The final
conclusion was that the alternator wasn't putting out enough juice. A fully charged, big battery would see me ok for a longish
run out, with lights on (and later off as the battery started to complain).
When that battery finally completely gave up the ghost I decided I really had
to find out what the problem was and correct it.
I hadn't
had the bike all that long, or at least hadn't yet used it all that much
though I enjoyed riding it enormously; it's a bike with bottle, revvy and
agile. ....but, there was the electrical
problem that needed fixing: Getting to grips with the electrical setup was the first challenge, what is
where, what components are in there and so forth. I worked out where the
alternator is .. not as obvious it sounds (to me anyway) .. with the help of
exploded diagrams on the Internet. The exploded diagram that I originally had
for the bike, a Mototrans 250 narrowcase, didn't actually include the
alternator in the diagram. The bike has Motoplat electronic ignition, seems to
work very well when there's enough power for a spark and
rectification/regulation has been achieved by a rectifier connected to a zener
diode all bolted to a hashed up metal plate under the seat. A little unorthodox
perhaps but could work fine. All electrics are 12V.
I ran a couple of tests:
- rectifier
diodes seemed ok, +ve and -ve faithful to their definition.
- turning
over the engine seemed to generate a little bit of ac voltage life out of the
yellow alternator leads. Would that be enough? wasn't sure yet
I figured
that I wasn't sure if the zener would regulate properly and that possibly
overcharging had 'baked' the old battery. As I had a spare, modern solid-state
regulator/rectifier unit I thought I'd plumb it in to see how it went. I used a
much smaller battery, all started and ran ok, lights and all but after a
shortish test ride it was clear that the problem hadn't been solved as the battery stopped having enough life for lights and spark. Now that
there was new battery and regulator/rectifier in the bike, the culprit was
looking like the alternator. I managed to dig out some more test papers from a web and put
a voltmeter over the alternator. The output reading was coming out at 10V at 3000
rpm when recommended reading was 15V.
The
suggestion was that the alternator flywheel was probably demagnetised. That
seems to be quite common. Re-magnetising is a difficult business and can be
expensive and anyway, the best you’re going to get is the original spec of the
alternator, 40 or 60W depending on the model.
On the
other hand, the manufacturer Electrex supplies an alternator that claims an
output of 100->120W at 2000->3000 rpm. I decided that would be the best
solution in this case.
Back to ‘where
is the alternator?’ It’s under the left cover of the engine but unfortunately
it’s partially covered by the clutch so the whole clutch needs to come out
before you can get access to it.The alternator flywheel requires a puller to remove it. There is an official cone-shaped puller that threads onto the flywheel and exerts pressure on the centre-shaft to release the flywheel from it. The new Electrex flywheel won’t need this kind of puller so for me it looked as if it would be a once-only use. The puller costs more than £40 so the idea of buying it for that one time hurt a little. Looking around the forums most people recommended against trying any other kind of puller but I thought I’d better give it a try. Damaging the old flywheel wouldn’t necessarily be disastrous as it wasn’t going to be reused. I used the puller in the photos, the puller hooks are just long enough to get around the back of the flywheel and past the bevel that would cause smaller hooks to slide off.
But my, it was tough to get off. The official puller exerts its pressure on the centre core of the flywheel, a steel centre with little flex. An external puller, such as the one I was using, pulls on the outside of the alloy flywheel, there’s probably some flex there which means that you’re giving it some with pressure on the shaft and nothing is happening. I put a propane flame to the core of the flywheel around the shaft and kept pressuring and pow, off flew the flywheel, sailing out onto the garage floor. It wasn’t damaged but I was lucky.
Recommendations
after trying this:
- -
If
you’re thinking of reusing the flywheel, know when to back-off. It’s just not
worth breaking it
- -
Could
be worth leaving the flywheel nut well threaded on the shaft, that would stop
the flywheel flying off uncontrollably
- - Heat with confidence to the centre but don’t prolong it, there is a risk of damaging the coils
behind the flywheel
