SOHC.co.uk Forums > CB750
750K2 Coils
AshimotoK0:
All I can say about pattern coils is check their primary resistance. Honda ones are around 4.5 Ohms. Some pattern ones are 3 Ohms' ish and this puts a lot more current through the standard points than they were designed for. If fitting CDi then that's another matter and a 3 Ohm coil will probably be fine. The other thing about the low resistance coils is that with the ignition switched on but without the engine running, the contact breaker that is in it's closed position will cause the coil to consume 48 Watts, so will overheat rapidly.
If I recall correctly there is a horror picture in Hondaman's book on the CB750 of the results of using low resistance pattern coils with standard points.
philward:
Thanks for that Ash! that will worry me to death! seriously, how do I check the primary resistance and if they are 3 ohms, can the circuit be modified to alter the resistance (I'm no electrician but putting a resister 'inline' externally).
AshimotoK0:
--- Quote from: philward on December 01, 2018, 01:07:39 PM ---Thanks for that Ash! that will worry me to death! seriously, how do I check the primary resistance and if they are 3 ohms, can the circuit be modified to alter the resistance (I'm no electrician but putting a resister 'inline' externally).
--- End quote ---
Don't have sleepless night as they say on Crimewatch ;D ;D . You can check the primary resistance with a decent multimeter on Ohms resistance range ... or put a meter set to 10A current range in series with the coil and it should draw no more than 3.5 A Plus some aftermarket coils will be absolutely fine. You could put a resistor in series of say 1.5 Ohms but that would reduce the HT. However they may still be enough energy to give a decent spark.
philward:
Thanks Ash - I'll have a go later if the grandkids will let me!
philward:
Just checked coils Ash - 4.2 Ohms, so I guess that's close enough. (4.9 Ohms on old coils)
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