15 amp main fuse on 74 CB750 blows

[m. ishmael]

The bike starts and will idle but when put into gear and release of the clutch and gas applied the 15 amp fuse blows. While in neutral, the engine can be rev'ed with no problems. Lights work etc.

[TOOLS1]

Welcome to the forum. The only thing I can think of. Would be that the clutch cable is moving, and pushing against a wire. Causing it to short out. Check your wires, for bad insulation.
TOOLS

[Piute]

hello n welcomelets check the basics and get them out of mind;Are you sure the fuse your replacing is blown?, or could it be a bad fuseholder ( common ) ?? Was the fuse visibly broken or blackened ?
Fuse holder clips on these old bikes are often weak and tarnished to where they will not pass the current demanded by the bike for long and heat-up and fail....have a close look at the fuse holder, front and back, and brighten the fuse clips nice and shiny and , carefully, squeeze the clips closer together to grip the fuse better...try that and report back, por favor.

[Piute]

hello n welcomelets check the basics and get them out of mind;Are you sure the fuse your replacing is blown?, or could it be a bad fuseholder ( common ) ?? Was the fuse visibly broken or blackened ?
Fuse holder clips on these old bikes are often weak and tarnished to where they will not pass the current demanded by the bike for long and heat-up and fail....have a close look at the fuse holder, front and back, and brighten the fuse clips nice and shiny and , carefully, squeeze the clips closer together to grip the fuse better..

ON SOME... sensor under the shift lever would be the Neutral switch and is only a ground wire..i.e. if it was loose or touched the case of the motor could not cause the fuse to blow....

[shinyribs]

Hmm...have you checked your throttle fuse??  Seriously though,that is mind boggling.I was gonna say clutch switch,but after reading Piute's post,maybe not.What all is on the circuit that the fuse is on?

[shinyribs]

Does it only do it when applying throttle AND releasing clutch lever?Will the clutch lever alone blow it?

[TOOLS1]

Only if it is an electric clutch. However it does bring up a question. Does it have the switch on the clutch handle that only lets it start if the clutch is dis engaged?
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[m. ishmael]

Thanks to all who have sent in an idea.

Here is a bit more info. The problem only seems to occur after engaging the clutch and applying some gas. I have gone as far as 10 yards to 20 belore the engine goes dead. The first time it happened was after a turn out of the driveway so after replacing the fuse and starting the bike i truned the wheel back and forth thinking a crack in the old wires might be causing a short. (The ignition switch is on the handlebar.) The theory didn't prove out. The next time I didn't make it to the end of the drive....no turns.

I have tested using the center stand and engaging 1st gear and letting out the clutch and letting the rear wheel turn with no problems.

I will check the contacts around the fuse area and clean etc. Is there something like a magnito that might short out under loadd?

[TOOLS1]

Are, you turning on a turn signal as, you leave the drive?
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[Piute]

  Rather then retype info I find ,I just copy , sorry about the double(my bad)

  Read and went over lots of different sights and post before going through my wireing and others fixes AND test come to mind(help to have someone that can use a tach n dwell)whole lot faster(thanks Paul ie)
                             This I also found useful..Good-luck,,,>>>Piute<<<N>>>Judose<<<
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Fuses blow when they melt from heat generation.

The fuse material is selected so they internally heat enough to melt the material when passing above rated currents.   They still heat a bit while below their rating too, just not enough to melt the fuse link.

The fuse will also melt if you simply heat the fuse material, say, in an oven.

If the fuse clips heat, that heat is then applied to the fuse material from the ends.  That, coupled with the the fuse's normal heating at say, 2/3rds of it rating, is enough to melt the fuse material near the end caps.
Therefore, if a fuse is "blowing" near the end caps, it is a fuse clip/holder malfunction rather than a circuit over-current event.

[shinyribs]

Is there a possibility that there is a wire getting pinched/shorted inside the sprocket cover?

[brianmac]

frequent cb750 issues
1. regulator overcharging due to your friend the nasa electrical engineer adjusting to max output.
2. bad ground to a turn signal, or other circuit, make sure frame and engine have a wired ground from battery.  and there lots of wires in the headlight that can get grounded out too
3. corroded brass connectors,
4. worn insulation grounding out. disconnect, reconnect systematically until culprit found.
5. fuses can be purchased cheap from cb750supply.com(the sshort 15 a ones needed).
6. i noticed that complete wiring harnesses are available for like 75$ on some sites for some year cb750's

[LukeM]

Assuming your bike has a factory harness, and is routed like it came from Honda, there may be a routing guide in the Factory Service Manual.

If it doesn't do it when on the center stand, but does when both wheels are on the ground, my first guess would be the engine shifting in the frame.  If part of the harness is inbetween the engine and the frame, it's possible something's getting pinched.

Alternatively, the frame may be cracked or broken somewhere (maybe a weld or something) and after a while it abraded the harness.

Electrical problems can be tough to troubleshoot.  I'd recommend getting a DC ammeter, and putting it in line with the fuse that keeps blowing.  See what it reads with the engine off, engine on, engine on and in first gear, and when you take off.  Please be careful trying to read the meter when riding.

One more thing:  there's nothing better than a good visual inspection.  Pull the gas tank and seat and anything else that's covering up your wiring harness.  Look for signs of burning or sparking.  If it's been doing it a while, you should be able to see it even through the wrap they use to tie the harness together.

Good luck, and I hope this helps.