30a fuse blows when engine is hot

[Cmbutler101]

I have a 1980 cb750c that’s driving me bonkers, the bike has been blowing the main fuse after running for more than 10mins or when I rev it up over 7k. The bike will blow the fuse while riding then blows the replacement as soon as I turn the key and will keep blowing them until the bike sits for 40 mins then it will let me start it up and the problem will repeat itself. I’ve replaced the main solenoid, voltage regulator, ignition coils wires and plugs yet the problem still persists. I realized my spark units had melted all the potting out of them, new ones are on the way but I’m still not sure those were the issue because I can’t find a thread of anyone blowing fuses due to melted spark units. Could it be the stator and rotor possibility heating up and shorting until the engine cools? Any help would be appreciated!

 

[amc49]

Charge battery fully so you got an hour plus to drive it with no charging then unhook the alternator harness that goes into alt unit. Drive it then and see if fuse blows.

The rotors on those are well known for shorting out. The rotor can die and bomb the regulator too.

Spark units are OK, they used crap potting compound and it melts out to drip even if bike not used during the summer. They are still OK. Later ones changed to a different potting compound and the issue stopped.

If you are still running the crap thin sliver of aluminum for the big fuse then convert to a better ATO blade type fuse and holder, the OEM fuse gets very dainty with age and all you can get are super old now. They blow out easy as spit like that. I've seen them look good and crumble into dust when touched too.

 

[Cmbutler101]

I just recieved the new spark units and just like I expected they still didn’t fix the fuse blowing problem. Thank you for the suggestion to unplug the stator/rotor plug, I could actually turn the key without blowing the fuse! The bike started up and ran for 30mins uninterrupted until the battery died. I guess I’ll be buying a new rotor!

 

[amc49]

Nope, further checking needed to make sure stator is good. Charge battery back up and then locate the SIX pin alt connector and remove the black and white wires from it and tape them up to prevent problems. That removes the rotor only from the circuit leaving stator to determine which is at fault. Rotor usually but the stat sometimes is. Both expensive parts.

With the two wires taped up connect the alt connector back up and ride bike again, if problem still gone it is the rotor or the two wires going to it. If trouble shows back up it is the stator.

The wires simply insert into the plastic connector to clip into place, you can use a fine slot screwdriver or dentist pick to look up closely inside the connector and locate the hooking tang and then press down on it and the wire comes right out. Pry on the tang from the connection side, not the outside of the plastic. The wires simply insert back in and they hook back into place when done. Insert and then pull back on them to make sure they are clipped in.

Or, remove the alt cover and then remove the brush plate (KEY OFF!) with both rotor brushes to do the same test as above but you MUST insulate the bare wires with tape or you could burn something up. You may even find a problem when you remove that cover, the plate is plastic and can break and be careful when attaching it back up! They are delicate and you will never find one on the net if you break it.

Rotor dead 8 out of 10 times, stator maybe 2.