AJS & Matchless Owners Club

Hi all, the bike in question is a 1961 Matchless G3, having stripped, checked refurbished & rebuilt I decided to fit a fuse between the battery and frame earth, this is something that I have done on Velocette and AMC bikes in the past to prevent the shed catching . After 75 miles completed in several runs a slight miss fire occurred? panic set in and I returned home, upon inspection the 15amp blade fuse that I fitted was blown, with a replacement fitted the horn worked, but the fuse blew on rear brake activation, the insulation on the live side of the switch supply looked poor and this may have earthed out on the switch spring blowing the fuse. All the light bulbs (dipped beam, instrument illumination & rear light) had blown. Number1; am I correct in assuming that this would be because the blown fuse took the battery out of circuit and all the filaments received the extra power/voltage ??
Number 2; is a 15 amp fuse adequate for the load?? ( with out the complexity of the mathematical equation) Many thanks in advance & Kind regards, Ian Wilkinson.

I think that if you are charging at more than 15 amps your fuse will blow .
It is not just the load.
Also if you have a short to earth in your wiring the earth side fuse will not protect your bike .

Hi Alex, I understand that the fuse is fitted (by myself) to protect the wiring & prevent a insulation melt down by breaking the circuit, but I THINK the battery in the circuit acts as a buffer? a solid state regulator is fitted but I think it also needs the battery in circuit? Thank you for your swift reply, I am a retired Engineer with only a basic electrical knowledge, I'm sure an automotive Sparkie will correct me soon, kind regards, Ian.

I would check your rectifier/regulator. When I did the lecky leg conversion I put a resistor in line to control the current more effectively.

Thanks for your reply, I will put a 5amp fuse in and check every wire individually (after removing the tank & seat) and try to make it blow, Paranoia starting to set in kind regards, Ian.

I would start to suspect that the regulator may be letting through too much voltage, would you be able to list what your wiring is, i.e. 6v/12v, original wiring bringing in coils with headlight or modified for the regulator, the make of regulator rectifier, It may help to understand what is going on. If you have a multimeter try using the continuity function to look for any short as it may take a few fuses.

Thanks Duncan, but have found the fault the switched live to the brake light (brown wire) had been pinched by the nut that holds the top Girling shock absorber mount, this caused the fuse to blow, but I stand by my statement that the bulbs blew because the battery was out of circuit due to the blown fuse & I have found written information to back this up by F. Neill, if I knew how to attach images I wound need such long messages, kind regards, Ian.