Author Topic: Keep blowing main fuse, any ideas? (Read 18842 times)

NL8485 · « **on:** October 11, 2010, 07:45:11 pm »

Hi there,
I'm new to the forums and was hoping someone here might be able to help me with a problem. I have a 1978 CB750F and my main fuse keeps blowing. It blows whether I kickstart it or use the electric starter. Could it be a bad coil? Or maybe condenser? Has anyone else had this problem? I'm at a bit of a loss as to where I should start. Any advice would be very much appreciated.
Thanks so much,
Nick

voch · « **Reply #1 on:** October 11, 2010, 08:01:00 pm »

fuses blow due to shorts... check the wiring.

Spanner 1 · « **Reply #2 on:** October 11, 2010, 08:25:21 pm »

Is the fuse hot when it blows?... have you touched it immediately after it blows?... usual cause is oxidised fuse clips, they may ' look ' fine but will not pass the required current to work the bike and heat-up to where the fuse melts ( good ). Cure would be to shine the inside of the fuse clips on all 3 fuses and carefully bend 'em in to where they are getting a firm grip on each fuse, also look at the back of the fuse holder for bad connection... good luck.

kevins750 · « **Reply #3 on:** October 11, 2010, 08:30:55 pm »

unplug all components that correspond with that fuse, then plug them back in one at a time until it blows----there's your problem

TwoTired · « **Reply #4 on:** October 11, 2010, 09:22:29 pm »

Does it "blow" in the middle or melt near the end caps?

y2kc0wb0y · « **Reply #5 on:** October 11, 2010, 10:56:34 pm »

Just had this same issue and it was the fuse clip. Strangely enough the fuse only blew when we turned the lights on so that threw us a curve for a few hours. Sanded out the clip, no more issues.

bollingball · « **Reply #6 on:** October 12, 2010, 12:17:23 am »

Don't forget to download the manual it has a good electrical print. Listen to TT and Spanner. TT has a brain that works as does Spanner

Does it blow after you turn key on but before you start or after you start? If after how long does it run before it blows? The start circuit is short and simply run circuit is a little longer but not to difficult to figure out.Study the print and get back to us.You can do it,you can do it

NL8485 · « **Reply #7 on:** December 04, 2011, 08:57:04 pm »

First off, thanks for all of you guys' help. I realize it's been a long time, but I just thought I'd post this in case someone later on is having the same problem.

Here's what happened; after taking all your advice, and blowing many glass fuses, and spending hours checking the wiring harness, I got frustrated and shoved this bike to the back of the garage and moved on. It wasn't till a few weeks ago that I pushed her outside and started working on her again that I finally solved the issue. I started looking at the regulator, took it off the bike, and opened it up. After playing with some of the gaps inside, I put it back on the bike. The bike then ended up blowing the main fuse every time the key was switched on. Bingo. Replaced the regulator with a new solid state one, and no more fuse issues so far!

Now I'm onto the long road of getting it to run properly!

grcamna2 · « **Reply #8 on:** December 04, 2011, 09:16:46 pm »

Nice fix! NL,
That year away from it gave you some new gumption to have a REAL big go at it again...it sure must have been nice to have that ignition on for a while.

Spanner 1 · « **Reply #9 on:** December 05, 2011, 09:29:36 am »

Hope your bike doesn't end-up back in the garage for another year 'cos you didn't address the fuseholder issue.... IMO

.... and does the bike charge the battery when running ?

CoachDoc · « **Reply #10 on:** December 05, 2011, 11:40:37 am »

Here's a a tip from Hondaman re. cleaning the fuse clips. Use a .22 cal rifle bore cleaning brush. It fits the dia. of the fuse clips perfectly, and will shine it up quickly. I have .17 caliber brush set that you can use both on the fuse holders and female bullet connectors.

NL8485 · « **Reply #11 on:** December 23, 2011, 03:16:20 pm »

Thanks, spanner 1, I should have said this in my last post. I did check, clean and sand the fuse clips a few times while trying to figure out what was going on with it. I also just yesterday went out and tried the .22 cal brush tip just in case (thanks CoachDoc), and also adjusted the clip prongs.

As for the battery charging, there are a few things going on with it. First, the bike doesn't run very well, so I don't think it's getting over the right RPM to keep it charged. Also, I might have fried the battery using a car battery charger on it so it could be shot at this point.

Currently I'm trying to figure out why it is popping through the carbs (I know it means lean, I keep upping the jets with no luck at 122 right now) and won't rev up/dies before 2000 RPM. Getting the feeling she just wants to sit in a garage and never go out.

grcamna2 · « **Reply #12 on:** December 23, 2011, 04:54:09 pm »

Are your spark plugs fouled out ?

racemx51 · « **Reply #13 on:** December 23, 2011, 05:03:18 pm »

I had this happen and it was all because the indicator lights were touching together.. Check there too..

TwoTired · « **Reply #14 on:** December 23, 2011, 05:18:00 pm »

Quote from: NL8485 on December 23, 2011, 03:16:20 pm

Currently I'm trying to figure out why it is popping through the carbs (I know it means lean, I keep upping the jets with no luck at 122 right now) and won't rev up/dies before 2000 RPM. Getting the feeling she just wants to sit in a garage and never go out.

Popping back through carbs is early ignition timing, intake valves adjusted too tight, or carbs not vacuum sync'd. Can be combinations, too.

Does the popping occur with throttle advancement or just while idling?

rb550four · « **Reply #15 on:** January 01, 2012, 08:50:21 pm »

I have had a few 550's with wiring trouble that the last owners couldn't get right.I found the trouble in 3 machines where on the backside of the fuse holder the wires are bent severely to be tucked into the plastic case, time, shrinkage, and PO s tugging on them left bare wires that made the electrical system operating inconsistently blown fuses regularly. Don't know if this info helps...but it can't hurt. Good Luck

Spanner 1 · « **Reply #16 on:** January 02, 2012, 07:16:35 am »

No quick fix to get your bike running, OP !....... a bike that has sat for a year ( and thru' a CO climate ! ) will have to have a 3,000 mile service performed on it along with a carb take-off , dissasembly and clean.... hope it's not back in the garage for another year

... Old bikes are not for everyone and a 750F is a great bike.... thought about selling it to someone who will fix it and ride it ?... just sayin'

klinger44 · « **Reply #17 on:** March 10, 2013, 04:49:05 pm »

My cb550 keeps blowing the main 15 amp fuse, use to be like once a month now 5 fusses today alone. I have checked all wiring and had an amp clamp on the main fuse and nothing, it only blows when i come up to a light and almost at a complete stop then it blows. I have heard the fuse clamp theory before and that will blow a fuse but the fuse would be just as hot as if it were not the clamps. If the fuse was lets say running at 14 amps for an hr then hot 15 and blew it would be really hot. My fusses are blowing and or melting. Any ideas other than the fuse clamps. Note that my charging system works sometimes and other not. I can always kick the bike to start then ride it for and hr an it will electric start but i leave the bike for an hr or 2 the electric wont start it. Its a brand new battery. Also only the main fuse blows none of the rest.

this sounds like it could be me. "I have had a few 550's with wiring trouble that the last owners couldn't get right.I found the trouble in 3 machines where on the backside of the fuse holder the wires are bent severely to be tucked into the plastic case, time, shrinkage, and PO s tugging on them left bare wires that made the electrical system operating inconsistently blown fuses regularly. Don't know if this info helps...but it can't hurt. Good Luck"

Proto · « **Reply #18 on:** March 13, 2013, 09:50:29 am »

Had a similar problem with my 76 CB750F1. It had been in storage for many years by the PO. After buying it from him and cleaning carbs, I got it to run but started immediately having main fuse issues. Could not find a short and fuse box connections were good. Sometimes it blew immediately when key was turned, sometimes I would make it a block or two. At times I could do a whole ride with no problems. I traced wires and checked connections, all looked good to me.

One day I turned the key and had no power to the starter button. Battery was good, all connections were tight. I unplugged the harness from the ignition and figured out what two wires to jump and got power to starter button and she fired right up. I rode the bike off and on all day without blowing a fuse. I picked up a cheap ignition switch at the auto parts store, wired it in and temporarily taped the switch to the handle bar and it worked perfectly for 10,000 miles (so much for temporary!) at which point I replaced the switch with an original style one. It continues to work fine.

I am assuming my original switch had a short in it causing my problems. The key was pretty worn too.

Brookesy in Oz · « **Reply #19 on:** April 27, 2013, 06:18:27 pm »

I possibly have something useful to add

Just trying to get my 750K1 going and keep blowing fuses - very limited number of wires connected so far & after reading these posts...
I have a new wiring harness & some of the sub-harnesses are new. The starter, starter solenoid, right hand switch block & regulator were the only things connected.
Disconnected the regulator & starter wires. Switch on. Fuse OK, Switch off.
Connected the starter. Switch on, Fuse OK, Switch off.
Connected the regulator. Switch on, Bingo!

No more fuses. I'll have to get some. I want to get her running today.

I'll check Hondaman's book to see if the regulator can be fixed. maybe I should just get a new one...

Thanks for posting fellas.

nikcorg · « **Reply #20 on:** April 28, 2013, 07:17:06 am »

I had the same problem on my -77 CB750K. (Main fuse kept popping.) It turned out to be corroded contacts at the ignition lock base. Also, the main fuse box had really dirty contacts, which most likely didn't help. When it eventually died on me on the road, the fuse box was melted around the 15A fuse clamps. (Before this, I really thought it was only the dirty contacts in the fuse box. Cleaning them up wasn't enough.)

I replaced the ignition lock and haven't blown a fuse since (on the bike, some of the ones in my head have popped occasionally.)

Billy Bones · « **Reply #21 on:** July 02, 2013, 11:15:44 pm »

76 CB550 keeps blowing the main fuse. I'm a hack with the wiring. With the ignition switch off, and a test light connected so that one end is on the main fuse and one end is on the negative terminal is my light supposed to light up? Currently it does. IE, should there be power to my main fuse even when the ignition is turned off? I read somewhere that it shouldn't have power with the switch off and that I should just keep disconnecting things until the light goes off. Is that the case?

PeWe · « **Reply #22 on:** July 03, 2013, 03:10:15 am »

Short to ground?

Follow all wires and measure if they have direct contact to ground. Measure with a multimeter (Volt-Ohm meter) to see the difference of 0 Ohm (short) or something connected.

It was very popular here to route the cables from the electrical switches on the handle bar outside the handle bar when changing to another one, not thru holes as OEM. This was done by grinding an opening on the left/right switch housing and cables will be squeezed between handlebar and housing. Begging for blown the main fuse. OK for +30 years ago, today sabotage

Kenzo1979 · « **Reply #23 on:** July 03, 2013, 05:23:19 am »

In regards to the regulator, it's possible that it just needs to be adjusted properly. Out of curiosity, I opened the regulator on my 500 and tweeked the one adjuster screw a bit per the manual. Put it back on and blew the fuse. I only turned the screw about 1/4 turn, turns out this is a lot. The adjustment seems to be very sensitive inside, so I backed it off an 1/8 turn and got the best charge without blowing the fuse. Could be the stock regulator just needs to be readjusted, or maybe it IS faulty. But the adjustments seem to be Pu$$yhair sensitive, so look out.

Billy Bones · « **Reply #24 on:** July 03, 2013, 07:22:33 am »

I had a 59 dodge where I played with adjustments on the regulator for weeks and never got it right, but I still think it could have been done. I build guitar pickups and the coils on those use really fine wire. Those coils seemed like a snap with that big chunky wire.

How about the question of should there be power to the main fuse with the ignition off?

News:

Author Topic: Keep blowing main fuse, any ideas? (Read 18842 times)

bollingball