Coil spark strength

[kaliona]

1978 165 hp Mercruiser. Trying to diagnose a no start condition. Originally, with cold engine, boat would start just fine, idle smoothly for about 5 minutes, then start to shudder and shake and finally die and can't restart. Next day, cold engine, same thing. . . starts, idles fine, then shudders, shakes, and dies. Now I'm at a point where I can't even get it to start. Rebuilt carb thinking was a fuel issure, no go. Compression is 125-135 psi all cylinders. Now Im checking spark from coil. So my question is, what does a strong spark look like. Mine is yellow, not blue (to my eyes) like I'm reading it should be. I have bad, old eyes, so I videoed the spark and hopefully its clear enough for someone to tell me whether or not it is a good spark. Thanks for your input.

 

[kaliona]

1978 165 hp Mercruiser. Trying to diagnose a no start condition. Originally, with cold engine, boat would start just fine, idle smoothly for about 5 minutes, then start to shudder and shake and finally die and can't restart. Next day, cold engine, same thing. . . starts, idles fine, then shudders, shakes, and dies. Now I'm at a point where I can't even get it to start. Rebuilt carb thinking was a fuel issure, no go. Compression is 125-135 psi all cylinders. Now Im checking spark from coil. So my question is, what does a strong spark look like. Mine is yellow, not blue (to my eyes) like I'm reading it should be. I have bad, old eyes, so I videoed the spark and hopefully its clear enough for someone to tell me whether or not it is a good spark. Thanks for your input.

Sorry, just realized I posted in wrong forum. Should be in the stern drive forum. Any way to move this?

 

[gm280]

Are you positive you are getting fuel? It should start if you have fuel, air, spark, compression AND correct timing. JMHO

 

[Chris1956]

Spark looks OK. Do you get spark out of distributor, thru sparkplug wires, at the right time? Did you remove the distributor, when servicing the carbs?

Spark plug wires, dist cap and rotor are normal maintenance items on Sterndrives.

 

[Texasmark]

Nothing else to do this AM so I will post this:
My experience with sparkplugs in a pressurized environment (internal combustion engine cylinder) is that a 0.030 gap will light off around 10kV. Old ignition system with coil and points, ran around 18kV unloaded STP...outside of the engine. Rate of rise of the spark and ultimate voltage are controlled by inductance of the coil and the rate of change of current: V = L di/dt. Since the ratio of 12v to 18kv was high, the coils required a high turns ratio which meant lots of turns on the secondary which, when considering this device has to be of reasonable size, meant small diameter wire which increased the resistance which slowed the current for a given input voltage making for a slow rise time and easily bled off (plug contaminants) charge.

Along comes CDI (Capacitor Discharge Ignition) systems whereby the input voltage to the coil (high voltage transformer connected to the spark plug) is 20x and then some...of the old system, which significantly reduced the required turns ratio and thus allowed for larger diameter, fewer turns of wire in a smaller package, thus reducing the circuit resistance. Reducing the resistance with higher voltage made for faster di (current change) numbers so dt (time of change) was significantly reduced....plug dissipated the energy rather than loose it to bleed off.

This gave you Mercury's 40kV (open circuit voltage) at a few Microseconds rise time, vs Milliseconds for old points systems plus doubling the unloaded voltage, helping the plugs to decide to fire rather than foul out. This fast rise time helped Merc. ignition systems satisfactorily fire contaminated spark plugs and not have the energy bleed off that occurred in older systems.

The energy in the spark is stored in the capacitors and is ½ CV exp 2....C being the size of the capacitors in the CDI and V the voltage to which they are charged prior to firing. Once dumped by the CDI trigger switch, this energy is transferred to the high voltage coil whereby it is stored as ½ LI exp2, being the inductance of the transformer and the applied rate of rise of current.

Since energy is "stored power disippation waiting to happen", the voltage spike sent to the spark plug quickly rises till it can break down (short out) across the plug gap......gap voltages have been measured as low as 100v during the discharge. Once that happens, the engergy that was initially stored in the capacitors is discharged in watt-seconds......meaning, the more voltage across, and current through the gap, the higher the "heat" (intensity of the spark) and shorter the duration of the spark.

Heat is color and you can go to a color spectrum and determine the heat intensity of the spark of your plugs, realizing that inside the engine and out in the open are two different numbers due to static pressure differences. Orange sparks are low intensity, long duration...for a given watt-second pulse whereby going on up to blue-white you have high intensity lasting for a shorter period of time. The latter has proven to be the most reliable method of firing spark plugs.

So backing the truck up and reading through this "waste of your time" can give you an insight as to where to start looking for your problem.....in my humble opinion.

 

[Faztbullet]

Spark looks weak to me....Spark intensity depends on coil voltage from battery. If spark starts out strong then starts to weaken the resistance wire or coil maybe failing. Need to monitor voltage at the + side of coil to see if it drops below 9volts...

 

[kaliona]

Well, got her started and running fine again. I usually don't like to just throw parts at a problem but for the price of a new condenser, I thought what the heck, it's been about 5 years anyway. Fired up and idles fine again. Thanks for all your input.

 

[Texasmark]

Bored again this AM so lets try this:
The energy that is consumed in the spark (voltage x current when the spark actually jumps the gap) obviously is produced/stored in the electrical system. In a conventional breaker points system, the coil (high voltage transformer) has primary winding current current flowing through it when the ignitioni points close. That current and the voltage across the primary winding, fill the coil with energy.....watt-seconds expressed WL = ½ L (inductance of the coil) x i exp2 (current flowing at the time of interruption squared)..

As the coil is filling, the voltage across the coil is low and at a slow rate due to the inductance of the coil and the voltage is low in both the primary and secondary whereby the secondary voltage isn't high enough to jump the spark plug gap.

When the points open, the current in the primary is supposed to stop but the nature of coils/transformers, is that they don't stop current abruptly once the circuit opens. They start discharging some of that stored energy which, when coupled with the points opening "open circuit" cause a voltage drop to occur across the points, and it grows as the points are opening, allowing for more voltage across them. VxI is watts and watts consumed across the points are heat and heat burns up points.

So the lowly little "condenser" (electrical capacitor) is placed across the points to shunt some of the current that otherwise would jump the points. They are designed to slow the rise and limit the amplitude to a value that is too slow and too low in voltage to allow the points to develop enough voltage fast enough to produce an arc.

The C, once filled, in milliseconds, ceases to absorb current and the primary circuit is finally open. The transformer is building voltage all this time trying to get rid of this energy stored in it...V = L di/dt and as this voltage increases it finally gets high enough to jump the gap on the spark plug and the secondary, high voltage circuit disposes of it's energy in the spark gap. BANG!

If the lowly little capacitor, shunting the points, doesn't shut off in time....aka it's "leaking" current internally, the two plates and dielectric making up the capacitor are no longer isolated by the dielectric, but little holes in such are allowing them to touch and conduct current, the coil voltage does not rise to the disired amplitude and some of the energy that was supposed to be dissipated in the spark gap is disssipated in the capacitor.....making for a low intensity, long duration orange spark like you saw in the video.