Motoplat alternator

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Reworking the Motoplat system on my 1974 350 Mk3 has been a rabbit-hole I have happily dived down over the past couple of years, and I think this is the final bit of digging I shall need to do. For those who have read my ramblings before, this latest post is quite long and occasionally detailed, but I reckon it could be useful for folk who are working to a tight budget – like me!

I started a couple of years ago by asking Steve Hardaker of Bradford Ignitions (who happens to be conveniently located a mere 45 minutes from my home) to check out the three alternator stators I had acquired over the years. He removed the red potting from the ignition exciting coils on two of the stators, and rewound them with about 25% more wire, to give a fatter spark, and thereby counter the familiar complaint of poor starting with the Motoplat system. He also gave them new output cables, and re-potted the ignition coil.

So this was the starting point:

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When fitted, it did exactly what was needed, and I very easily converted it to 12 volts by using a standard cheap rectifier/regulator, and taping off the red wire (which is the centre tap on the stator wiring, giving 6 volts when connected to the original Logitron rectifier).

So I happily started running the bike with the oft-derided Motoplat alternator and CDI, and all was very well – as waffled about in previous posts.
Being a bit of an anorak, I’m determined to keep an eye on the health of the engine while running, to the extent that I’d love to have an oil pressure gauge even though I know that would be both pointless and difficult to achieve! What I have done is on the electrical side, and that’s a small digital voltmeter set into the headlamp shell, plus a wonderful tricolour LED from https://www.improvingclassicmotorcycles.com which is green for good charging, red for not charging and yellow for overcharging. Easy to fit, and a real asset.

Over the next few hundred miles, thanks to both the LED lightshow and the voltmeter, it became clear that the stator was not always charging. After about 10 miles, when the engine had reached operating temperature, the voltage would drop erratically down to below 12, then bounce between 11.9 and 13.5. After another 20 miles or so, it might well settle down and charge consistently, then drop off again with no warning.

Internet wisdom, as well as a call to the now retired Steve Hardaker, confirmed thermal degradation. The charging coils, certainly 50 years old, and way past their expected life, were vibrating within the red potting that had kept them safe in years gone by, but the wire had had enough and was either cracking or shorting out with temperature and engine vibration.

Despite optimism, all three stators exhibited the same symptoms, so I started to scratch my head.

First stop was to contact a couple of alternator-rewinding companies, one of which said definitely not to the Motoplat product, and the other simply didn’t reply. Rude, I thought.

Then I found Roger Lovelock of https://www.bbbikeshop.co.uk, who is encouragingly old-school, and outstandingly helpful. He also has several YouTube vids showing what he’s up to.

Since all three of my stators were knackered, I thought I’d have a go at removing the red potting as best I could. You’ll see from the photo that I managed to get the potting off (phenol resin, I’ve discovered, which lets go quite easily with gentle use of a MAPP torch). The original plastic bobbins did get a bit melted, but I wasn’t too worried about that as I planned to replace them. I did notice they are of two different heights, 3 short and 2 tall. There’s probably a good reason for that, but I’ve no idea why.

I left the ignition coil as untouched as I could, together with the diode that stops all those volts coming back from the CDI “conversor” to earth. Steve’s rewound ignition coil has probably got 2000 turns of very fine wire on it, showing a resistance of 228Ω, and that’s worth preserving. Fingers crossed.

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I sent this photo of the naked stator to Roger at BB Bikeshop, and he came straight back with the information that it’s exactly the same as those fitted to Morinis, and he’s done several of those – as you can see on his website. It seems the Morini ones were not potted, but the metalwork is certainly Motoplat or made to their design. I guess the Bologna factories all knew their colleagues in Barcelona.

Roger also told me he had 3D-printed bobbins for his Morini rewinds, and kindly sold me a set of those. I had reckoned on building the bobbins with a mix of 3240 epoxy board and Nomex tape, ready to set about rewinding the charging coils. But with Roger’s bobbins, things were looking up.

Motoplat originally wound 20 AWG wire onto each of the bobbins for the charging coils, before assembling them onto the stator star, in a “bifilar” pattern – one set of turns in one direction, then some sort of paper insulation tape, then another set of turns in the other direction, They would then assemble the bobbins onto the star and join the wire as appropriate by twisting and soldering. The centre tap was added at the far end, so that the whole thing is a bit of a cat’s cradle when you uncover it.

Since I didn’t need the centre tap, having already converted everything to 12 volts via a nice new Electrex Mosfet reg-rec, I thought I should be ok winding a single length of 17AWG (1.15mm) the same number of turns as the double-wound original, as long as I wound each coil in the alternate direction – clockwise, anticlockwise, clockwise and so on.

My first mistake was to think that I would be able to hand-wind 1.15mm wire tight enough to fit the stator. The end result was that Roger’s 3-D printed bobbins suffered – a lot. The horizontal top and bottom plates broke off, given the rough handling I was forcing them to accept!

But the centre cores of the bobbins were fine, of course, so I re-used them with fibreglass sheet tops and bottoms, all JB Welded into place, and this time I used 21 AWG (0.8mm) wire.

I calculated that I should be able to get about 300 turns in total, and that would give a reasonable voltage at decently low rpm. I’m not worried about the current, since everything is LED and the continuous load with the lights on is less than 20W.

Wound as a single clockwise/anticlockwise run across the five charging poles, that wasn’t too hard to do. Being a novice at this, the winding isn’t perfect, but I read that it’s OK to be a bit scruffy, provided the total number of turns is what it should be, and the whole thing fits inside the rotor without touching. It worked out at 80 turns for a tall bobbin, and 60 for a short one, winding four layers of wire on each. Total about 340 turns.
(The reason for using a single length across all the coils was simply that the “magnet wire” as it’s known has a very effective coating of insulation, and it’s hard to be sure you will get a good connection with solder no matter how carefully you scrape the wire, so the recommended way – if there’s room for it – is to rewind with a single length of wire.)

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In order to counter the heat-degradation of the output cables, I used fibreglass-sleeved cable rated for military and maritime use. Not cheap, but I only had to buy a couple of metres, and it will be worth it. Also high-temp fibreglass sleeving for the joins. And proper solder with lead in it!

The output cables got locked down with JB Weld as appropriate, and then it all got liberally painted with the red high-temp windings varnish. I was a little over-enthusiastic, perhaps, but it won’t be seen and better too much than too little.
I checked and rechecked the resistances and continuity points, and felt reasonably confident that it would do the job.

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Rebuilding the engine was therapeutic, as everything went back together nicely (yes, new tab washers for crankshaft and clutch shaft, plus a new gasket), and making the connections to the loom was straightforward.

And the result, astonishingly, was first-kick starting and a proper charging. 14.3 volts at a fast tickover, or 14.1 volts with the lights on. And that’s the green LED charging light, as well as the voltmeter to confirm it.

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Across this wonderful forum, there have been several well-informed experts who have asserted quite vigorously that the Motoplat gear was only good for throwing in the bin, or maybe as a paperweight, and the only good way is to buy a complete system from Electrex.

Well, I beg to differ. This whole adventure has cost me a touch under £60, as against £430 for the alternative. I leave the last word to Roger Lovecraft, who wrote this to me: “..., it is easy (if you have the money) to bolt on a £430 replacement system, but not half as much fun and sense of achievement as fixing the original system at a fraction of the cost.”

Best wishes to all,
Pete.

[Duccout]

Hi Pete,

Thank you for taking the time to write this up! As for the Morini connection, if Bill is reading this, I wonder if it may help with his Morini ignition woes?

Cheers,

Colin

[blethermaskite]

Well! I will eat my hat against all my odds and yours well done! Cheers George

[ranton_rambler]

I also have a 3-colour LED voltage monitor although from a different source. I have converted my bike to 12V with an Electrex reg-rec. The first unit failed completely and the FOC replacement was OK at first but then ceased to control the red light in the headlamp shell properly although it still manages the battery voltage as it should. Hence I bought the little monitor to keep an eye on charging in lieu.

A side benefit is that it's permanently connected to the battery so when I see it start to flash yellow in the garage I can connect the trickle charger for a few hours.

[Jordan]

Rebuilding a CDI for fun is a real labour of love, and not a little time-consuming.

These coils are made in India for Vespa scooters: https://tinyurl.com/5dch2dy7
They are dimensionally the same as the ones in Ducati made and Motoplat CDI charging coils for Ducati singles.
I've been using one in mine since 2016.
I stuck one on a stator from a points model bike, after removing one of the battery charging coils. Some metal was removed from the mounting plate to make room for the fatter coil.
Bargain hunters might want to check out what other electrical parts are still available on ebay.

Reason for different sizes of battery charging coils is because they wouldn't fit on the stator if all were the bigger size.

[themoudie]

Aye Colin, that wee bundle of trouble is STILL ongoing!

Rather than ignoring the ignition power coil and trigger coil mounting poles on the stator and just using the 4 charging coils (35W output) to charge a 12v battery system. The stator is going to be rebuilt, with 2 additional battery charging coils.

I'm also going to have an external trigger coil mounted to the stator by a homemade extension plate and activated by a shaped blob of weld on the outside of the rotor. The blob will need to be built up slowly, so as not to overheat the internal rotor magnets, then shaped to achieve a trigger signal 5°-6° BTDC. Then use a CDI whose ignition curve can be tweaked!

A steep learning curve!

Good health and keep wearing them out, BillR