I wonder if the forum can help me out? My M20 seems to be suffering a timing problem. Here's the story.
Reconditioned magneto, M01 with slack cable advance.
Contact breakers set to 10-12 thou.
Lapped in the spindle/pinion taper, nice matt grey surface, no scores or pits.
Set the timing to 7/16 BTDC at full advance using the Rizla method.
Tapped on the pinion, nice mesh, gear teeth not too loose or tight. Tighten.
Took several goes but got it to 7/16 eventually.
Recheck, all OK.
Start up runs nicely, solid tickover, revs rise and fall according to position of ignition lever, all as expected.
Then out for a ride, all well, pulls like a steam train up a 1 in 4 hill
Until:
After about 15 miles, loss of power and backfiring in exhaust. Most noticeable on slight hills. Picks up on the flat. Gets worse through the next mile or so, followed by total loss on the next slight hill. Conked out, impossible to restart.
Fortunately within pushing distance of home. Sit down, cup of tea, think about it.
Spark plug is a nice pale brown colour, dry, no oiling or carbon. Good spark. Plenty of gas in the tank, reaching the carb.
But, static timing has now advanced by at least 3/16 to 10/16 BTDC! No particular symptoms of pre-ignition during the conk-out, in fact the backfiring made me think it was over- retarded.
This is the second time this has happened, so not a random thing.
Can anyone suggest a cause or remedy? It seems that the mag pinion is slipping on the taper but I thought that a good pair of mating faces made this near impossible?
Any suggstions gratefully received.
Thanks,
John

email (option): John.childs1@btopenworld.com

It sounds to me like one of your tapers is wrong? Easiest thing it to try a different pinion, or maybe grind a tiny bit off the back of the pinion boss in case it's bottoming out. Ron

email (option): ronpier@talk21.com

Just a thought but years ago I had an issue with the irregular firing of an Enfield and discovered that the shellac on the armature had decided to melt and was thrown into the mag body which was sufficient to cause the armature to stick and then slip. Have you checked the inside of the mag body for cleanliness - any previous shellac once warmed up is likely to cause the armature to slip.

When it happened to me I cleaned the shellac out with petrol and a wooden stick (cut out of a hedge) at the side of the road - re-timed the beast and headed for home only for it to slip again - in fact I re-timed the machine 4 times to get home that day.

This happened to me some years ago on my B33 It turned out to be a hairline crack through to the taper. A new gear solved the problem.

I'd certainly look at the gear first...I've also had a cracked gear as Ian describes (though only once in 45 years)...The more common problem is that the back of the gear is contacting the magneto clutch nut and preventing it going fully 'home' onto the taper..

The solution is just to grind a little off the back of the drive gear to restore clearance...Ian

email (option): ian@wright52.plus.com

Thanks all, good suggestions, I will check the gear and taper area closely. The gear and mag ran together without a problem for several years and thousands of miles until the mag lost its spark. Given that the mag has been completely rebuilt I really hope it's not the shellac thing!

email (option): John.childs1@btopenworld.com

I have seen this scenario before on a B33 mag, the crack in the taper on the gear is so fine you can hardly see it. Obviously it opens up as the gear is tightened down on the shaft taper allowing it to move position.
I would recommend looking at this just to be sure.

Rick.

Thanks Rik, I will do that. It would explain why, when tightening the shouldered nut, it felt a tiny bit 'soft' yet the threads are good.
J

email (option): John.childs1@btopenworld.com

That fellow that used to write a column for Frank Westworth's magazine (something about traveling around to other folks' sheds) finished his final column with the solution to a mysterious long-standing issue he had with his rigid 5T Triumph ("Daisy" I think it was) that sounds very similar to your dilemma. Turned out to be a cracked ignition points plate, I believe.

FWIW.

Bruce Cooper
Houston

email (option): btcoop77069@aol.com

If the magneto's been rebuilt I presume the contact breaker unit is a snug fit on the armature spindle? If loose the timing can vary.
Unfortunately mine's a bit loose necessitating frequent checking.

My friend had this problem on his 3HW after the mag had been rebuilt by Brightspark. We were on the Beamish Trophy trial and the timing kept slipping. It turned out to be the pinion bottoming onto the Magdyno clutch nut instead of gripping the taper. as this had never happened before the rebuild we reckon that the rebuilders must have not used the original armature from his mag, possibly they had some rewound ones in stock,a bit off the back of the pinion and it was fine. The lesson? Don't enter an event on a newly worked on bike without a good road test first.

Good call gents, problem identified.
Contact breaker gap still as set at 10-12 thou.
No rotational slack between the breaker and spindle, nice and firm. Just a tiny little axial movement, a half thou snitch.
Pinion boss does not bind on mag nut, at least 20 thou clearance.
But, there is a hairline crack right through the pinion boss. Just visible on the attached photo at the furthest point of the taper. Under a x8 magnifier there is no doubt, it's cracked right through.
Also I noticed that the shouldered nut is belled out by 10 thou or more at it's inboard end. I suspect a previous owner graunched it on tight, either causing the problem or trying to solve it. My thousands of trouble-free miles were living on borrowed time.
So, thank you all for the suggestions.
Now to find a good replacement pinion....
Cheers,
John

email (option): John.childs1@btopenworld.com

Autocycle Engineering have some good quality UK made replicas...Many that are out there for sale originate from India and the usual caution should apply when looking at those...

I haven't tried one so can't vouch for the quality, but personally consider purchases of anything from there to be a gamble..Sometimes things are good, often they are not...Generally, I won't take the risk with engine components...Ian

email (option): ian@wright52.plus.com

Hi all’
I had the same problems as John with hairline cracks on the mag pinion, so spoke to Chris Williams at Autocycle the only ones he stocked were Gold Star pinions with the cotter pin for the timed breather so bought one and my timing has never slipped since, That was about 10 years ago so I can safely say they’re ok,
Regards Tom.

email (option): rustytomm20@hotmail.com

Chris Williams at Autocycle - top man.