Spark Color

[Deleted member 12676]

from http://www.ankn.uaf.edu/publications/vs/magneto.html
The strength of the spark is revealed in the color. A red or yellow spark is weak and probably will not spark in the cylinder. A blue or white spark is strong and has enough voltage to fight across the spark plug gap even under pressure within the cylinder.

http://automateinfo.com/spark-plug-care-in-your-two-wheeler/
Notice the color of the spark, it should be blue/purple in color.

http://www.pwctoday.com/showthread.php?t=342678
A weak yellow spark is not very good and in most ignition systems means trouble. A bright blue is your most desireable and is usually a sign of a healthy ignition system. The hottest is an ultraviolet .

 

[Deleted member 12676]

How to check your spark

Take the plug out
connect it to the spark plug cap
put your gloves on and hold the spark plugs threads against the cylinder head fins
(best to do this with little light. best at night.)
position the plug so that you will be able to see the spark
push the bike while watching for spark
observe the spark color

You can increase spark plug gap to see the spark better. Just don't forget to return it to the normal .025" afterwards.
(as a side note- spark gap has a slight affect on ignition timing. larger gaps cause a slight delay.)

 

[OCCStingray]

(as a side note- spark gap has a slight affect on ignition timing. larger gaps cause a slight delay.)

WOW, so let me get this straight what your saying is that the further the spark has to travel the gap the slower the spark will arc? In effect retarding the timing?
Man that is deep.

 

[bmg50cal]

Taking from one source that says the speed of lightning is 2,485 miles/sec, which is a tiny fraction of the speed of light at 186,000 miles/sec.

Convert it to inches/sec and we get 157,480,315 inches/sec. Run that against a 0.025" gap and the time to cross the gap is 0.00000000015874 seconds.

How about we reduce the spark speed even to 1,000 miles/sec (it's a slow tired spark on a Monday morning badly in need of coffee) which would convert to 63,360,000 inches/sec. It would cross a 0.025" at 0.0000000003945703 seconds, we could double the gap to 0.05" and it would take 0.0000000007891 seconds to cross the gap.

Looking at these numbers, does one really think that a wider spark gap would have any real effect on timing?

 

[max350]

Simple tool (Vitage Mobelec Spark Tester) to check how strong the spark is

 

[Deleted member 12676]

I like that tool

The delay has nothing to do with the speed of electron travel. The output voltage from the high voltage coil isn't instantaneous. It ramps up to the high voltage. So as the spark gap is increased it takes longer to spark because the time it takes to ramp up to spark voltage is longer. Whether or not the time delay is worthy of consideration in the practical world is unknown though. I doubt it makes much of a difference when comparing small spark plug gap differences.

 

[HeadSmess]

A bright blue is your most desireable and is usually a sign of a healthy ignition system. The hottest is an ultraviolet .

so, please explain how i see a spark thats in the U.V range? without having my cornea removed? (old wwII trick, people that have had cataracts removed can see UV)

following that link, im with the purple with green polka dots...

my theory is...unless its an ancient points system... if i cant see a spark in daylight....something needs tweaking.


length of wiring/resistance of cables will have more effect on timing than any spark gap.

whats more important in CDI is some kind of resistance in the HT side of things.

why? the gap is considered "open circuit", until the spark occurs. at this point the gap is considered "short circuited" allowing full current flow.

now this seems irrelevant, except...CDI has an issue in that the spark, while very hot, powerful and clean, is also very very brief. so brief, it can cause issues with actually lighting the fuel mixture!

some resistance in the circuit simply reduces the current flow, allowing the spark to stay "on" marginally longer.

um. i have to quote jennings here again...wait while i go track it down again




meh. more forum referencing. more people with no idea telling each other whats what

 

[HeadSmess]

here we are, direct quote. (p.160)


But the capacitor-discharge system's best
feature is that it is tremendously fast, faster than even the conventional magneto:
Something in the order of 50-100 microseconds are required for a single electrical pulse
from a conventional battery-and-coil to reach sparking strength; a magneto requires 35-
40 microseconds (which is just below the wet-fouling level of 45 microseconds); but a
good capacitor-discharge system takes only a couple of microseconds in climbing to full
voltage, which means that it is so far below the wet-fouling level that it may be
considered virtually immune to that disorder. Moreover, its spark is delivered so rapidly
that firing will occur even when the plug is very badly fouled with oil or anything else.
The single disadvantage of the capacitor-discharge ignition system is that it can deliver
this terrific spark so briefly that ignition of the air/fuel mixture in the combustion
chamber does not follow. Magnetos, for instance, keep their sparks going for about 0.5-
1.0 millisecond, while the capacitor-discharge system's spark may last less than a tenth of
that time. It has become common practice to add a resistor in the circuit to prevent the
capacitor from dumping too fast and delivering a too-brief spark to the plug, but there
still are systems being made without this feature - which is to be regretted - as cold-start
problems will occur even with everything else in order, including a fresh, clean plug. A
too-brief spark is also, likely to be unable to fire the kind of rich mixtures needed by two-
stroke racing engines.



how come noone ever mentions that important aspect when hyping up their super duper aftermarket CDI unit?


USE RESISTOR PLUGS, CAPS AND/OR LEADS.

note that this resistance does NOT delay any thing. it simply extends the duration of the spark. while the voltage builds, as long as the gap remains "open", the potential across the resistor will be zero. only when a current flows, ie, the spark is occuring, will the resistor have a potential across it...

you dont want resistors on the LT side of the cdi, as the coil produces its sudden voltage rise by allowing the capacitor to discharge through it with no restrictions. slow the discharge down, and the voltage produced will be much lower, as EMF is directly proportional to number of flux lines cut in a given time by a conductor... more lines cut in less time...more volts

 

[Deleted member 12676]

thanks for the input HS.
I think the person that I quoted meant to write "violet" instead of ultraviolet.
"It has become common practice to add a resistor in the circuit to prevent the
capacitor from dumping too fast and delivering a too-brief spark to the plug"
HS what he is saying here is not about resistance in the high voltage wire or cap. The voltage duration of the high voltage coil output has nothing to do with resistance since its duration is an exact copy of the duration of the voltage spike applied to the primary side of that coil. Adding resistance only lessens the current, therefore making the spark less hot and less likely to disturb radio communications. The needed resistance to keep the charge capacitor in the CDI from dumping too fast has to be in the current loop of that part of the CDI circuit. Now that you have made me aware of this slight problem I will experiment with different resistances in that loop and see if I can get a power increase as a result of less misfires (which are common in 2 strokes).

 

[HeadSmess]

oh yeah. i misread but i still think its better in the HT side. in fact, im going to go raid the mechanics bin and use some silicone lead, get rid of the 22KV neon sign cable that is, well, highly conductive (it seemed like a good idea five odd years ago )

also....worth investigating multi sparks, or "ringing" cdi systems. more than one spark! if the first one fails, maybe the second one will get it instead of the cap simply discharging, it "rings" as cap and coil forms an LC circuit. really makes a noticeable difference on BIG cylinders. ie, audi twin spark (it is audi, isnt it? though they use a different system as CDI does have a cap failure issue, something not wanted in a car that should do a few hundred thousand with no issues...)

also read into the jaycar/silcon chip cdi replacement, where the designer has used a diode in a certain place so some of the reflected energy is used to charge the cap back up slightly.

cap discharges into a coil(with as little steel in the core as feasible so it saturates quickly, unlike points). magnetic field builds (very rapidly in CDI). creates voltage in secondary winding(standard transformer). pop, sparks. then the magnetic field collapses, just like in a points system. obviously CDI uses stored charge, points use stored magnetic field.(without enough dwell, there isnt enough magnetic field produced, weak spark) this back EMF is usually dumped to earth or neglected altogether... USE IT!!!!