Science of the spark

All ignition related articles
Post Reply
64duo
Site sponsor
Site sponsor
Posts: 125
Joined: Wed May 11, 2005 4:07 am
Bikes: 2001 Road King w/sidecar
1947 Servi-car
Location: Redmond, OR

Science of the spark

#1

Post by 64duo » Thu Dec 22, 2005 1:51 pm

Description: How big of a spark does it take to fire the pan

In an effort to chase other avenues for hard cold starting, what can be said about spark. How big of a spark does it take to fire the pan? My experience is very limited in knowing what a good spark should look like, mine is blue and not all that dramatic. Does the spark look different in 12v vs 6volt? Does the battery amp have anything to do with the quality/quantity of the spark. How can one eliminate weak spark as a reason for hard start? I'm using 12v & electronic ign. Thanks in advance.
Jonathan 64flh



fourthgear
Posts: 1365
Joined: Mon Apr 11, 2005 1:12 pm
Location: north florida

Re: Science of the spark

#2

Post by fourthgear » Thu Dec 22, 2005 3:35 pm

64duo
I believe the amperage of the batt. is probably only relevant to a electric starter and in some ways the size or gage of wire being used. You should have a crisp blue spark , the coil has more to do with your spark than anything else as long as you have good voltage to it , 12 or 6 V. Your ign. only sends a signal to be boosted for spark, points or electronic. Coils are notorious cause for hard starting problems , they are hard to diagnose and harder to tell if they are giving you what voltage boost is required to start properly in all conditions. Some times you can do a resistance check on them to try to tell if they are up to par with specs , but even with a good multimeter its guess work at best because of the diff. manufactures of coils and there specs.( coils don't come with resistance specs on primary or secondary coils , some tech. manuals have them for stock coils and when were they written?)Pretty much if you have a lazy yellowish spark , something is not right. Plugs , spark plug wires , ign. timing , point cond., and condensers all have an adverse effect on starting, not to mention all the mechanical things that have to fall in line , Ya have to experiment with diff. combos to get things right. I'm sure others here can say more about this subject, just my two

King
Posts: 373
Joined: Fri Apr 02, 2004 5:05 pm

Re: Science of the spark

#3

Post by King » Thu Dec 22, 2005 7:19 pm

While we are at it what about spark plug gap. I am running STD heads that use the standard Evo plug which comes from Harley gapped @ 0.040. I still run 6V points ignition and the stock square coil. The question is can I still keep the0.040 gap and have an adaquate spark or do I have to reduce it to the 0.025 recommended in the old "Riders Handbook". Visually it still looks good at 0.040 with a blue spark but I'm wondering??

Thanks

King

fourthgear
Posts: 1365
Joined: Mon Apr 11, 2005 1:12 pm
Location: north florida

Re: Science of the spark

#4

Post by fourthgear » Thu Dec 22, 2005 7:25 pm

King
I would run the point gap , the larger gap was for electronic ign. , they can and do run a higher voltage coil with resister wires and plugs, but hay if it runs good that way I would not change a thing.

kell
Posts: 369
Joined: Sat Jun 19, 2004 2:58 am

Re: Science of the spark

#5

Post by kell » Sun Dec 25, 2005 3:41 am

About visual spark tests,
Taking the plug out and seeing what the spark looks like won't serve you very well as a diagnostic test because sparking a gap in an aerosol under several atmospheres pressure requires much higher voltage than getting a spark to jump the same gap in open air.
This is a guess based on experience, but in open air, I would say your spark should jump a quarter inch gap, at least.

harleydoug

Re: Science of the spark

#6

Post by harleydoug » Fri Dec 30, 2005 5:02 am

Just a few quick points to ponder. The spark that jumps the plug is AC voltage, not DC battery voltage. The spark must be blue and crisp to fire under compression in the cylinder. Breaker points systems need a smaller gap of .025 to work. The electronic systems can handle the .040 gap. Be sure that the coil matches the system though. A coil with 5 ohms is required for the points system, while a coil of 2 or 3 ohms is used in the electronic systems. You may want to test the primary resistance of your coil to see if it matches. A mismatched coil will be hard to start and leave you along the road at night in the rain. Been there, done that!!
Keep those old dogs barking!!:-)

64duo
Site sponsor
Site sponsor
Posts: 125
Joined: Wed May 11, 2005 4:07 am
Bikes: 2001 Road King w/sidecar
1947 Servi-car
Location: Redmond, OR

Re: Science of the spark

#7

Post by 64duo » Sat Dec 31, 2005 2:12 am

Dec 30, 2005, 5:02am, harleydoug wrote:
Be sure that the coil matches the system though. A coil with 5 ohms is required for the points system, while a coil of 2 or 3 ohms is used in the electronic systems. You may want to test the primary resistance of your coil to see if it matches. A mismatched coil will be hard to start and leave you along the road at night in the rain. Been there, done that!!:-)
Well, now i feel i'm pushing the envelope of my really understanding, and spark & electricity, would not be the only things I have trouble understanding.
But, fortunately I'm also not afraid to say so.
So given that i'm in over my head, anyone willing to help direct me with testing the resistance of my coil to check what ohm it may be? I'll need some basic step by step source. Hers where i'm at so far.
Step 1) I have a multi-meter.
Step 2) I have a multi-meter.
Thanks in advance
Jonathan 64duo

fourthgear
Posts: 1365
Joined: Mon Apr 11, 2005 1:12 pm
Location: north florida

Re: Science of the spark

#8

Post by fourthgear » Sat Dec 31, 2005 9:57 pm

64duo
The primary of the coil is what you connect your 12V power and point or ignition source to . The secondary is of course where the plug wires go. You should have a GOOD digital multimeter to do this test, just select your multimeter on ohm setting and put the red lead on one terminal and the black lead of the multimeter to the other and make sure the wires from the points(ign. ) and 12 V power are completely off the terminals to do the test and the plug wires too while your at it. The should give you your ohm reading, just don't expect it to be exactly what may or not be published in any book. You can check your secondary the same way , I used to check a lot of coils to see if they were any good by checking resistance ( ohms ) but found ( like has been said here ) they sometimes work a lot differently under driving conditions and temperatures. My philosophy is when in doubt , through it out. Have fun.

kell
Posts: 369
Joined: Sat Jun 19, 2004 2:58 am

Re: Science of the spark

#9

Post by kell » Sun Jan 01, 2006 12:17 am

The symbol for resistance is the greek letter omega. Your meter may have that symbol next to one of its settings. That would be the one you want.
There are two kinds of meters: analog (needle) and digital (readout).
The digital meters will measure low resistances like a coil primary, but the average cheap digital meter will have an error or about half an ohm or an ohm, maybe more. You can see what the error is just by shorting the leads to each other and seeing what it reads out. Anything other than zero is error. It's like the tare weight on a scale. You can subtract this error from a reading to get a more accurate result. So if it reads say 0.7 ohms with the leads shorted, and 5.2 ohms when you measure the coil, your coil resistance is 5.2-0.7=4.5 ohms.
The really cheap analog meters only have one resistance setting. It will have X1K or something like that printed on the selector. That means it will only measure very large resistances (thousands of ohms) with any accuracy. If you have a meter like this, you won't be able to measure the coil's primary resistance with it.

Post Reply

Return to “Ignition”