Spark Plug Colors

[arbs_53]

As some of you know, I just had my engine rebuilt. Last night I was doing some minor adjustments to the valves, (some of you may now be wondering why am I adjusting valves on a brand new engine! well so am I!) so I took out all of my spark plugs, so that I could turn the engine over easier. I started with No. 1-it was black and sooty. No. 2 was perfect-nice brown, evenly coated. No.3, 4 and 5 were white, as if they were leaned right out and #6 was like #2 - a nice even brown. Is there something here I should be concerned about?
I had the early TR250 head ported and polished and flow-bench tested; used the original intake manifold and running twin downdraft Webers.
I'm concerned the fuel is not being emulsified and/or is being distributed unequally to the cylinders. That's my guess, anyways.
Is this a plausible explanation or is there something else going on here?

 

[kindofblue]

Chilton manuals for most cars have a guide in them to spark plug identification. (as far as what the colors and buildup on the tip mean). Check at your libray for an old manual. (not the general guide, for a specific car.)

 

[RobT]

I would always use spark plug colour as a guide to fuel mixture (nice light tan colour was ideal), but I thought I read somewhere that with modern fuels, the colour should be mutch darker - a bit of soot was O.K. Am I correct or just dreaming?

Rob.

 

[Dave Russell]

Hi Dave,
The only way to tell is to start with clean plugs, run it under load for a bit, shut down immediately & check plug colors. If the plugs have experienced starting, idling, low speed running, the colors may be confusing to say the least.

There could very well be mixture distribution problems during start up, idle, & low speed. While this may be annoying it shouldn't be completely unexpected. On the other hand, uneven mixture distribution when the engine is loaded IS a serious problem & mixture should be set to give the leanest cylinder a reasonable color.
D

Rob,
I have experienced the same thing. The new fuels don't color the same way as they used to. I would much rather err on the side of too rich.
D

 

[arbs_53]

"...uneven mixture distribution when the engine is loaded IS a serious problem..."

I pulled the plugs after nearly 175 miles of driving, mostly at 65-70 mph. I was expecting to see a color that was uniform across all plugs, but it's really quite a difference. So it has me concerned, though I'm not so sure if I should be. If it's due to the stock intake manifold not emulsifiying the air/fuel or not directing it evenly to each cylinder, then I could live with it until I can afford the upgrade to the three SU's that Richard Good has available. I would also increase the air/fuel mixture tso that the car is running richer in those middle cylinders.
But if it's due to a botched job with the guy who ported the head, then I will have to address that problem immediately. But he has a good reputation and he does a lot of stock cars in the area, so I doubt if it's anything he did.
Another thing I noticed while on that short run Sunday was a slight misfire when the engine was under load- at lower rpm's. I couldn't really tell when I was at cruising speed if it was still doing it.
When these TR engines are built up to anything above stock horsepower, how does one know what the optimum timing should be set at? Is it trial and error or is there a way to know for sure taking the guess work out of it?

 

[Mickey Richaud]

[ QUOTE ]
Another thing I noticed while on that short run Sunday was a slight misfire when the engine was under load- at lower rpm's. I couldn't really tell when I was at cruising speed if it was still doing it.

[/ QUOTE ]

When I first read your post, I thought about problems on the ignition rather than the carburetion side. Maybe totally "out there", and I don't mean to muddy the water, but have you checked your plug wires and distributor cap?

Mickey

 

[Dave Russell]

[ QUOTE ]
"...uneven mixture distribution when the engine is loaded IS a serious problem..."

I pulled the plugs after nearly 175 miles of driving, mostly at 65-70 mph. I was expecting to see a color that was uniform across all plugs, but it's really quite a difference.

[/ QUOTE ]
One problem at a time, though timing, plug heat range, & mixture are all related.

To get a true, engine loaded, plug reading at 70 mph, you must start with clean plugs, run the car, put the box in neutral as you switch the engine off, coast to a stop, pull the plugs , & read them at road side. This is sometimes called a "clean plug chop". Any operation such as coast down, or idling after the 70 mph run will present a different mixture to the plugs & mask the color readings. Even a couple of minutes can change a clean plug to sooty.

Plug heat range can have a large effect on this process. Too hot a plug can mimic lean mixture, a cold plug can appear as a rich mixture.

Ignition timing can change the picture. In general, the timing should be something like 20 to 25 degrees at 2500 engine rpm, & a maximum of 32 to 36 degrees at 4000 rpm. Any vacuum advance units disconnected.

A common cause of uneven mixtures is manifold air leaks or unmatched carbs. Uneven cylinder head cooling can have an effect.

Cylinder head porting that is intended for a competition engine can be all wrong for a street engine. Large ports & or valves will reduce mixture velocity & can cause separation of air fuel which gives uneven fuel distribution to the cylinders. "Porting" that works for a particular type of engine can be all wrong for another type. A lot of experience with "your" engine type is needed to get the porting correct. In general, small but well shaped high velocity ports will work much better than large ports on a street engine. The port size/shape, cam type/timing, compression ratio, intake & exhaust manifolding, & ignition timing, are all interrelated & must work together for the intended usage of the engine. I stress that what works for "other stock cars in your area" may be wrong for your engine & it's intended usage.

I suggest that you consult the person who ported the head about your concerns.
D

 

[Alan_Myers]

Hi,

Analyzing engine performance by spark plug appearance is somewhat iffy. The "clean plug chop" method already mentioned is important and improves your odds of getting some usable info. (Just be wary of doing it on modern cars with power steering and brakes!)

I'm not sure about Weber downdraft carbs, but sidedraft carbs such as DCOE pretty much require a dyno tuning session after installation.

An alternative might be a portable exhaust gas analyzer used while driving.

Before any testing and carb tuning can be done, many other things should checked: valve adjustment, cam timing, ignition system, fuel delivery system from gas tank to float bowls, manifold leaks, crankcase ventilation, compression test, any emissions equipment, etc. Heck, even the exhaust system can have an effect.

One key thing is that most Webers use a pretty low fuel pump pressure, a pressure regultor is a common accessory. I believe 3.5 psi is the max.

A newly rebuilt engine might still have rings that are in the process of seating, may give some uneven compression, cylinder-to-cylinder, that draws fuel unevenly. For that reason, a re-tune is often needed once the motor is past the break-in period.

Cheers!

Alan Myers
San Jose, Calif.
'62 TR4 CT17602L