1973 CB500K2 rebuild, strange electrical or starter problems

I've got a '73 CB500K2 I had rebuilt professionally, including a top and bottom half of the engine. Everything has been put back together, with some professional machine work so I could integrate a Ducati swingarm and CBR600RR fork up front. Installed the electrical loom on the outside and connected everything so I could test it before installing it permanently, put some fuel in the tank and tried to start it.

Motor turned over and almost started but then the battery ran dead. Charged the battery up and then turned the key over so I could press the start button and heard a 'pop' come from the battery. WTF? Pressed the start button, just heard a loud metallic "Thunk" from the starter area. Tried several times, nothing happens except the thunk noise. The battery terminals begin to heat up and melt anything plastic or rubber touching them (at least one of the terminals, dont remember which one). Removed the starter wire from the solenoid and it clicks like it should. Tried another battery and different solenoid, same behavior. Any thoughts? ???

Sounds like your starter motor has seized / shorted out and all the current from your battery is melting wires.


Maybe take the starter motor off and just test it touching the lead to a battery.

I think you are right. I just tested everything again and made sure the leads to the battery terminals were all secure and tight and that nothing was improperly connected. I turned the key and there was no pop or noise. Turned the key off and back on and this time I heard it come from the starter. Pressed the start button several times, heard the clunk/thunk, and then watched some wires melt near the battery with a puff of smoke.
I think its the starter cable with insulation worn off and its grounding against something, or the starter seized. Should be relatively simple to fix.

I had a starter motor "stop" on me after I re-connected the +ve lead to the starter motor - what happened was the terminal inside the motor casing had spun around and grounded on the starter motor case. Took me a while to work that one out.

Well, it looks bad. I jacked the bike up and put jackstands under the rear sets. Took the spark plugs out and tried to hand crank the motor by rolling the rear wheel in each gear, wouldn't budge. I even lifted the bike's rear end up by one of the mag wheel spokes and it would not move. Took the points cover off and tried to hand crank the motor with a socket wrench, and it "turned" but the rear wheel did not budge. Then the small (long) bolt that holds the large nut over the points cover snapped and is stuck inside the crank shaft now.

I can hit neutral twice when shifting gears too; so I think the transmission was reassembled improperly and that's what caused the motor to lock up. Its obvious that they did not handcrank the motor after reassembly to check for interference or function check.

Pull the starter motor and starter clutch off and then see if the motor turns over. Most likely a starter failure or clutch fell apart and jammed. At the risk of stating the obvious, continuing to push the button and watch wires melt and listen to steel parts breaking is not a good idea.

If the motor does not turn after the starter and clutch are removed, suggests that the problem is deeper inside, so let's hope it's an easy fix.

BTW, shifting gears with a motor not running will produce false neutrals because the gears need to spin to shift easily, so let's not worry about that just yet.

sadtaco said:

Took the points cover off and tried to hand crank the motor with a socket wrench, and it "turned" but the rear wheel did not budge.

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If you are saying that when the transmission is in neutral, you can turn the motor over, your problem lies in the trans area.