Topic: Keep blowing main fuse

[2tall]

I have a 76 550 that keeps blowing the main fuse. I filed the points down a little on my lunch break and adjusted the clutch.

I took it for a couple of rides then, and all was well. Now, when I want to head home from work it just blows the main fuse as soon as I turn on the ignition. Any ideas?

I pulled the timing cover off, and all looks well in there. No burn marks, or anything obvious. This is my first electrical problem on these bikes, so I'm pretty clueless as to where I should look first.

Help!

Thanks,

Paul

[OldSchool_IsCool]

Is the fuse blowing by melting the middle of the reed or is the fuse going bad but LOOKS good (i.e. the metal reed looks OK)?

[OldSchool_IsCool]

I think you are right, there is a dead short somewhere.

Here is how you can troubleshoot without spending a paycheck on fuses.  Remove the main fuse.  Take a 12v light bulb and clip it across the fuse holders.  When you turn the key on, the light should go BRIGHT.  Now, pull the other fuses and unplug connectors.  When the bulb goes dim or dark, you have found the plug or fuse that will lead you to your short.

[Leino]

Turn the kill switch to "OFF".
Does it still blow the main fuse?

And I mean turn the kill switch before you turn the key.Otherwise it doesn't make a difference... 

[jakecb420]

Check and make sure that the contacts where the fuse sits are clean. thats what did my bike in one random evening. cleaned all three up and so far so good a year later.

[Skonnie Boy]

By any chance, are you using newer fuses that are slightly longer than the holders?  Once the original fuses on my bike went, I never got the longer fuses to last very long, despite cleaning the holders and such. 

It probably is a short though.

[1974CB750rider]

Check your ignition switch. There is a metal collar right where the wires go ito the switch. If any of the wires there are naked and touch the collar that will  blow the fuse. That was what happened on my CB450. On my 750 the same thing was happening but it turned out to be a short in the brown wire coming from the switch. I replaced the switch and the problem was solved.

[KurtK]

My 500f kept doing this.  I replaced the condensors and the points and it has never happened again.

[Laminar]

I'm using the proper fuses, and the contacts are clean and shiny. Thanks for thinking of that though.

I will check the switch first. I worked on it about a month ago, so it seems to be a good candidate at this point. The street where I work is an old concrete street with some pretty bad bumps. It could have easily caused a failure.

Let me ask a dumb question though. If the fuse blew while riding, would I notice it during the day? Or would the bike die? The reason I ask is that when I got done riding after yesterdays test run, the bike idled for about 10 seconds before I killed it with the switch. So it was seemingly all fine until I went to start it. No real indications of failure earlier in the day.

Off to look at the switch...

Paul

The bike would die immediately - this happened to me while riding. The main fuse blowing prevents any power to the coils, which means the bike instantly shuts off.

[KurtK]

Paul ...I am far from being an expert....   But my 500 f would kind of miss at 5-6000 rpm and then I would back off the throttle....and it would go dead  ...the main 15 am fuse would be blown.  I replaced the condensors and the problem with the fuse went away  (over 3 months now)  but I was still getting a miss at 6000 rpm the points looks fine...filed them etc...but boy when I replaced them the motor smoothered right out.

I think it would be cheap insurance to rplace both and see if this helps.

[KurtK]

Someting else I did was to install in a new fuse holder that holds the blade type fuses.  I don't think this had anything to do with the problem however.

[OldSchool_IsCool]

2Tall,

You are chasing a red haring with the points.  Points a SUPPOSED to ground out.  The coils get 12v via the black/white wire.  The yellow and blue wires provide a path to ground.  The grounding path is complete when the contacts are closed.

Someone mentioned earlier if the fuse blows when you turn the key or when you switch on the kill switch.  Try this, kill switch off, key off, new fuse.  Turn the key on.  Did the fuse blow?  If yes, then it's nothing in the coil circuit.  Does the fuse blow when you turn the kill switch on?  Yes, then the coil circuit is suspect.

There is a handy interactive wiring diagram here. 

www.oldmanhonda.com

Use it to turn off circuits once you verify they are operating correctly and thus focus on the circuits that remain.  Also, consider the light bulb trick so that you don't go broke replacing fuses.

Once you get it all sorted out, consider getting one of these:

http://forums.sohc4.net/index.php?topic=53063.0

[OldSchool_IsCool]

Headlight bucket is another gremlin-rich-environment.  Check any cables that feed into the bucket looking for any chafing.  I protect mine with a little plastic wire loom available from any auto parts supplier.

[HondaMan]

A common place on these bikes where shorts develop is the (now hard and fractured) corner where it goes around the steering head on the way into the headlight. Now that the plastic covering is hard from years, it tends to fracture in sharp edges and cut through the wire's insulation, letting wires short together or to the frame. I have seen this MANY, MANY times. Turning the steering head a certain way often blows the fuse, as a good indicator.

If yours is in this condition, you can replace that sharp, shredded plastic sheath with far better materials in the proper sizes, from www.vintageconnections.com. Del there is a great guy to work with, and has kits for just one bike or a whole showroom full: I rebuild all the Hondaman Specials with his stuff.

[HondaMan]

BTW: it is not possible for shorted condensors or points to blow a fuse, if you have the right fuse installed: these ignition systems only draw a maximum of 5 amps at 13.8 volts, if both condensors and points were somehow a dead short...a shorted coil could theoretically make it happen, but this would require physical crushing damage to the coil that was more than 1" deep, as the primary windings are deep inside. Then, I suppose the problem would be pretty obvious.. 

[Leino]

BTW: it is not possible for shorted condensors or points to blow a fuse, if you have the right fuse installed: these ignition systems only draw a maximum of 5 amps at 13.8 volts, if both condensors and points were somehow a dead short...a shorted coil could theoretically make it happen, but this would require physical crushing damage to the coil that was more than 1" deep, as the primary windings are deep inside. Then, I suppose the problem would be pretty obvious.. 

I had a very practically shorted coil that blew the main fuse instantly.

[Leino]

I talked to my Dad about this last night, and he (EE degree who's owned about 20 bikes) said it wasn't likely (not impossible) to blow a fuse due to a coil circuit failure. It would drain the battery, but not kill a fuse. May I ask what exactly was shorted in your coil that caused the failure? Just curious...

I don't know.It was years ago.I replaced the coil and got back on the road.
It wasn't a a wire issue, that much I can tell.

Oh yeah and I've never had points in my CB, Newtronic when I bought it, Dyna 2000 now.

[Leino]

How do you like your electronic set up? I've been tossing around the idea of getting a Dyna for awhile now. I guess I'm waiting for a good excuse like a bad condenser or something.

Newtronic was nice, never gave me any trouble.
Don't know about the Dyna yet, my CB has been sitting for a while now...(Read: TOO LONG)
I've mostly been messing around with GS Suzuki's last 4 years, drag racing etc.