Author Topic: M21 Ignition timing  (Read 256 times)

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Hobbs

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M21 Ignition timing
« on: 10 January, 2019, 14:55:14 »
Good afternoon and a happy and prosperous new year to all Forum contributors.
I am about to have the Mag-Dyno from my 1958 M21 refurbished.
When it returns I will have to re-set the ignition timing.
The original BSA figure of 7/16" BTDC was probably ok in 1958, however using modern fuels I wonder if any other members have found better results with different settings.

Cheers
Hobbs

berniej

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Re: M21 Ignition timing
« Reply #1 on: 11 January, 2019, 09:45:33 »
Good point - and on higher compression engines with an auto-advance that's often necessary to stop pre-ignition.
However I don't think it's so critical on these old thumpers. You don't say if yours is manual or auto-advance - I'd guess manual?

Mine are both manual and I time to 7/16" with full advance at the lever and run on standard UK unleaded with no additives.

The actual setting when riding is mainly done by feel and how the engine responds.

The important thing is not to set it any more advanced than this as it could result in valve burning when advanced. In practice I rarely have the lever fully advanced except when running at full throttle (which is quite rare!).

Cheers,
Bernie

'49 M21/B31 hybrid
'56 M21 combination
Bantam D7
C15 project
B40 Sportsman project

Mike Farmer

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Re: M21 Ignition timing
« Reply #2 on: 11 January, 2019, 10:53:29 »
 :)

If you increase the fully advanced angle, you will also increase the fully retarded angle which of course means when fully retarded your motor may well fire before TDC and kickback.

This is entirely personal but I always time fully advance about a degree or so less than what the book says. It doesn't make a noticeable difference but it will alleviate the danger of kickback.

Mike 8) 8) 8)

Pete C

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Re: M21 Ignition timing
« Reply #3 on: 11 January, 2019, 11:39:14 »
Received wisdom from the WM20 site is to leave the ignition as standard. In any case donít retard as this can result in overheating.

The conclusion part of this link explaines it  http://wdbsa.nl/ignition_timing.htm

I have run my M21 with  standard timing with whatever fuel is around and no problems.

And you don't want to kick it over if too far advanced...it hurts!

Cheers
Pete


Hobbs

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Re: M21 Ignition timing
« Reply #4 on: 13 January, 2019, 11:08:18 »
Thanks for the replies. However there seems to be some confusion regarding the terms "Advance" and "Retard".
An advanced spark is effectively further away from TDC. A retarded spark is closer to TDC.
On the M 21 with the adv/ret lever pushed fully forward (slack cable) the ignition timing is fully advanced.
As the lever is moved anti-clockwise (cable tight) the ignition timing is progressively retarded.
The article indicated in Pete C's link explains this admirably.
BSA's recommendations state that excessive use of retard should be avoided as this may cause overheating and possible valve burning. They also recommended that retardation should only be used for cold starting
and also when under heavy load as is in top gear inclines and hard acceleration.

« Last Edit: 13 January, 2019, 11:13:23 by Hobbs »

DEAN SOUTHALL

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Re: M21 Ignition timing
« Reply #5 on: 13 January, 2019, 11:47:15 »
I'm not sure what the confusion is that you are referring to mate.

One source of confusion is that there are both slack and tight advance mags around.
I has been argued that slack advance has the advantage that if you cable snaps while riding the timing will remain advanced.
BSA: turning ordinary men into mechanics since 1910

DEAN SOUTHALL

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Re: M21 Ignition timing
« Reply #6 on: 13 January, 2019, 15:34:56 »
Here's a good detailed description of sorting the M20 igntion

http://www.wdbsa.nl/ignition_timing.htm
BSA: turning ordinary men into mechanics since 1910

Hobbs

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Re: M21 Ignition timing
« Reply #7 on: 13 January, 2019, 20:01:01 »
Fair point Dean. The M21 mag changed postwar from MO1 to MO1L  though I'm not sure if the manual cable entry point changed sides.
When viewed from the Points side a clockwise rotation mag with a left hand cable entry will be fully advanced with a slack cable and vice versa.
The confusion arose from Bernie j's reply where he states he hardy ever uses full advance.
After forty five years experience of motorcycle maintenance, restoration, competition and road use I have yet to see a engine that has overheated or burned a valve due to running the correct ignition advance.
Changing the subject slightly, do any members have experience of using  EasyCap condensors and are they available for
lucas face cam type mags ?

Pete C

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Re: M21 Ignition timing
« Reply #8 on: 14 January, 2019, 13:01:11 »
easycap for M01 mags here http://www.brightsparkmagnetos.com/images/C03%20pic.JPG

No experience myself but a friend has used them and is happy.

The Price of 13 quid or so against a mag rebuild to replace a Standard condenser, it is worth a shot anyway.

Not sure about your comment that you haven't seen damage resulting from correct timing, don't you mean "incorrect" timing?

Cheers
Pete


 

JulianS

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Re: M21 Ignition timing
« Reply #9 on: 14 January, 2019, 13:29:29 »
I used an easycap on my A10 for a while and was quite happy with it.

However it will not help if the old windings are a fault.

Also fitting is more than just fitting one to the points assembly, it is necessary to disconnect the old condenser. Quite detailed instructions on Brightsparks site, but if the "quick snip" detailed is not possible a full dismantling of the armature, especially removing the bearing, is not not easy.

Worth reading up;

http://www.brightsparkmagnetos.com/condensectomy/index.htm

Photo show one fitted to my A10 a few years ago.

berniej

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Re: M21 Ignition timing
« Reply #10 on: 14 January, 2019, 14:34:25 »
Hmm, not sure why it should be thought that my comment introduced any confusion - the intention was quite the opposite.

Both my mags are pull-to-advance but my comment would be equally applicable if it were t'other way round.

I find when riding that the engine doesn't seem to like full advance even when at cruising speed. Maybe I just never reach the revs that demand it before my teeth start rattling!

I have done a full condenser change on the M01 and it's not the easiest of DIY jobs. I found that the correct mag bearing puller is a must - I couldn't find a way to easily fabricate a suitable DIY tool.

Cheers,
Bernie
'49 M21/B31 hybrid
'56 M21 combination
Bantam D7
C15 project
B40 Sportsman project

DEAN SOUTHALL

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Re: M21 Ignition timing
« Reply #11 on: 14 January, 2019, 17:33:56 »
I have used Easycap on the Sloper. Bantam and the Trike V-twin and am pleased.
I have not put thousands of miles on any of them yet though ;)
BSA: turning ordinary men into mechanics since 1910

Hobbs

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Re: M21 Ignition timing
« Reply #12 on: 15 January, 2019, 11:37:00 »
Dean, Was that your Sloper at the Romsey Classic Show on boxing day?
I spent a while looking at this one and it's certainly one of the tidiest i've seen.
What are they like for riding ?