1975 350 MKIII Motoplat generator?

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themoudie
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Joined: Sat Jun 14, 2014 12:44 am
Location: Scotland

1975 350 MKIII Motoplat generator?

Postby themoudie » Mon Jun 12, 2023 1:13 pm

This is the first time that I have come across a 'MOTOPLAT' generator installed in a Ducati single (1975 350 MKIII) and I suspect that there is a difference between the thread on this rotor and the 'DUCATI ELECTRONICA' generators that I have seen in the past. Hence, Laurie Richardson's excellent puller couldn't get a grip of it. :(

So, having made up my own puller, using bent M8 threaded rod and nuts, to hook behind the rotor; a piece of M10 threaded rod, with nuts to act as the central pressure screw and a piece of 65mm x 35mm x 3mm MS box section, drilled to keep all the threaded bits at the correct distances apart, I was able to "pop" the rotor off the taper on the crank, with relative ease.

It appears that this stator, from it's position, controls the ignition timing using the exposed coil, on the right, to acheive this; the rest of the stator appears to be encapsulated. There is a single cable coming from the stator, with 2 yellow wires that I assume are the feed to the regulator/rectifier for the battery and a single red and blue wire that I assume are for the ignition coil/CDI? However, the rubber insulation is perished, cracking and exposing tinned copper multi-strand cores on all 4 coloured wires. :(

There is pitting of the encapsulating material and as I obtained the whole machine in "big lumps", I have no idea whether the generator/ignition was functioning, or the reason for the demise of the machine. There is also no sign of a CDI or coil in the tea chest, but there is a regulator/rectifier.

So, I was going to replace the entire generator/ignition system, using an Electrex set up. Does anybody have any opinions/experience of the Motoplat set up that would disuade me from going down the Electrex replacement route?

I have attached images of the Motoplat unit fitted at present, for others future reference/information.

Thank you for your time.

Good health, BillR
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blethermaskite
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Location: northern ireland

Re: 1975 350 MKIII Motoplat generator?

Postby blethermaskite » Mon Jun 12, 2023 4:04 pm

Bill I assume your engine is one of the last 350mk3 singles and probably made in Spain? you can ID these motors by the fins above the exhaust port being about double the height of those on an Italian built engine, then there is a broader fin round the head from the back oil drain banjo bolt to the inlet flange, and finally the clutch inspection cover does not say 'made in Italy' on it (sorry if you already know all this :) ) so my 350mk3 engine is a Spanish one and came with Motoplat ignition and I assume the solid encapsulated gen stator (I don't know because I have never needed to remove the rotor) but your wiring colours are the same as mine. I was always advised that the Motoplat ignition was inferior to the Ducati electronica system found on the late Italian built engines and in practice I have found this to be true, My generator still works ok ish but the ignition failed long ago and was replaced with a Lucas rita system which is of course independent of the generator fed only by the battery. My generator wiring is still sound and undamaged and has not needed replacement (so far :roll: ) I suspect this would be a problem due to the wires being encapsulated in the stator construction?........If it were me I would junk the lot! and go for the proper 12v conversion.
Cheers,
George

themoudie
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Joined: Sat Jun 14, 2014 12:44 am
Location: Scotland

Re: 1975 350 MKIII Motoplat generator?

Postby themoudie » Mon Jun 12, 2023 8:09 pm

Aye George,

Thank you, I had picked up on the late 350 MKIII data from your previous post in the "Widecase rotor thread pitch" thread. Link: Widecase rotor thread pitch and that is what made me suspicious when I couldn't get Laurie's puller to take a full thread on the rotor hub.

The points of difference that you list, in my eyes, appear to be largely cosmetic and as this is a complete strip and rebuild I am not too worried about them. Hopefully, there aren't too many dimensional differences!

Shall I hold onto the rotor and stator in case you need "emergency" spares? Or, would you update, if yours when "PHUT"?

Good health, Bill

blethermaskite
Posts: 486
Joined: Thu Mar 01, 2018 1:06 am
Location: northern ireland

Re: 1975 350 MKIII Motoplat generator?

Postby blethermaskite » Mon Jun 12, 2023 8:40 pm

Bill, as far as I know the differences are just about 100% cosmetic (I'm sure Nigel would know better than me)......thanks for the offer of keeping the motoplat bits but frankly if mine goes "phut" (before I do) I will just fit the proper electrex 12v conversion and be done with it.
In my experience with that engine.....(about 50K miles) chrome flaked of the rockers at about 1K miles, clutch plates shot at 15K miles, and motoplat ignition failed around 30K miles.......thats the lot, and I have been revving the brains out of it for years, still on original big end, bore, piston and rings.......great engine...... and in now 49 years has never once broken down.
Cheers,
George

themoudie
Posts: 649
Joined: Sat Jun 14, 2014 12:44 am
Location: Scotland

Re: 1975 350 MKIII Motoplat generator?

Postby themoudie » Mon Jun 12, 2023 11:43 pm

Aye George, nae bother, the stator is going in the re-cycling and the rotor could make someone a classy paper weight! :D

This engine has screw adjusted rockers, so these may have been replaced at some time, if yours are the solid, shim type?

The piston appears to be a high compression type, but I haven't measured it or the bore and it appears to have been cleaned up using emery cloth!

The crank is going to Nighel for a wee "looksee" and I'll make some decisions afterwards. I think a cooking motor is out of the question, as I'm not going to race the machine. But, something with a good spread of mid-range power and a bit quicker on the acceleration, would be nice. I have a rough (welded up) 2 x 10mm plug head, with 32mm inlet port that might work quite well with Nigel's 173 cam, one of which is sitting in a drawer, handy! Also a 32mm AMAL MKI Concentric or if I have too a MKII.

May I ask what sort of revs are you taking yours too?

I attach images of the impact driver combination needed to remove the M5 x 15mm cheesehead slot screws that had been stud locked and spring washered into position! :evil:

Also the cracked stator potting material (highlighted) where expansion/contraction from a cable has been sufficient to crack it. One of the 5 charging coils has also started to loose its wrapping, on the inner face, opposite the aluminium backing plate.

Finally, the perished insulation on the exposed ends of the stator feed cables that have been exposed to sunlight. The cables where they are exposed as they pass through the stator back plate and inside the outer insulation sheath appear to be OK.

Thank you and good health, Bill
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Last edited by themoudie on Tue Jun 13, 2023 9:30 am, edited 3 times in total.

blethermaskite
Posts: 486
Joined: Thu Mar 01, 2018 1:06 am
Location: northern ireland

Re: 1975 350 MKIII Motoplat generator?

Postby blethermaskite » Tue Jun 13, 2023 9:28 am

Bill, yes mine has solid rockers, I think some early widecase 350s may have has screw adjust rockers (the scramblers or the sebring?) my engine has a green/white camshaft and 32mm mk1 concentric, I have been running it to 8K all the time with odd bursts to 8.5K.......I think the factory red line was 7.2K ?
Cheers,
George

themoudie
Posts: 649
Joined: Sat Jun 14, 2014 12:44 am
Location: Scotland

Re: 1975 350 MKIII Motoplat generator?

Postby themoudie » Tue Jun 13, 2023 1:57 pm

Thanks George, I hate to think which parts bin combination has been used for this engine then! :shock:

So I may have to go in search of shimmed rockers, if I am going to use Nigel's 173 cam. :( Could be pricy?

As for the 350cc rpm ceiling, 8,500rpm doesn't sound unreasonable, I believe 350cc race engines will take up to another 1,000rpm before it gets messy. ;)

I occasionally take the 450 up to 6,000rpm; most of the time 5,500rpm is plenty to "make progress" on the roads that I ride on around here and I understand that for a 450cc road motor 7,000rpm is probably plenty; race motor maybe 8,000rpm. ;) But, you have to remember that there is an 86mm or larger piston whizzing up and down in there. ;)

No doubt, if Nigel and or Eldert sees this they will provide chapter and verse, if I'm talking cobblers! :oops: ;)

Watch out for the storms, we have been too far to the east to benifit from the rain, but get the easterly breeze.

Good health, Bill


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