Author Topic: Cranking vs Firing Voltage Question  (Read 848 times)

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Offline Locomotiveman

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Cranking vs Firing Voltage Question
« on: February 28, 2018, 11:12:46 pm »
WHY does a fully charged battery seem to better get an engine with Carbs, Coils and no Computer to 'Fire Right Up'? I'm trying to visualize the combustion chamber, ionized gases and the spark with a good battery cranking an engine over vs with a poor battery. Spark Voltage is White Hot vs Orange?  Cranking at higher RPM's ? Or is this a figment of my imagination. 
If the TRUTH is crystal clear..I must need glasses.

Offline VTconnie

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Re: Cranking vs Firing Voltage Question
« Reply #1 on: March 02, 2018, 12:10:47 am »
Haha okay i'll bite.

A battery that is not at maximum potential likely fails even worse under load, as many do. When the bike is trying to start, it's sending power to the coils for ignition, and now a high amp starter motor. So just as you suggested, a poor battery will give weaker spark, and slower cranking speeds won't help a high RPM inline-4 to cold start.
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Offline JimBob

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Re: Cranking vs Firing Voltage Question
« Reply #2 on: March 02, 2018, 03:16:08 am »
One word: Starter.


That sucker draws a LOT of current (75+ amps). That reduces the amount of current available for spark. Additionally if it’s weak, the starter will crank more slowly, reducing the efficiency of fuel atomization. It’s already a struggle at the low RPM of a starter, reduce it more and the air/fuel charge is just really poor.


If you ever get a chance, look down a carb while a vehicle is starting and you can see the fuel being sucked in by the vacuum of the pistons. Then look at one with a weak battery - you can sometimes actually see the gas dribble out because the vacuum is so weak.

Offline jettawreck

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Re: Cranking vs Firing Voltage Question
« Reply #3 on: March 02, 2018, 11:09:36 am »
Even leaving the ignition spark voltage out of the equation, a slow crank makes for lower compression numbers and as mentioned poor fuel ingestion and dispersion (?).  Cold temps makes for even more noticeable results.
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Offline Locomotiveman

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Re: Cranking vs Firing Voltage Question
« Reply #4 on: March 25, 2018, 10:33:41 pm »
I was often baffled when I could Kick or Push-start my '67' Electra-glide to life, but trying to get it to fire using the starter was a waste of time. Coupla good, fast spins seems to always be better than R-r-r-r, but I've never quite visualized WHY. I'm gonna say 'Higher RPM's' is key to a sweeter start-up. Maybe. :) My neighbor's new computer'd Cadillac plain wouldn't start once Voltage dropped to some inferior level.
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Online SteveJ.

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Re: Cranking vs Firing Voltage Question
« Reply #5 on: March 25, 2018, 10:54:22 pm »
I was often baffled when I could Kick or Push-start my '67' Electra-glide to life, but trying to get it to fire using the starter was a waste of time. Coupla good, fast spins seems to always be better than R-r-r-r, but I've never quite visualized WHY. I'm gonna say 'Higher RPM's' is key to a sweeter start-up. Maybe. :) My neighbor's new computer'd Cadillac plain wouldn't start once Voltage dropped to some inferior level.
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Offline Brooke_Benfield_OR

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Re: Cranking vs Firing Voltage Question
« Reply #6 on: March 25, 2018, 11:14:36 pm »
When a lot of current is being demanded from a battery that is resistive (old and failing) then the voltage across its terminals will drop down. This affects the rate of spin the starter can provide and also weakens the spark. Slower cranking means less compression and less fuel/air mixture being drawn into the cylinders. Couple that with weak spark and you have a machine that doesn't want to start with the electric starter but might do fine with the old kick start method.
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Online Nosmo

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Re: Cranking vs Firing Voltage Question
« Reply #7 on: March 26, 2018, 01:38:22 am »
Or, as my mother used to say, "Just BECAUSE...now shut up and eat your dinner." 
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Offline GeorgeRYoung

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Re: Cranking vs Firing Voltage Question
« Reply #8 on: March 26, 2018, 12:07:41 pm »
In the dim distant past of foggy memory, cars had a solution to this problem. There was a ballast resistor in series with the ignition coil primary. During cranking, this ballast resistor was shorted out by a contact, to deliver the full (reduced by cranking) battery voltage to the coil. In the same era, there were of course other ignition problems, e.g.:  ignition points, single coil/distributor.

I've never heard of this applied to motorcycles.


Offline Locomotiveman

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Re: Cranking vs Firing Voltage Question
« Reply #9 on: March 26, 2018, 02:13:42 pm »
GEORGER YOUNG, Exactly, I had a 70's Dodge Pickup with a 'Ceramic ballast Resistor' on the Firewall. Always carried a spare. Older farm tractors...same deal. CONCLUSION: Don't fight a weak battery after a few years...move on to a new battery. ps I notice it's hard to find a true 'Voltage Tester' for $cheap$ that will handle 20,000/Volts etc. Spark plug tester lights...yes. Multimeters, yes. But old school 'YEOW!!' spark Voltage Tester that FLUKE made...Not so cheap nowadays.
If the TRUTH is crystal clear..I must need glasses.

Offline MAN OF BLUES

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Re: Cranking vs Firing Voltage Question
« Reply #10 on: March 26, 2018, 07:36:31 pm »
In the dim distant past of foggy memory, cars had a solution to this problem. There was a ballast resistor in series with the ignition coil primary. During cranking, this ballast resistor was shorted out by a contact, to deliver the full (reduced by cranking) battery voltage to the coil. In the same era, there were of course other ignition problems, e.g.:  ignition points, single coil/distributor.

I've never heard of this applied to motorcycles.


That brought back fond memories of all the cars passed down to me by my dad, who was a strict Chrysler/Dodge kinda guy...
I was trying to put into words the actual process the ballest resistor has and also explain in a manner it applies to the spark, when I defided to use modern (internet...) sourcing for my wording... well, lo and behold I found this, which is exactly what I wanted to say, in even clearer terms...
http://www.imperialclub.com/Repair/Electrical/coil.htm

Cleaning out pop's garage, after he passed away years back, I probably went thru and tested a couple hundred coils, resistors, and associated ignition components, to see what was what, and dispose of the non functioning "replaced parts he just had to save forever..."
 >:( >:( >:(

I do the same thing now, its a strange inherited disease... :rotflmao:

Anyway, that article splains it all... enjoy.

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Offline Locomotiveman

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Re: Cranking vs Firing Voltage Question
« Reply #11 on: March 26, 2018, 10:39:21 pm »
Stay with me here, boys. Re: Spark Voltage. Nuthin' will whack yer wee-wee to life/death better than a Magneto zap. Old timer local Mechanic would, in the depressing dead of Minnesota Winters, jolt himself to a cheerier perspective on life by intentionally grabbing end of a plug wire from a running Farmall or John Deere. BAM!! "I feel GOODer now." My Dad saw him do it. I mentioned this to a retired Medical Dr. who worked at the State Mental Hospital. "Yes, of course; we charged the County $75 for the very same thing."  I'm just sayin'......
If the TRUTH is crystal clear..I must need glasses.

Online Nosmo

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Re: Cranking vs Firing Voltage Question
« Reply #12 on: March 27, 2018, 12:39:09 am »
At one of the airplane shops I worked in, another mechanic was fooling around with a Bendix magneto, and had an ignition lead in the #1 cylinder position, ( I won't go into the technical reasons why  here) and spun the shaft to activate the impulse coupling, but didn't realize the other end of the high tension lead was touching the metal zipper of his coveralls, down by his crotch. Yeah, 30,000 volts or so right to the goolies.   :)) :)) :D

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Offline Locomotiveman

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Re: Cranking vs Firing Voltage Question
« Reply #13 on: March 27, 2018, 04:36:51 am »
OMG!!  :-\  Some things a fella only does once..
If the TRUTH is crystal clear..I must need glasses.

Offline Bud

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Re: Cranking vs Firing Voltage Question
« Reply #14 on: March 27, 2018, 12:57:49 pm »
When my dad was a boy hanging around his fathers generator repair shop, he thought it was quite funny to wire the cane bottom stools of the employees, to an old crank telephone magneto.  He'd hide so that he could watch the reactions of those poor unsuspecting guys.  I'm sure those guys wanted to strangle him numerous times.  When we had our Western Auto, he used to charge up ignition condensers and then toss them at someone, hoping to catch them off guard.

Offline MAN OF BLUES

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Re: Cranking vs Firing Voltage Question
« Reply #15 on: March 27, 2018, 07:37:53 pm »
When my dad was a boy hanging around his fathers generator repair shop, he thought it was quite funny to wire the cane bottom stools of the employees, to an old crank telephone magneto.  He'd hide so that he could watch the reactions of those poor unsuspecting guys.  I'm sure those guys wanted to strangle him numerous times.  When we had our Western Auto, he used to charge up ignition condensers and then toss them at someone, hoping to catch them off guard.

 :rotflmao: :rotflmao: :great: :great:

My dad tought me that very same trick, which I continually pulled on my gear head pals as a kid, hours of fun for sure, you don't need to do it more than once to know if someone tosses you something small and silver, don't catch it....
Its even funnier when your older brother, whom I am sure was tought that very same trick, 15 years prior, fell for it repeatedly... :))

Dad also had an ignition tester box, it was ancient, and made out of wood, and I actually think it was made by Bendix, had a metal tag on it, and it was used to test spark plugs, and coils... It had to be Model T era for sure... I'm sure it contained a magneto, dad told me never to mess around with it cause it would knock me on my azz.... so I never messed with it.... gotta dig it up outta the remains someday, and resurect it, I'm sure its worth $$$$ today. That along with the old AC/ pre Delco(?) Spark plug cleaner/sandblaster tank would make a nice display.. of ancient garage junk in modern times...
It looked like this...




  oh poop.... now I have to go dig up this stuff, because I know he also had awesome signs like this..



« Last Edit: March 27, 2018, 08:20:59 pm by MAN OF BLUES »

30 YEARS OF KAW.....Rich R. (the other one..)  COG 5977  JUSTAMEMBAHNOW
and if you are gonna call me names... it's MR. Analdweeb if you please...