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blowing main fuse

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david451:
my cb750k 1974 has blown the main fuse twice, blew today after the mot but has been fine until then it has only blown twice since I started restoring her. checking the wiring diagram it lists two items but was hoping there was a common fault or its just a case that they hate stop start repeatedly like during the mot, rode there fine and back.  Checking the wiring diagram and its the main fuse that supplies ignition lights and start function that I lost, tracking it back it goes from the fuse to the ignition switch and off in a different direction from the fuse also to the rectifier obviously for the charging. Charging ok though.

That's one down just to start the other one for restoration now, what a nice bike to ride. Nice sound and rides fairly well for its age and the brakes are fine, I thought they would be rubbish but nothing that you would be concerned with. Guys taking pictures at the mot station made me feel special. My other one came in a red colour 1976, fuel tap on left side but its a shed so its gona take the two years my last one took to get right. In 1976 it came in red and a nice blue do you guys think it would be wrong if I did it in the nice blue instead of the red, I always wanted a blue one in the 70s but like most of us had no money

Lobo:
Hi Dave,

Is your bike a K2.... how many fuses does it have? (mine has just the one)

Very first thing to confirm is that the main is a 15A fuse... it should be. If 'Honda' fuses are in short supply a common 13A household fuse is fine for troubleshooting.

So many possibilities... until a guru chimes in with "check the fuse holder for corrosion" etc I can only suggest this is what you indeed do... ie one by one of the electrical areas, looking for corrosion, bared wires, burned wires, loose connections and the like.

It is NOT normal / acceptable for a fuse to blow due stop/starting....

I'd start by putting in a fresh 15A fuse, and methodically working from the front of the bike aft with the ignition on. Check all bulb holders for corrosion, grab wires / loom parts / earths and wiggle 'em. Have the headlight off & shake the rats nest within. Put steering full lock each way, operate the front brake as you do so, indicators, horn, kill switch etc too. Take the tank off.... check for chafed wires, particularly as the loom goes towards the headstock. Check for PO DIY gaffs. Close check of all connections / corrosion within side panel electrical area - inc that fuse holder. Rear brake switch, indicator holders, (wiggle the blue / orange wires where they enter the stalks), rear lamp...at any point the fuse blows stop and investigate further.

Yea.... I know, all bloody obvious.

If you have an ammeter, put it in series (ie between) with the positive of your battery & the battery lead. (DO NOT CRANK the starter... you'll knacker your meter). Select it to the 10A range, with ignition off. If current is flowing, you've some kind of earth leak which might explain. If no flow apparent, select a lower range. There should be no current flowing at all... unless you have an alarm fitted.

Remove / replace (each?) of the fuses in turn to narrow things down to a circuit... if it cures the 'leak' concentrate on that area - disconnecting components until you stop the leak.

Gremlin probs are the worst to find...

Simon.

mike the bike:
Did the fuse blow violently (black or copper coloured) or did the fuse wire just part (fuse wire still visible but showing a break)
If it blew violently, it's more likely that there's a short to ground. 

david451:
Hi guys I have had this bike in a trillion bits everything cleaned and then put back together, I remember sticking in a house fuse way back when either it blew a fuse or it was blown and I couldn't get it to start but cant remember it was years ago.

I hate electrics two minutes or two days phew but clearly something when the bike was tested. I don't know if it blew during the test and he just stuck it out but when I went to it it was dead, but its a case of wiggle and observe I guess, off with the tank again headlamp out and check that birds nest as mentioned phew.

Anyway its mot ready.

Anybody know of somebody that will chrome plate my exhausts at a reasonable price quoted 200 each but need the baffles out and that's impossible.

Lobo:
Hi again,

If you've had the bike in a trillion bits - ie everything - inc the various components / switches etc - are now beautifully clean / buttoned up by the book - then I'd be thinking your 40yr old loom might have a wee internal issue?

Simon.

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