean

[Member 10617]

I understand that running rich can have the following consequences, among others: lumpy engine and poor gas mileage. But what are the other signs or consequences of running rich? (fouling?) ... And what happens if you run "too rich"?

Same for running lean. I understand that running lean can result in lumpy engine and backfiring (although a bit lean can result in better gas mileage). But what are the other signs or consequences of running lean? (overheating? engine destruction?...) ... And what happens if you run "too lean"?

I realize this is a very basic question, but so much is made of avoiding overly rich or overly lean mixtures that I am wondering what problems this might cause, and I have a feeling that other newbies like me may have the same question.

 

[tomgt6]

My GT6 always runs to rich. It leaves a black film on the rear of the car. In some spots it has even turned the paint color.

I fixed most of the rich issue but it still runs on the rich side then right on.

 

[TR3driver]

Another sign of running too rich is black smoke from the tailpipe.

The effects from either one can depend heavily on other conditions at the time. But in general, just a little rich won't hurt anything except fuel economy. Richer than that can cause fouled (or glazed) plugs (leading to rough idle, misfiring, hard to start, etc), and potentially rapid engine wear. E10 seems to make the problem worse.

Too lean is much more dangerous, IMO. Slightly too lean just reduces power and possibly increases the tendency to knock, but leaner than that can cause the plugs to overheat, or the radiator. In severe cases (or if the knock continues undetected) it can lead to total engine destruction.

I already told the story of Dad's TR3A swallowing a valve due to mixture problems, so I shan't bore you again.

 

[poolboy]

You think the other "newbies" might be afraid to ask ?

Seriously, you basically answered your own question.
.
You want to see what the spark plugs might look like from one extreme to the other, because it's a long way from one end to the other with lots of middle ground before you get into the real serious "too rich or lean"?
Then study on this.
https://www.dansmc.com/spark_plugs/spark_plugs_catalog.html

 

[Member 10617]

Randall,

"A little rich won't hurt anything." That's very good to know. Many thanks.

As for running lean, I had your story of your Dad's TR3 swallowing a valve in mind when I asked the question. I get conflicting information about running lean -- some tell me that I should run the car as lean as possible without the engine stalling -- but I have hesitated to accept this. Running a bit rich, without creating clouds of black smoke or fouling the plugs, seems to be the way to go.

Poolboy,

Your spark plug chart really lays it out clearly. I'll check my plugs from time to time. My answer to my question touched on just the obvious problems that can come from going one way or the other, but I was in dark on what could seriously go wrong running too rich or too lean. It helps to know that in Randall's opinion, "Too lean is much more dangerous."

Don't know if other newbies would be afraid to ask this question... but as far as I can tell... they haven't within recent memory.

 

[TR3driver]

Where's Dr. John when you need him? Pilots know all about this stuff, because they have an in-cabin mixture control and specific routines for adjusting mixture. Of course they typically also have an exhaust gas temperature gauge, so they know when they are running too lean.

BTW, I should have known the EGT was way out of bounds on Dad's car. The exhaust manifold would literally glow red after a run. And yet it seemed to run fine. In fact, Dad mentioned it seemed less peppy after the rebuild (although I changed several things that would cause that, including swapping the 4.10 rear for a 3.70).

 

[Member 10617]

Randall,

With the risk of further demonstrating my ignorance about these matters, did the overheating in the engine show up on the temperature gauge? If I am running my engine too lean, would it show up on any of the gauges?

What you say about in-cabin mixture controls reminds me of the very early cars that required the drivers to constantly adjust mixture and timing.

 

[Twosheds]

Here I am, Randall. I was taking a nap, something I've been doing a lot of recently.

All good stuff so far. You even wrote knocking instead of pre-ignition. You know how it makes me nuts when someone says pre-ignition when he means detonation or knocking, pinging, pinking...

The only thing I haven't seen mentioned much is that either too rich or too lean will have less power made than correct mixture. The cylinder is a fixed volume. Fuel and air are mixed and pulled into this volume in a ratio. They are combined in a process called combustion. If too much air is in the cylinder, there is not enough fuel to mix with it, and the fire doesn't produce as much pressure to push the piston. Same for too rich only unburned fuel will be left over.

If the right ratio is mixed, there will be just enough fuel to combine with the air with no air nor fuel left over coming out the exhaust. This will make the biggest fire and most power. Of course, in the real world, not all the fuel and air combine, but that is what we strive for.

I agree that it's better to err on the side of too rich than too lean as far as possible damage to the engine. I saw an aircraft cylinder that ran too lean from Tennessee to Maryland. Detonation or high temp had beaten the combustion chamber to a pulp and the valve seats fell out; not enough aluminum left to hold them!

Manual mixture controls adjust for changes in air density due to altitude and temperature. They are usually in Full Rich on the ground and at low altitude, then the pilot moves the lever or knob towards Lean as the altitude increases. This keeps the mixture more or less constant as altitude changes. If the panel has an EGT, this is used to set the mixture. Usually, the pilot leans until EGT peaks, then richens a little. This is because air-cooled engines need to run a little rich for cooling. If you don't have an EGT, you have to use the tach or your ear.

 

[TR3driver]

Then study on this.
https://www.dansmc.com/spark_plugs/spark_plugs_catalog.html

One thing to note about that chart, what it calls a "plug with too high heat rating" is what most people call a "cold" plug. Likewise "too low heat rating" is a "hot" plug. Seriously confusing!

Nelson gave a smaller, but IMO much clearer chart at https://www.buckeyetriumphs.org/technical/Carbs/CarbsIII/CarbsIII.htm

Scroll down to the plug photos, and then scroll down some more for some excellent engineering-type info on "reading" the engine.

 

[TR3driver]

With the risk of further demonstrating my ignorance about these matters, did the overheating in the engine show up on the temperature gauge?

In that case, yes. The temperature would be fine around town, but it would slowly creep up at freeway speeds and eventually boil if the weather was warm. Actually, it didn't show on the gauge, because the gauge was ruined from overheating

But it might not always show up. We attributed the cooling problems to the old, original radiator (but they disappeared after replacing the carb jets and repairing all the damage).

 

[poolboy]

I thought that the "load conditions" in those descriptions was important in understanding what the author was trying to describe..no ?

 

[SkinnedKnuckles]

Also - if too rich you will likely have more vapors in the crankcase. That may increase oil consumption if the increased vapor traffic entrains oil out of the valve cover breather.

And of course there is the "dilution factor" in the cylinders where excess fuel dilutes the oil film with resultant increased friction/wear on the rings.

That's why I like to look at the plugs every once in a while and make sure things aren't too rich.

 

[TR3driver]

I thought that the "load conditions" in those descriptions was important in understanding what the author was trying to describe..no ?

Yes, when you are trying to evaluate the "heat range" of the plug. Heat range is always compromise, you want the plug to get hot enough at light load to burn off deposits, without overheating at maximum load. So light load is where you see signs of a plug that is too cold; while heavy load is where you see signs of a plug that is too hot.

Your ignition system may play a role as well; I found that when running a MSD 6A ignition, using a colder range plug would greatly extend the life of the plugs. The hot plugs didn't seem to be overheating, but the center electrode would just evaporate in a few thousand miles. And the MSD 6 would fire right through any kind of fouling, so the cold plugs worked OK.

 

[CJD]

Planes...now you are in my field!!

It is true that planes lean mixtures, but only at cruise. Under full throttle they always richen the mixture before going to full power. Which brings up the point...lean will take a long time to damage an engine at part throttle, if it ever does. It's at full throttle that a lean mixture can damage parts very fast...we're talking like seconds. Most general aviation engines produce very low horsepower per cubic inch and aren't loaded all that much. At altitude they produce even less power, so it is very difficult to hurt them by leaning at cruise, at altitude. If you lean a plane too much during cruise, they cut out completely and get very quiet...immediate feedback that you leaned too far!

So, relating to cars. If you think you are too lean, don't hammer it! If you baby it, you likely will not hurt anything until you get a chance to change the mixture. Going further, you can set the idle mixture as lean as you want so long as you get a consistent idle, and you will not hurt anything. Part throttle is more inmportant, but full throttle lean is deadly to the engine. You will start melting parts at full throttle.

Rich is just ugly. It fouls your plugs and makes the engine run badly pretty fast. Long term rich will gum the valves, carbon the chambers and exhaust, thin the oil, and can cause the valves to stick.

John

 

[Member 10617]

Wow... this question has really brought out the experts. Always very interesting to read responses and analysis from guys who know these cars.

What I take away from all this is:

(1) Check the mixture carefully following standard instructions found in most of the manuals and other sources.

(2) Not too lean; not too rich. But a little richer is better than a lot leaner.

(3) Check the plugs from time to time for tell-tale evidence of too lean or too rich.

(4) Watch what comes out of the tailpipe.

(5) Strive for a consistent, even idle.

(6) Don't baby the engine, but don't give it, as one friend once advised me, an occasional "hot meal" by approaching redline in order to "clean out the carbon."

Once the carbs are balanced, the mixture is "right", and the idle "proper", the engine should be good to go for some time. But it seems to me that, in fact, all these have to be checked periodically and "fine-tuned" for changing conditions, changing seasons, and changing fuels. I haven't found the case to be "set it and forget it."

Randall: The plugs shown in your link are really helpful. Many thanks.

Got to quit now, I'm startng to have vapors in my crankcase....