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Author Topic: 72 CB750 - no spark  (Read 825 times)

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Offline mca2

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72 CB750 - no spark
« on: August 30, 2025, 03:18:28 PM »
After a carb rebuild and reinstalling the tank, the motor won't fire. Double checked the kill switch and fuses ad all ok. So I put a spare spark plug in the nr4 HT leads held it against the engine and no spark.

I measure 11.6V on the ingoing b/w wire and also 11.6 on the outgoing yellow and blue wires. Primary coil resistance is 4.5ohm and i forgot to measure the secondary circuit.

Any suggestions where to look next?
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Offline scottly

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Re: 72 CB750 - no spark
« Reply #1 on: August 30, 2025, 04:19:52 PM »
When the associated points are closed, you should read zero volts on the blue or yellow wire.
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Offline newday777

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Re: 72 CB750 - no spark
« Reply #2 on: August 30, 2025, 09:31:40 PM »
Was it running before the carb rebuild??
Did you file, clean and set the points gap and static timing?
Did you unscrew the plug caps from the plug wires and cut off 5mm of the plug wire to get new connection wire for the caps to screw into? The wires get loose connection after a while of pulling the caps off and on the plugs and need refreshing.
« Last Edit: August 30, 2025, 09:33:49 PM by newday777 »
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Stu
Honda Parts manager in the mid 1970s Nashua Honda
My current rides
1975 K5 Planet Blue my summer ride, it was a friend's bike I worked with at the Honda shop in 76, lots of fun to be on it again
1976 K6 Anteres Red rebuilding project, was originally my brother's that I set up from the crate, it'll breath again soon!
Project 750s, 1 K2, 4 K6, 1 K8, 1 F1, 1 F3
2008 GL1800 my daily ride and cross country runner

Prior bikes....
1972 Suzuki GT380 I had charge of it for a year in 1973 while my friend was deployed and learned to love street riding....
New CB450 K7 after my friend returned...
New CB750 K5 Planet Blue, demise by ex cousin in law at 9,000 miles...
New CB750 K6 Anteres Red, to replace the totaled K5, I sold this K6 at 45k in 1983, I had heavily modified it, many great memories on it and have missed it greatly.....
1983 GL1100A, 1999 GL1500 SE, 1999 GL1500A

Offline M 750K6

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Re: 72 CB750 - no spark
« Reply #3 on: August 31, 2025, 12:36:00 AM »
Tank off and carb removal, you may have disturbed the wiring to the coils held by the left side of the frame crossbar. On mine, I have connections there.
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Offline bryanj

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Re: 72 CB750 - no spark
« Reply #4 on: August 31, 2025, 01:20:14 AM »
Or the connectors in the rubber boot by the rear brake light switch where the brakelight switch wires go as well, they can pull out easily
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Offline mca2

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Re: 72 CB750 - no spark
« Reply #5 on: August 31, 2025, 02:55:16 AM »
Ok, checked some more and summary is:
  • primary coil resistance 4.5, inf to ground => primary coil ok
  • secondary coil resistance measures through caps 22k, inf to ground and primary => coil, cable and caps ok
  • 11.6V at b/w feeding side of primary coils and 11.6 at blue and yellow exit points => power goes correctly through primary
  • toggling the kill switch lets the power drop to zero at b/w cable => kill switch is ok
Furthermore the bike ran before carb overhaul so I’m thinking I nicked a cable or connector somewhere but based on the above, all seems fine.

Only thing that could have spontaneously died, is the accent ignition system. So I’ll see how to test that.
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Offline scottly

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Re: 72 CB750 - no spark
« Reply #6 on: September 12, 2025, 08:59:16 AM »
Did you solve this?
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Offline mca2

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Re: 72 CB750 - no spark
« Reply #7 on: September 12, 2025, 12:51:40 PM »
Quote from: scottly on September 12, 2025, 08:59:16 AM
Did you solve this?
Not yet, i'm suspecting faulty connection between coil and ignition. Hope to find some time this weekend to look at it again.
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