Topic: Main fuse keeps blowing

[golden-viper]

The main fuse on my 75 CB750F keeps blowing when I really get on the throttle. It only does it in gear on the road, not in idle, so I don't think it's a short in the switch housing like I originally thought. Any ideas? It's aggravating not to be able to pass anybody on the road! Thanks!!

[Bob Wessner]

Does it happen if you rev it while standing still also, or only when the bike is in motion?

[golden-viper]

Only when the bike is in motion.

[hawkeye]

Could it be a problem with the generator?

[eldar]

THis would be hard to say but I would check both the regulator for proper operating range and just to make sure it does not have some weird short, and check the rectifier.

After that, I would say that I think ignition systems require more power to rev up when the bike is under load than it does when there is no load.

Do you have stock coils of upgrades? Are you still using the points of do you have an electronic ignition?

Have you gone through your wiring harness to make sure all connections are good? You have to actually pull the connection apart to check this. Also clean your fuse contacts and make sure that your amp draw is not actually more than 15 amps. This can happen if you have hotter coils or electronic ignition.

[golden-viper]

Everything is stock on the bike. The connections should be good, they have actually been hard wired. I know that's pretty weird, but the connectors were so corroded the previous owner just eliminated them. It'll be a bear if I ever have to replace the harness!!

[eldar]

If everything is stock, then do some checking on the regu/rectifier. Otherwise, you could have a coil going bad but I have not seen what could happen there.

[Bob Wessner]

the previous owner just eliminated them

Doesn't sound stock any longer. Did the PO do it right??

[golden-viper]

All of the parts are stock, only the wiring has been altered. He was a electrician, so I am assuming it was done right. All of the wires were soldered and look good.

[eldar]

Well since the wiring was altered, there are a couple possibilities.

1: There may be a "cold" solder joint and this can cause poor conduction.
2: Insulation somewhere may be cracked or missing and it could be hitting the frame as you accel.
3: He may have wired in some kind of relay from the battery to the coils, this subject is somewhere on this site.

So really, it is a matter of tracing down the problem.  Have you pulled off the gas tank and checked the coils? Are those stock? Have you looked at the ignition to see if it is stock?

Gotta ask cause you just never know. What can look stock may not actually BE stock.

[jsaab2748]

If your wiring harness is still strapped to the "backbone" of the frame, check to see if it is getting chafed by that large bellcrank that the throttle cable is connected to. The harness passes right by
there, in very close proximity, on its way towards the back of the bike (least mine is that way). Pull the tank and open the throttle all the way thru it's entire operating range. Just an idea..good luck
jim

[TwoTired]

The main fuse on my 75 CB750F keeps blowing when I really get on the throttle. It only does it in gear on the road, not in idle, so I don't think it's a short in the switch housing like I originally thought. Any ideas? It's aggravating not to be able to pass anybody on the road! Thanks!!

Are you still using the stock fuse box?
Is the fuse hot to touch?  The physics of the fuse is to melt when when current is exceeded by heating the internal link.  However, it can be pretty hot when it is at say, 95% of rating.  Just before it melts it gets plastic and rather soft.  Sudden G loads might cause it to mechanically fail.  Another factor is fuse clip corrosion.  This is resistive and disipates energy as heat.  If all the connectors were badly corroded, the fuse clips probably had  oxidation, too.   After the bike has running for a while, you should still be able to put your thumb on the fuse and clips without discomfort.  Hotter than that, and you should probably abrasive clean the fuse contacts.  Secondary problems can be melting of the plastic around the clips and loss of tension in the spring clips.  Poor alignment and low contact pressure against the fuse ends can also cause heating.

Maybe not your problem, but, it is easy to test.
Cheers,

[Lumbee]

sup all...I have a 78 750F...took her out for a romp on Saturday, and she dies about 30 miles out.  I replace the main fuse, go about 100ft, and the same thing.  Out of fuses, so I call the wife/truck to come bail us out.  Once home, it seemed to ride OK if I removed the headlight and tail fuses.  Then I notice the wires on the clutch sensor are frayed and exposed, so I tape those up (decided I didn't need it as I have no sodering kit).  I took it a few miles down the road and it seems OK.  So my two questions are...

#1-Because it ran OK with the headlight and taillight fuse out...what exactly does that mean, that the short was in the head or taillight?  And if this is the case, why was the main fuse blowing, and not the head/taillight fuses?

#2-Do these symptoms mean the clutch sensor was not the culprit?

[TwoTired]

You may wish to look at this thread.
http://www.sohc4.us/forums/index.php?topic=2206.0

The fuse can fail for a couple of reasons.  Too much current or too much heat.  Too much current through the link generates enough heat the melt the link.  But, you can apply enough heat to the fuse to melt the link with NO current passing through it.  Obviously a combination of heat sources can make it melt well below it's rated current capacity.
Clean/ polish all the oxidation off the fuse clips and check to make sure there is good contact pressure and alignment between fuse and clip.  Oxidation is resistive, and passing current through a resistive element generates heat.  Both the other fuses draw current through the main fuse and any oxidation between fuse and clip.  If you cannot hold your thumb on the fuse while it is passing current within its capacity, you have a clip heating problem.

Cheers,

[oldbiker]

TwoTired hit it right on the head.

[Lumbee]

...none of fuses was hot to the touch after blowing?  As I said I found exposed wiring in the clutch sensor, and I hope that was my problem.  I'll put some miles on it over the next week and see how it performs.

[eldar]

Well if the wiring was frayed, it may have caused a short.

Do as tired said. Clean all fuse contacts.  Go over your harness and clean or replace any bad or corroded connectors and replace any frayed wires.

[Killer Canary]

Doesn't hurt to check if any of the light housings are full of water.

[cub]

I had similar problem earlier and thought I found it. Found out about 6 km from home I was wrong... Check where the lamps for neutral, oil lamp etc sits. On my bike there was corrosion in the lamp holders there that some times caused a short..but only sometimes and always a long way from home.

[Lumbee]

...just thought I'd follow up with this post.  I was out last night tooling around and found where the short was.  The last time I pulled the engine I neglected to put the front drive sprocet cover back on...I know...I'm a lazy bastard!    Any way it seems without the cover on that nice bundle of wires that comes from the engine was rubbing against the chain/sprocket for the last couple of months.  It wore through the black sheath on the outside and about 90% through one of the wires in the bundle.  Moral of the story?  The front sproket cover does more that make the motor look nice and neat.  It also keeps those wires away from the sproket and chain!!!

[Bodi]

is it worth it?
Depends, as usual, on what it costs and how much you want it.

is it hard to do?
I've heard of replacement fuseboxes that plug right in and mount with the same screws. That would be easy, making some other generic fusebox work will be harder of course.

is there any particular type of blade fusebox i should look for?
The stock one has worked apparently well for 30 years. Why do you think it can be improved upon?
Blade fuses are a bit smaller. The main reason they've been adopted is that the "live" terminal is not exposed as with the 30mm straight fuses we have in our bikes. That makes it harder to short out the system and cause trouble - hopefully this isn't a big risk in your covered fuse box under the side cover.
So unless your fusebox is ruined, just remove it (unscrew and unplug) then clean the fuse holder ends and bend them a bit tighter with pliers. Open the back and make sure the wire connections look good. If a fuseholder end is badly overheated (the metal loses its springiness if it gets super hot), you can rewire the box to use one of the "spare" holders as a working fuse or even move the "spare" metal bits to replace the burned one.

[Bodi]

Fine steel wool or a scotchbrite pad are good. You want to get a shiny copper colour showing. Clean the fuse end metal as well, it's probably plated steel so just get the dirt off - not the plating or the end will rust pretty quickly. Dabbing a bit of dielectric grease on the terminal and fuse will help prevent future corrosion.
Clean the fusebox connector terminals as well if you can. I don't think there is power there with the ignition switch OFF but maybe disconnect the battery to avoid surprises.
Occasionally old fuses go weird as wel, and heat up even though not overloaded. Getting a real brandname (like BUSS) fuse rather than some far-east version is a good idea.

[HackNslashCustoms]

Thanks for asking this question. I'm having the same problem. I tried cleaning the "clips" and the top ones broke right off. So I'm forced to either find a used one that will probably do the same thing eventually, or a replacement.

If anyone has any links, or brands that would help us, Post please!

[ofreen]

I wired in a spade type fuse holder to replace the main buss fuse because of the same problem.  The bike would blow main fuses about twice a year from the beginning.  (I got the bike in 1977.)  About 7 or 8 years ago, the bike blew 3 fuses in the space of a week.  The last time, the end of the fuse got so hot that it melted a hole in the plastic fuse cover.  I figured it was time to quit screwing around and see what was wrong.  I found that there were only two strands of wire soldered to the connector on the back of the fuse panel.  It had come that way from the factory.  It was amazing it had worked as well as it did all those years.  It took about 3 minutes to find the problem once I was motivated enough to look.  Oh well.

There's been no problem since installing the spade type holder.

Greg
'75 CB750F

[Caferacernoc]

Well, the way my bike was wired, by previous owners, it's my only fuse. Anyway, here's the problem. I can ride around for about 20 minutes and then the fuse blows. After the fuse blows, if I put a new one in it will blow as soon as I restart the engine and try to take off. If I let it sit for a few hours it give me the 20 minutes of ride time before it blows. In other words, it works when the engine is off. But after the bike is warmed up it will blow the fuse almost immediately after starting the engine. especially if I give it some revs. When the problem first happenned I gave the wiring a once over and found some dodgy wires coming off the ignition switch, so I replaced them and thought it was fixed, but now I discover it just has this time limit. The bike runs fine until the problem occurs and turning lights on and off and whatnot seems to have no effect. I've looked at every wire, jiggled them. Nothing. It runs for 20 minutes then it's no good until it "cools off". What in the wiring system would be so heat sensitive? Is the regulator or rectifier acting up? Any help is appreciated. Thanks.