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Popping 15amp fuses

DocRootBeer

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Twice over the past week, I've experienced a loss of power after riding for about 20 minutes. Then, I notice I've blown the main 15a fuse. Any ideas? My hunch is an issue with the regulator/rectifier which leads to blowing the fuse, then the battery loses charge over time, ultimately impacting ignition. Thanks in advance! (Bike is a 1975 CB750 F0).
 

gunr

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Sounds like a short.
When you say a loss of power do you mean shuts off or just lags?
My nephew kept popping fuses and he put in a heavier fuse...burned the wire insulation from the tail lights to the headlight bucket. Had a short in the tail light wire .
Fuses are cheap!
 

DocRootBeer

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I think you're right. I suspect it's a short too. It doesn't just die, it sputters and then doesn't produce any power. I suspect the battery runs low after the fuse blows. I have plenty of fuses on hand. Trouble is, they pop about every other ride. There's a lot of old, crusty wiring. I'm sure there's a short due to some worn insulation somewhere.
 

madmtnmotors

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"There's a lot of old, crusty wiring"

Mine used to do that. The stator connectors often suffer the most due to the high current through them, especially the bullet connectors for the three yellow wires under the front sprocket cover. I usually replace these three bullet connectors, both M and F, as long as you use the proper connectors and crimp tools:


They can be done without removing the shift cover or stator cover if you take your time and work carefully. Don't go cutting an inch and a half off the end of the yellow wires and expect them to still be long enough. I start by carefully trimming the outer protector from these bullet connectors and then cut the connector off right at the crimp. When complete I only lose about an 1/8" in length on each wire. Most other connectors can be cleaned up:


EZPZ :cool:
 

barney905

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Twice over the past week, I've experienced a loss of power after riding for about 20 minutes. Then, I notice I've blown the main 15a fuse. Any ideas? My hunch is an issue with the regulator/rectifier which leads to blowing the fuse, then the battery loses charge over time, ultimately impacting ignition. Thanks in advance! (Bike is a 1975 CB750 F0).

Twice over the past week, I've experienced a loss of power after riding for about 20 minutes. Then, I notice I've blown the main 15a fuse. Any ideas? My hunch is an issue with the regulator/rectifier which leads to blowing the fuse, then the battery loses charge over time, ultimately impacting ignition. Thanks in advance! (Bike is a 1975 CB750 F0).
Scrolling through some of these older posts this was as close to my issue as I could find. I too have a 1975k5 with 96000 miles on it. It's never let me down and has been very reliable until a few days ago. I too kept popping 15amp main fuses. The bike crapped out twice in the middle of traffic. I determined that when I applied the front brake, I would blow the fuse. Thinking it was a direct short from a bare wire I checked everything in the circuit. It ended up being the pressure switch in the front brake hydraulic line. The little round booger with 2 pins next to the steering head. Ebay and $16 and we're back in business. After only 48 years and only 96k miles you would think I should have a more dependable product (snicker snicker).
 

madmtnmotors

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After only 48 years and only 96k miles you would think I should have a more dependable product (snicker snicker).

I know, right? ;)

Good work tracing down the culprit. Electrical issues can be maddening. I just recently tracked down the issue with the reverse lights on my Suburban. Turns out the trailer wiring adapter that was made specifically for my vehicle had the reverse lights wired straight to ground.

Phhbbbtttt....
 

bossboy302

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I struggled with what I thought was a short on my K5, after hours with a multi-meter, jumper wires, etc it ended up the battery was bad internally.
Yep, electrical stuff can be a bitch.
Not to derail the thread, but where are you getting the 5x25mm 15A fuses? Seem very hard to find...
 

barney905

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I struggled with what I thought was a short on my K5, after hours with a multi-meter, jumper wires, etc it ended up the battery was bad internally.
Yep, electrical stuff can be a bitch.
Not to derail the thread, but where are you getting the 5x25mm 15A fuses? Seem very hard to find...
I gave up on the 25mm 15amp glass fuses and soldered in a 4 slot spade fuse holder, the smallest I could find, I bought off of Amazon for $10. 3 spots for the bike and one blank. The best I could do finding the glass ones was auto zone and that was a small mixed package for $15. I couldn't mount the conversion back where the old fuse holder was so I packed some foam rubber around it and positioned it above the rectifier. Nice and snug. Works excellent and no worries about locating fuses.
 

brettp

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I thought about doing the same; relocating the fuses with a blade style distro block. But I haven't committed yet. I get the glass fuses from autozone, still.
 

barney905

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It's a easy conversion if you're into soldering. I used shrink tubing to isolate the connections. I don't like using crimp fittings. I would love to keep things original but I also want reliability. After 48 years and 96000 miles I've been doing a few upgrades with the electrical stuff.
 

brettp

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I do solder. I soldered a voltmeter to my DOHC's fuse box. I can see the charging voltage as I ride. I just haven't wanted to spend the time to stretch all the fused circuits from the seat back to the headlight. At least, not yet.
 

Jeffry Gagnon

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Scrolling through some of these older posts this was as close to my issue as I could find. I too have a 1975k5 with 96000 miles on it. It's never let me down and has been very reliable until a few days ago. I too kept popping 15amp main fuses. The bike crapped out twice in the middle of traffic. I determined that when I applied the front brake, I would blow the fuse. Thinking it was a direct short from a bare wire I checked everything in the circuit. It ended up being the pressure switch in the front brake hydraulic line. The little round booger with 2 pins next to the steering head. Ebay and $16 and we're back in business. After only 48 years and only 96k miles you would think I should have a more dependable product (snicker snicker).
I feel better now, barney. Thanks. Just hit 84k mi. on my '91 CB750 Nighthawk ("Tokyo Rose"). Still runnin' like Lady Rolex. Was beginning to wonder if timing and/or alternator chains were nearing the end of expected lifespans. Nice to hear that you're still breakin' in a CB750 @ 96k.
 

barney905

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These rides will last forever if you take care of them. Watch the electrical stuff though. Over time stuff starts to degrade as the above posts were saying. I've upgraded the ignition, coils, some wiring, switches, and now my fuse holder. Little things like the small 1 inch (25mm) fuses not being readily available anymore can get frustrating. My old k5 isn't my primary ride anymore but it's still a member of my family. I ride it on sunny days around the mountains here. Still a fun bike to ride.
 

Jeffry Gagnon

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Your blue (K5?) is beautiful, b905. From the pic and your mileage, I think we do about the same type of riding: mostly long distance highway. My Nighthawk evolved quickly into a long-distance tourer. Bought it used (Craigslist) and got a great one. Bought in 2011. Verified sitting in a shed in Queens, NY since its original owner quit riding in 1995. Only 19k mi. on the clock. Everything rubber was dry-rot. A bit of pitting on the headers. But everything else (except a split seat cover) checked out fine.

For me w/the 750, first came a National Cycle Plexi-fairing and some warm, wind/waterproof riding gear. Then a 16"x14x18" deep backrest-trunk on the pillion. Then an extension to the rear luggage rack to support more backpack-type riding and camping gear. (Fortunately for me, the orig. owner had purchased the only 2 options - the center-stand and the sissy bar / passenger backrest, which included the rear luggage rack.

Like countless other riders, first ride of any kind was a used 50cc Honda Cub, a 1963 which we called "Honda 50s" in the '60s - my first "escape" as a teen. Taught me that cars never see you on a bike. Then on to a '63 CB77 SuperHawk. I was in heaven. Based in CT, went from Provincetown to Montreal to D.C. to central Ohio, and to all points in between.
No cell phones then - parents would freak out not knowing where I was ;o)) As with the Cub, never a problem I couldn't remedy on the roadside.

Honda riders speak the truth when they say, "I'll see you there."
 

barney905

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Well, I have to eat a chunk of crow pie now. I thought I had it figured out why I kept blowing my main 15amp fuse every time I applied the front brake. I replaced the front hydraulic brake switch and thought that was the problem. Oh no. I've been replacing just about every electrical component on the bike from new coils, ignition switch, rectifier and voltage regulator along with a Dynatek ignition setup. The bike runs like a scalded ape. What I've narrowed things down to is the fuse blows when I'm coming to a stop in first gear and apply the front brake. If I come to a stop in neutral I don't blow the fuse. Most of you riders are smarter than me so before I scratch a bald spot in the top of my head does anyone have some ideas? This is a k5 with lots of miles. I'm running 10 lights. 5 in the front with the Windjammer fairing and headlight, 5 in the back with the tourpack and taillight. Good battery and good charging system. Only the front brake application does this and only in first gear coming to a stop.
 

brettp

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What happens when you are stopped in neutral then shift into first gear?
Have you cleaned or replaced your clutch switch?
Lastly, how does the bike behave when the fuse blows? Does it jerk? Does the bike simply cut out and die?
 
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barney905

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What happens when you are stopped in neutral then shift into first gear?
Have you cleaned or replaced your clutch switch?
Lastly, how does the bike behave when the fuse blows? Does it jerk? Does the bike simply cut out and die?
Nothing happens when I put it in gear to get going again. Everything functions as it should. When the main fuse blows everything stops of course. Only when coming to a stop in gear and applying the front brake. Rear brake application doesn't pop the fuse. There's no jerking or coughing. It runs fine. Clutch switch, that's interesting. I'll check that out. When coming to a stop in gear while applying the rear brake only it doesn't blow the fuse so I'm focused on the front brake switch circuit. It appears that the black power wire involves the ignition switch so I replaced it to be sure. Nope, that wasn't it. Today was especially frustrating in that my original side panel broke apart from taking it off and on too many times. Pretty brittle after all these years. More stuff to replace.
 
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