How to check ignition coil

[lc4]

Currently restoring a Honda MR50 and have no spark, fitted new points ,plug and cap re-using original old coil and rectifier.
Whats the process on checking that the coil is ok and also the rectifier?

[mitch75]

A resistance check on the coil.
You will need a manual for the specs and a multimeter to read the ohms.

[lc4]

Thanks mitch75, I ended up sorting it out, as usual something stupid 

[Freakshow]

an earth....

[pancho]

 Here is a question that I should know the answer too but never thought about till now-where does the high tension circuit from a coil complete its circuit?
 In other words the spark jumps the plug to continue into the cyl head, then continues to where?
 If the body of the coil is insulated from the frame will the spark stop due to open circuit?
 NO it doesn't!
 So how is the circuit completed.
 It may be a dumb question--- but???
cheers pancho.

[Lozza]

It's all about the plug gap, there is a voltage rise on one side of the plug after firing the voltage differential is back to zero. Bit like lightning where does all that voltage go? The earth, circuit is completed for that 'spark' and bring on the next bolt.

[Montynut]

Here is a question that I should know the answer too but never thought about till now-where does the high tension circuit from a coil complete its circuit?
 In other words the spark jumps the plug to continue into the cyl head, then continues to where?
 If the body of the coil is insulated from the frame will the spark stop due to open circuit?
 NO it doesn't!
 So how is the circuit completed.
 It may be a dumb question--- but???
cheers pancho.

Well the spark should stop if there is not a complete circuit on the secondary of the coil. An ignition coil is basically a simple transformer and both windings need to have a completed circuit to operate. Usually the coil is mounted solidly to the frame or has an earth wire that is mounted to the frame or engine cases. This earth is the ground point of the HT side and can also be the earth point of the LV side also. The primary and secondary windings can also be connected together at the ground point as they are effectively anyway via the earth system.

As in any electrical circuit you need a potential difference for current to flow an example is a bird on a 11000 overhead line they land on it and touch 11KV without any flow of current as there is no circuit to a point of lower potential. Now if the bird was to touch the cross member of a pole or a limb of a nearby tree bye bye birdie. There must be a link between the earth system and the neutral of the supply system. lighning strikes between two points of massively different static charge. Either cloud to ground (well actually ground to cloud but we won't worry about that) or between clouds as in sheet lightning.

[matcho mick]

what about twin output coils,1 plug does ground to cloud  ,

[mitch75]

Are we talking about a ignition that uses points because they are the earth.
Dual output coil fires both a the same time one is wasted.

[OverTheHill]

talking about twin lead coils i tried one once on my old dunga basicaly a Yam RT2 moded vmx, well before vmx started, with a cdi ststem i jacked up off umm an orange ag175 stator coil [last one was cdi] & a backplate that fitted the DT and due having two plug holes in the head i thought heres a go, [cz's have two plugs albiet two separate systems]. Did have it running but ran terrible so shelved that idea. Think i had the plugs shut right down in the gaps. Just another dumb idea about 30 years ago. Pretty sure it was a 12 volt TCI twin lead coil off a 4 cylinder bike wrecked at work & just earthed out one side of the primary & ran the cdi box orange wire [yamaha box to coil color] in to the power in on tci coil. Always wondered if more source coil voltage might have fixed it!! or the only real way is to run two separate systems on one stator plate plus two boxes & secondary coils. Now there's a plan, where's me bike.