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DT magneto


lotty
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I'm on my way to making my magneto good again. It is a 1975 dt 175. A component in the magneto had come out of place and so as the bike was kicked over it caught on the coils and damaged them and bent itself. So I have had the coils re-done, a good job at Rex's. So all there is now is to replace the component that did the damage; the trouble is that i have no idea what it is called, and neither can I find any pictures/diagram drawings of it. So I will try to describe it in the hope someone can know what I am on about and what it is called.

How can i begin? Well, I am taking about the actual flywheel with the magnets around. In the center of this is a tapered shaft, and with the slot an woodruff key it would snugly and accurately locate onto the stator plate, BUT whilst this happens, there is a sleeve with a cam which fits onto this taper and this spins freely on it. Sounds nuts because the cam needs to be fixed in position.....BUT in the back of the flywheel is a component, a sort of arm that rocks and is fixed to the bottom of the flywheel by a plate which screws it down in place. In the cam sleeve is a little cut-out square, and the rocking arm end fits into this cut-out square, loosely. So this is what holds the cam in place.

Does anyone have any idea what the hell I am on about?  Doug

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Hi Doug

Initially i thought you were referring to the lubricating sleeve arm.

As it's a points based flywheel (Not CDI). There is an arm which screws onto the backplate which is the carrier for the little pad which is soaked in oil, to lubricate the cam.

Go to cmsnl website and find DT175 parts list and under flywheel magneto image you will find ....

Part 11 and 12 in the above diagram refer to the pad and retaining screw, its not clear from this diagram whether the pad comes attached to the arm as a replaceble part. The photo appears to be a replacement pad only.

 

Edited by NE0
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Or Doug....

Having just re-read your post...

I think you're referring to the Advance and retard arm which is fitted INSIDE the magneto unit.

I looked on ebay and found a 1974 Magneto unit and there was a close up photo of the inside.

Unfortunately, the part in question is part of the rotor assembly and does not have its own part number.

You would need to source a replacement rotor (magneto unit) to obtain the part you are seeking.

NB. For many of you who might be reading this :- Generator, magneto, rotor, flywheel are all (rightly or wrongly) terms used by owners worldwide for the same thing!

Edited by NE0
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I'm now convinced the part you're describing is indeed the advance and retard arm as you mention there is a sleeve which spins on the rotor body.

For those that might be interested, this is the advance and retard mechanism for this bike.

Typical of early car engines the timing cam was directly driven from the crankshaft. which meant the faster the engine went, the faster the timing shaft would spin. If the contact breakers (points) were fixed in one position and opened & closed accordingly, as the engine speed increased, the engine would not run properly as it would'nt be able to 'keep up' with the speed of the shaft. It was discovered that if the points were 'moveable' then they could be adjusted to open at a different position relative to the position of the cam. On a points based ignition system, the timing of points opening relies on moving mechanical parts to contol the opening and closing of the points.  In an early 1970's car with an early LUCAS distributor, the points are attached to a moveable plate which is moved by centrifugal forces and is also attached to the vacuum of the carb. The faster the engine went the forces would move the plate altering the position that the points open.

With a motorbike from the same era, there is not enough room to fit all these moving parts, so they created a system which moved the position of the engine cam instead. Here on the Yamaha DT of 1974, the points are mounted in a fixed position on the backplate. Unlike a car the backplate is not adjustable with this setup. Yamaha subsequently came up with the idea to advance and retard the opening and closing of the points by altering the position of the engine cam. In this case a sleeve which can be moved on the magneto body. Centrifugal forces makes the free moving sleeve spin faster, the arm and spring hold it in check, but in essence the position of the cam, when spinning faster, will cause the points to open sooner or later (depending on engine speed)

 

Which hopefully answers your concern from your first post Doug:-

Quote

Sounds nuts because the cam needs to be fixed in position.

No it can't be in a 'fixed position' otherwise it wouldn't be able to alter the timing. It's designed to move... but not by much.

 

Hope this helps.

NE0

Edited by NE0
Clarity and brevity
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Hey lotty, posted a picture for you on the yoc fb page, (no copyright hassles) do you mean item 12?

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Hi NEO and Cynic.  Yes, the advance and retard arm. That picture of it in the flywheel it right. Cynic, sorry i couldn't find the picture on fb......it's me, I'm hopeless.

I need another advance and retard arm. At least i know what it is I'm seeking. Not going to be easy. If anyone knows of a possible one then please let me know.

On my damaged one (beyond repair), on the arm was some black plastic which I reasoned was debris fallen from the plastic coating of the magnets, so that is something to keep an eye on...chunks fall off and into the mechanism and could have caused the damage.

So thank you. The advance retard arm.  And thank you for explaining the reason for it and how it works.

PS. I used to have a 175 mx. Sold my AP50 and got the mx all ready for my 17th birthday. A superb bike. My 4 other mates all sold their mopeds and got MXs. We drilled an extra hole in the monoshock to get em a bit higher. They were great bikes. We'd all shoot off over Pendle Hill and stuff. Take our baffles out and blast round muddy fields firing mud all over eachother. Even in the snow going home from the factory, cars and buses skidding sideways, and we'd go back over the tops riding through drifts. Great days.  

 

Once again, thanks.  Doug

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You're welcome Doug,

Think you're going to struggle to get one other than ebay or similar.

You might have to think laterally , and instead of searching for a magneto.....search for another engine......just for the spare parts it will have.... to keep yours going.  Doesn't matter even if it's seized ...if its got a magneto it will be worth buying.

You won't need reminding that you're searching for 47 year old parts!...in a few years time it will be 50 years old!!

and the bad news is ....I can remember being at school in 1974....and it doesn't seem that long ago!......gee life's short!!

 

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Cheers pal. It's on its way. What a good do. I really appreciate your help. Nice one.

It does pass quick. Was all chatting the other day at my daughter's. Age came up. I looked at her and casually said, 'Well you're knocking on a bit now. what are you, 34'. Her face. Stunned. First time she'd ever heard anything like that about her. She said 'Yes'. I said, so your are only five and a bit years off forty!'. Suddenly she was almost 40! Ha ha. I did use this to make an important point, saying 'If there is anything you want to do, do it, don't hang about'.

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  • 2 weeks later...

Just a quick post to put this thread to bed. So the stator got rewound, the three coils and all cleaned up and re-wired and tested. A superb job by Rexs Speed Shop. I also purchased a new old stock flywheel magneto ( My old flywheel was stamped F-140, as is this new one, which means, according to the manual, that my bike is a UK series DT175. If it wasn't UK series it would be a CT175 and be stamped F-130) because i needed a new advance/retard which sits in the bottom of the flywheel. 

I wired it up and put it all together. Very difficult as always to set the points because so awkward and at the highest point the contact gap can be seen but the adjustment screw can't be got to! Anyway, I took a stab at it and kicked over for a spark.........which I got.....hooray. So it should work, BUT, with the plug in I kicked it over and eventually determined that no fuel is getting through at all, so I am having to service the carb before I can start the engine. But it should work. The stator coils rewind and re-wire is a good job and i'm sure it is right, just like new and all tested with the results.

Edited by lotty
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6 hours ago, lotty said:

Just a quick post to put this thread to bed. ...

It ain't to bed till the fat lady sings!!! lol
But, it's looking hopeful..! :D

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Well, the fat lady sang, but she didn't make the grade. 

Carb kit came. New o-ring on main jet and spang into life on 2nd kick.

It's been an ordeal.....for me....pushed and tested me. But at my age I've got used to how it can be. I am very grateful for the support here and rex's for the coils rewind. Life certainly throws 'em at a person. I'm drained, haha. Thanks everyone.

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2 hours ago, lotty said:

Well, the fat lady sang, but she didn't make the grade.

Maybe not, but YOU did!!!... You're top of the Leaderboard with a gold medal! 😄

Edited by NE0
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