SOHC.co.uk Forums > CB750

Cheap coils on ebay

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Bryanj:
DO NOT use 'Wing as they are ballasted coils

Piki:
Thank you Bryanj, but I do not know that is "Ballasted coils" 750 four are similar a 400/500/550...

Bryanj:
Physically yes but the resistance varies

Lobo:
Hi Piki, as I understand it... and not related / specific to Hondas... but 'just generally'....

A ballasted coil typically runs at a lower voltage .... say 6-9 volts, and this is achieved with the use of a resistor by-pass wire. It is all to help starting; ie when the starter motor cranks this resistor is shorted out ... allowing full available battery voltage to the coil (which will now be less than 12v due the starter load), and giving a better spark. Once cranking stops the resistor is put back into the circuit, and the reduced 6-9 volts now feeds the coil. Because the coil is basically designed to run at 6-9 volts the spark is strong / adequate and the engine continues to run just fine.

On your 400F, the coils are un-ballasted, and are designed to run at 12v. During cranking the battery voltage may likely drop to around 10v, and the spark will become weaker. Not ideal, but good enough in this case. Once your stater has ceased cranking the voltage will return to 12v or so and full spark will return.

So, as Bryan says, "the resistance varies", .... and according to starter cranking. Your 400F circuit is not designed for ballasted coils, and by swapping coils between systems you either run the risk of under-voltage / weak spark or running them at over voltage & burning your coil / points out.

I think...!

Hope this helps,
Simon

Cabilao:
Thanks Lobo,good explanation,i also had no clue about ballasted coils. You never stop learning ;)

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