Topic: Another blowing main fuse thread - '76 CB550K

[Keenankoso]

Hey all,

New to the forums but have done a ton of reading during my 2 builds, so I appreciate all the knowledge here! I know this is a pretty common thread (even found one from a few days ago with the same issue), but I've tried everything I've found related to it and the problem persists.

The main fuse on my CB550K is blowing intermittently. I believe I've narrowed it down to the black/white coil wire circuit somewhere. I'm running a Charlie's Place electronic ignition which I've taken the wire loom off and inspected for shorts (found none), stock coils, Rick's solid state reg/rec, and reproduction handlebar switch from previous owner. When this started happening, I just kept putting in new fuses and disconnecting things until it didn't blow as soon as the key was turned on. Turned out that disconnecting the black/white wires from the coils was the answer, but then suddenly it was working fine again. When they started blowing again, I noticed the fuse wouldn't blow if I had the kill switch turned to off, so I took that off and inspected to find the black power wire had come loose from is solder point which I repaired. That fixed it for a few more days, then it blew the fuse again while riding. This time, I bypassed the kill switch entirely and plugged the black/white wire from the coils directly to key on power so I could eliminate the switch from the equation and just use the key to kill the engine. This allowed me to turn the bike on without blowing the fuse, but 30 seconds into a test ride it blew again. Replaced the fuse once again, and now it turns on and hasn't blown, but I feel like it's just a matter of time before it decides to.

Is this a grounding issue somewhere at the coils? My understanding is that the coils don't ground to the frame at the mounting points but through the spark plugs/engine. Could this be a bad set of coils? Or maybe something is happening in the electronic ignition? Any more ideas or places to look would be greatly appreciated!

[scottly]

Take a very close look at the coil mounting brackets; if they are like a 750, the brackets have a relief on one side to clear the wires where they enter the coil body. If the bracket is reversed there is no clearance for the wires, and they may short to ground. 

[TwoTired]

Please describe where the fuse “blows”.  If the glass fuse blows near its center, you indeed have an overcurrent to track down.  If it is parting near its end caps, it is overheating due to a film on the clip contacts, poor grip tension, melted mount base or poor solder connections on the back of the fuse block that are causing the overheating.  If you can’t hold your thumb on the fuse with the key turned on, it’s overheating.

Cheers,

[Keenankoso]

Take a very close look at the coil mounting brackets; if they are like a 750, the brackets have a relief on one side to clear the wires where they enter the coil body. If the bracket is reversed there is no clearance for the wires, and they may short to ground. 

Appreciate this! Going to check this out tonight and report back. I'd be thrilled if this is the case.

Please describe where the fuse “blows”.  If the glass fuse blows near its center, you indeed have an overcurrent to track down.  If it is parting near its end caps, it is overheating due to a film on the clip contacts, poor grip tension, melted mount base or poor solder connections on the back of the fuse block that are causing the overheating.  If you can’t hold your thumb on the fuse with the key turned on, it’s overheating.

Cheers,

Apologies, I meant to include this in the original post as I've seen you recommend this on the other threads. It's blowing right in the middle of the fuse so looking like another indicator of overcurrent.

[Keenankoso]

Take a very close look at the coil mounting brackets; if they are like a 750, the brackets have a relief on one side to clear the wires where they enter the coil body. If the bracket is reversed there is no clearance for the wires, and they may short to ground.

Sorry for the double post, but just checked and you were indeed correct. One of my mounts was backwards and it was on the side of the black/white wire so I'm really hoping that's the issue. Trouble is that I won't really know for sure until I'm out for a ride and it dies on me 

[scottly]

The first time this exact problem was solved, the thread went on for 5 pages before we figured out it was the coil mount.  

[Keenankoso]

The first time this exact problem was solved, the thread went on for 5 pages before we figured out it was the coil mount.

Well, I was able to ride tonight without a fuse blowing so I'm hopeful this was the problem (and that I'm not jinxing myself by saying that). Thanks for the suggestion, I don't think I'd have ever caught this one.