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Coil Polarity

Bill Thomas

Well Known and Active Forum Website User
VOC Member
Very Good, Bruce, I was playing with a D distributor on the bench and somehow I wired it the wrong way, Silly Old Sod !, I thought the spark was not as it should have been, Then changed it round = OK Now I can check it with your way, Thanks, Bill. I must try it Len's way where you use a double coil and no rotor or dist' cap, Cheers Bill.
 

greg brillus

Well Known and Active Forum Website User
VOC Member
Interesting point about polarity with regards to a magneto, well on a twin this would be of no concern as the firing intervals are 205 degrees apart ( or 180 on a parallel twin ) which ideally suits the mags polarity......however....on a single it is much more of an issue....consider that on a typical Lucas K1F mag that the end housing, on the points end, can be rotated 180 degrees and the engine once timed correctly will run ok.......think about this....If you run a mag on the test rig at say 1000 to 1500 rev's which is 2000 to 3000 engine crankshaft revs and check the primary voltage with a peak voltage meter, if the polarity of the mag is correct, then the voltmeter will show around 200 volts.....if you rotate the end housing 180 degrees and perform the same test at the same rev's the volt meter will show about 120 volts...!!! The primary winding has 200 turns of heavy guage insulated wire surrounded by 12,000 turns of very fine wire for the secondary, which is a ratio of 60:1....200 volts X 60 = 12,000 volts at the spark plug....wrong polarity = 120 volts X 60 = 7,200 volts at the spark plug.....are there any Comet owners out there who's mag's are suffering this condition without knowing.....I would say definitely....YES. Polarity on a mag for a single is very important. A hot engine with a magneto running the wrong polarity will most likely need bump starting......most people would be unaware of this condition.
 

vince998

Well Known and Active Forum Website User
VOC Member
My D ran with this setup for many years very well. I´ve still got the coil somewhere (it´s a sealed coil so your stuck with the HT leads fitted). It looks japanese. The reason i changed it (apart from going twin spark) is beacause i read an article stating that the cylinder under least pressure gets the biggest spark. this would be the "empty" cylinder or "waste" spark and not the one meant to fire?
Very Good, Bruce, I was playing with a D distributor on the bench and somehow I wired it the wrong way, Silly Old Sod !, I thought the spark was not as it should have been, Then changed it round = OK Now I can check it with your way, Thanks, Bill. I must try it Len's way where you use a double coil and no rotor or dist' cap, Cheers Bill.
 

Hugo Myatt

Well Known and Active Forum Website User
VOC Member
Interesting point about polarity with regards to a magneto, well on a twin this would be of no concern as the firing intervals are 205 degrees apart ( or 180 on a parallel twin ) which ideally suits the mags polarity......however....on a single it is much more of an issue....consider that on a typical Lucas K1F mag that the end housing, on the points end, can be rotated 180 degrees and the engine once timed correctly will run ok.......think about this....If you run a mag on the test rig at say 1000 to 1500 rev's which is 2000 to 3000 engine crankshaft revs and check the primary voltage with a peak voltage meter, if the polarity of the mag is correct, then the voltmeter will show around 200 volts.....if you rotate the end housing 180 degrees and perform the same test at the same rev's the volt meter will show about 120 volts...!!! The primary winding has 200 turns of heavy guage insulated wire surrounded by 12,000 turns of very fine wire for the secondary, which is a ratio of 60:1....200 volts X 60 = 12,000 volts at the spark plug....wrong polarity = 120 volts X 60 = 7,200 volts at the spark plug.....are there any Comet owners out there who's mag's are suffering this condition without knowing.....I would say definitely....YES. Polarity on a mag for a single is very important. A hot engine with a magneto running the wrong polarity will most likely need bump starting......most people would be unaware of this condition.

Ah-Ha! So this would explain why the cam ring lobes in my two Comets are in two different positions and always have been whilst in my possession despite both magnetos having been 'rebuilt' at various times. Both bikes start and run well but which is the correct position?
 

greg brillus

Well Known and Active Forum Website User
VOC Member
Without having a volt meter of the correct type, it is very hard to tell. A modern digital one will not show peak volts ( unless it is a really good one ) the old analog type with a moving needle might work ok. If you put the bike on it's rear stand, and with mag cowl and end cap removed you can check it while it is running by attaching the negative lead to the casing and touch the possitive lead to the centre bolt holding the points onto the armature. Simply rev the engine to say 2000 to 3000 revs and the voltage should read around 200 volts, however if the polarity is wrong then this figure will be significantly less....secondary voltage ( at the spark plug ) is directly proportional to primary voltage. I know that many magneto rebuilders simply refuse to believe this condition exists, but the facts are there to see. An osciliscope works to show this as well, but not too many Vincent owners out there own one.....surprisingly. :)....Greg.
 

Big Sid

Guest
On the matter of coil polarity , coil ign . To my best recollection . With plugs out , one plug wire out of its plug cap to bare the wire end . Held close to a fin with a sharpened wooden pencil in between the wire in the fin , the graphite of the pencil positioned between these two . Have some one spin the motor or flick the points creating a spark . The spark to jump to the pencil lead ,,,, a spark from the lead on the other side able to leap to the fin . The lead in between .
Closely observe the spark appearance . One side will look normal while the other side will be much different , having a wavy aura look to it . Weird looking .
If the aura side is seen to be between the plug wire and the graphite this means the polarity is correct . If the aura is seen to be between the graphite and the fin , it's wrong . Sid .
 

Big Sid

Guest
Many Vincent twins suffer from a kick start lever that jams solid when the starter quadrants first tooth fails to engage properly with the ratchet teeth . It enters only if one rocks the back wheel while in gear .
We have had much success correcting this by removing the big quadrant and gracefully grinding the lower first tooth down by one half in height and then grinding it's remaining leading slope back towards the second tooth , creating a sharper thinner and shorter tooth when done . Nicely polish to complete the mod . No promises , but it usually works for us .
This USUALLY helps it slip into normal engagement as it nears the ratchet . Grease all teeth and those on the ratchet outer teeth and the tiny ones on its ratchet side also . This extends its life . Firm up the spring too . Sid .
 

Big Sid

Guest
Where both plugs fire together on a vee twin it's called a " wasted spark system " and has seen fine service since God made green apples . One cylinder only firing . Works on vertical twins too I think .
Where the motor is tuned radically for great power , cams etc then it's usually abandoned in favor of a separate firing system .
Where there may be a burnable mixture in the other cylinder and the hot cams allow , that idle cylinder can flash fire weakly , not desirable in a performance motor . Contaminates the next desired firing cylinders mixture . Sid .
 
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