If you read the Britbike internet forums, sadly, this is a regular occurrence. However, it wouldn't have happened on your bike if it had the original wiring harness or a well-made replacement; sadly it has a crap replacement (Wassell "Genuine Lucas"?
Risking telling you something you understand already, the standard "positive ground" of your bike's electrics simply means the metal parts of the bike's structure - frame, engine, bodywork, etc. - are connected to battery positive.
The snag with then putting just one single fuse in the wire attached to battery negative
is it doesn't protect against all possible short-circuits, including the ones your bike has just experienced/is experiencing.
The shorts on your bike are almost-certainly something metal touching both the battery negative terminal
and an aforementioned metal part of the bike's structure, a/the fuse in the wire attached to battery negative
won't blow, because the shorts aren't through a/the fuse there.
thing that protects against a short from the battery negative terminal itself on any 'positive ground' bike is a fuse in only a single wire actually attached to the battery positive terminal
. This also protects against all the possible short-circuits that a fuse in the wire attached to battery negative protects against.
Your bike's original wiring harness, or a well-made replacement, would've had the single fuse in a single wire attached to battery positive.
Other things to check on your bike now:-
. What is/was touching the negative terminal of the new battery, as your post starts off "Fitted a new battery" ...
.. If you're using standard automotive blade fuses, neither the new fuse in the one single wire now attached to battery positive nor the fuse in the wire attached to battery negative exceeds 20A, better is 15A.
.. Otoh, if you're using tubular glass fuses with metal end caps, check the rating very carefully - 'British' should be 35A (unlike all the others, it's a 'blow' rating, not a 'continuous' rating
), US/Japanese again shouldn't exceed 20A, better is 15A.
A simple mod. you might want to consider now - '68-on, the battery sat on a rubber mat (82-8091) in the bottom of the carrier; buy one and fit it over the top of the battery, held in place by the standard "Retainers".
A second mod. for the future - an educated guess says you've been sold the 'standard'
9Ah (Amp-hours) replacement battery 'recommended' for most Britbikes with 12V electrics. Next time you buy a replacement, consider buying the similar 7Ah one - has the same length and width but is a little shorter. Btw, (cold) "cranking amps" (ccA) has absolutely nothing to do with anything on a kickstart-only bike.
measured the (tri spark) regulator and it seemed to be faulty.
Depends how it's connected to the harness but unlikely to be anything to do with the original problem; however, could be buggered now. Post how the new regulator is connected.
now she just doesn't run right.
Points or electronic ignition? If electronic, make/model?
'Fraid the new battery could be buggered - you've shorted it twice, shorts draw a huge current from a battery, the battery is relatively small and easily knackered by heavy current draw it was never designed to supply.
Measure across the terminals with a Voltmeter, if the meter shows less than ~12.5V, put it on an overnight trickle-charge.
After an hour or so off-charge, measure across the terminals again; if it still shows less than ~12.5V, it's buggered (consider my advice for a smaller replacement battery?).
Otoh, if measuring across the terminals shows the desirable ~12.5V, connect it to the bike's electrics, connect the meter across the terminals, watching the meter:-
. first turn on just the ignition, meter might just a little lower but it shouldn't be much (if the meter indicates 12V or lower, battery's buggered);
. if the meter is still indicating ~12.5V, turn on the lights including the headlamp; again(if the meter indicates 12V or lower, battery's buggered.
The above is just a quick-'n'-dirty load test, only definitive if the battery fails; if it 'passes' unfortunately it doesn't necessarily mean the battery is 'good', still have it properly load-tested by an auto-electrician with the kit.
The other area to examine carefully is the wiring harness itself; regrettably, other Red 'ground' wires within the harness might've been damaged by the shorts.
Hth; post how you get on?