Ignition timing

The book says 32 deg BTDC. However, with present-day petrol being rather lacking in octane rating, you might be better with 30 degrees. If you don't have the original dished pistons and have later Commando pistons, you should retard the ignition a couple more degrees. I currently run 28 degrees BTDC, having Commando pistons (I couldn't get original dished top ones). Gordon.

I use a 2BA socket on a 1/4" drive to turn the points plus the skinniest feeler gauge I can find for points opening. A head torch helps. The art is in carefully tapping the A/R unit onto the magneto shaft - I use a rubber mallet - then tightening the nut without upsetting the timing. It can take 2 or 3 tries to get right. Don't forget to check the timing on both cylinders. When I was a callow yoof, I didn't do that on mu 99 and eventually found that one side was at 30 Degrees btdc, the other at 57. A duff cam ring which cost me a piston. Gordon.

Previously wrote:

Hi All,i have a 750 Atlas motor in a Matchless (hybred g15) ,i removed the magneto for repairs and would like to find out the timing on this engine (degrees before t.d.c.) Thanks For Any Help .Steve(Norton Commando's also)

Hello Stick with 32% fully advanced before top dead center you need a 9v battery and two crock clips and some electric wire and a bulb fit one end to earth and the other to points , you get a light when the mag is turned until the light goes out then turn it back and till it come on . the just get to flicker on and off .then nip the lock nut up you timed up

Anna, any justification for sticking with 32 degrees in view of my earlier comments? My Atlas has Commando pistons and is unusable through pinking on unleaded petrol on standard ignition timing - hence my observations. Also, nipping up the lock nut without tapping the A/R unit onto the tapered shaft will almost certainly disturb the ignition timing. Gordon.

Tapping the A/R unit runs the risk of 'Brinelling' the mag bearings.

You would have to be brutal to manage that. Note the reference to a rubber mallet. Perhaps I should have said small rubber mallet. There will always be someone who picks up a lump hammer instead...

Previously wrote:

Anna, any justification for sticking with 32 degrees in view of my earlier comments? My Atlas has Commando pistons and is unusable through pinking on unleaded petrol on standard ignition timing - hence my observations. Also, nipping up the lock nut without tapping the A/R unit onto the tapered shaft will almost certainly disturb the ignition timing. Gordon.

Hello Yes My Manxman 650 Is set at 32% Fully Advanced And has a smooth tick over , and lot of power through the rev range . You have to hold the points end when nipping up with care But though you guys would of worked is out for your selfs .I know has much about mags as these pro's. As for Nipping up the Mag shaft as a double taper tread , you do not need to tap it with a hammer rubber or other wise . I have made a locking tool ! Yours Anna J

Delighted your Manxman runs well on 32 degees. My Atlas would not - unless I just pottered along on 1/4 throttle... So the ignition had to be retarded. Don't forget, the octrane rating of petrol has decreased rather a lot since these bikes were new, and my Atlas is running higher than standard compression ratio.

How about some detail of your locking tool? Without one, I find the light tap before tightening the A/R onto the shaft works a treat. Gordon.

You people are comparing apples and oranges. The Manxman is a 650 while the Atlas is a 750. When the Atlas was launched, a big thing was made of the fact that it would run on 2 Star petrol. This was due to the low compression ratio. Another reason for the low compression was an attempt to calm the horrific vibrations for which the Atlas is famous.

Atlas pistons have not been available for a long time. Since the 750 Commando has the same bore and stroke as an Atlas, the pistons will fit. The Commando has a much higher compression ratio, and the recommended timing for Commandos is 28 degrees BTDC. The Commando prior to (And after) the Combat engines is the same as the Atlas one, profile wise, although of course later Commandos had a cam change to mount the contact breaker on the end of the cam.

All in all, the way to do timing is to start at a reasonable figure, and here I'd say 28 degrees BTDC, and ride the bike. When it is warm ride it up a hill and see if the engine pinks. If it doesn't, advance the timing a degree and repeat. When it starts to pink, retard it to the previous position and Robert is your Mother's Brother!

Couldn't put it better myself. Gordon.

Previously wrote:

Delighted your Manxman runs well on 32 degees. My Atlas would not - unless I just pottered along on 1/4 throttle... So the ignition had to be retarded. Don't forget, the octrane rating of petrol has decreased rather a lot since these bikes were new, and my Atlas is running higher than standard compression ratio.

How about some detail of your locking tool? Without one, I find the light tap before tightening the A/R onto the shaft works a treat. Gordon.

Gordon good engineers do not let on about there special tools , I have made them for my clutch and a the exhaust locking ring . C spanners slip , The one I have made dose not slip My great uncle Freddie Dixon Made Special parts for his motorcycle and fitted a metal cover or box over them so folks could not see or copy what he had done , and its the same with tooling , I am going to do some experimental work on a balancer that will fit to the engine sprocket made from alloy and fittings for a wight of around 80gamms set at 76 degrees after top dead center to help with the reciprocation factors and help to make for smoother running at speed , we see what works , yours Anna j

"Gordon good engineers do not let on about there special tool". I could be wrong here, but isn't this forum about helping other owners keep their elderly Nortons on the road? Does it really help to rubbish someone's method of timing the ignition by stating you have a superior method and then refusing to say what it is? I am actually rather saddened at this. Gordon.

"Special tools"Have you had them patented? Will it make you a fortune?

All good engineers make special and adapted tools and if it makes life easier then share the experience with the rest of the members. Don't behave like a child. i.e. "I'm not showing you" Get a life.

Anna your posts usually interest and often amuse me. Unlike other members I have not found them irritating; until now!

The ADT As a Double Locking Tread , You need to take care doing this up , Its a sleeve nut, And you need to lock the engine once you have got you engine turned to 32 degrees Before top dead center . Your ADT should be open to full advance by putting a small wedge or bolt to keep it open , then hold the points end with a small spanner , the idea of the getting you timing right it by rocking the engine back and forth ,I use a big spanner on the main shaft nut, to do this , with a small touch you can see were the point start to open now when you see this you can then take it back so it just closes then lock the engine with a wedges ether side of the chain were the clutch is or put it in gear , lock the back wheel once you have no movement anywhere you now hold the point end with small spanner and do up the ADT Sleeve Nut . you feel the first threads then it get tight, keep going it will come slack now your on the second thread this with lock up as you tighten it up , you are now time up you can then check every thing first to see if its right you should have the points open at 32 degrees Before top dead center fully advanced and 8 Degrees after TDC if you have this you now can fit every thing back and do not forget to take out the small wedge or bolt to keep open the ADT. Yours Anna J

Anna, your instructions seem all wrong to me. Having got the piston to the required degree (whether 32 degrees, or 28 as Gordon prefers) the crank should not be moved until the timing has been set. Having wedged the auto advance,at the fully advanced position, the points carrier is rotated, until the contacts just open, Not the engine. I can see no reason to lock the back wheel, the amount of slack in primary, and rear, chain would not lock the crank with any accuracy. Apart from all this, the timing instructions are quite simple to follow. The original question was not "how to do the job", but, "what is the correct timing, in degrees before t.d.c.". + or - a couple of degrees probably will not make much difference under normal conditions, so, maybe 30 degrees may be a good starting point. John.

Previously wrote:

Anna, your instructions seem all wrong to me. Having got the piston to the required degree (whether 32 degrees, or 28 as Gordon prefers) the crank should not be moved until the timing has been set. Having wedged the auto advance,at the fully advanced position, the points carrier is rotated, until the contacts just open, Not the engine. I can see no reason to lock the back wheel, the amount of slack in primary, and rear, chain would not lock the crank with any accuracy. Apart from all this, the timing instructions are quite simple to follow. The original question was not "how to do the job", but, "what is the correct timing, in degrees before t.d.c.". + or - a couple of degrees probably will not make much difference under normal conditions, so, maybe 30 degrees may be a good starting point. John.

Well it all works out for me And I have had Norton's for 40 years And the Correct Timing is 32 Degrees Before TDC Fully Advanced , For machines fitted with a Magneto , Electronic devices have a different setting !

Anna, just because the 'correct' ignition timing was written in a book 50 years ago, it does not necessarily mean that it is correct today. You should really read some of Harry Ricardo's papers on the subject. He has internal combustion pretty well sussed years before either of us were born. My point is that compression ratios (because low compression Atlas pistons are no longer available) and fuel octane ratings have changed since the Norton manuals were written and these both have a significant effect on maximum possible ignition advance before detonation or pinking occur. Which is why 32 degrees advance for a Norton Atlas with Commando pistons is inappropriate. Gordon.

Previously wrote:

Anna, just because the 'correct' ignition timing was written in a book 50 years ago, it does not necessarily mean that it is correct today. You should really read some of Harry Ricardo's papers on the subject. He has internal combustion pretty well sussed years before either of us were born. My point is that compression ratios (because low compression Atlas pistons are no longer available) and fuel octane ratings have changed since the Norton manuals were written and these both have a significant effect on maximum possible ignition advance before detonation or pinking occur. Which is why 32 degrees advance for a Norton Atlas with Commando pistons is inappropriate. Gordon.

Well Gordon There is something called a Compression plate comes to mind And what was good for the goose is good for the gander , or what was good 50 years ago will still be the same now the Norton motor as not changed and you can get Atlas pistons or BHB pistons the originals, if You look for them 650 Pistons BHB are even harder to get But I managed to get some and the piston ring to , they are out there so try looking in the right places. and I have no trouble with Fuel Octane in my bike it does not pink , the only thing about ethernol fuel is it take the lacquer off the tank if it get on too it ! so be careful when you fill up and always put the bike on the Main stand and Not the Side Stand like I did , and had petrol running down the side of it witch took the lacquer off the side . Now I have a tank to respray. well we all learn the hard way yours Anna j

As you say Anna, the Norton motor has not changed - except of course for the lack of original dished pistons for the Altas - but petrol has. Octane ratings are now rather lower and the means of measuring octane ratings has changed too, so modern 97 octane is pretty much the same as old fashioned 94 octane. Which is why I maintain that what was originally correct for ignition timing is not necessarily correct now. And yes, I know about compression plates too. I fitted one to my son's 88 after he melted his pistons using unleaded petrol on the Autobahn. Incidentally, is your secret special locking tool the wedge or the spanner? I would really like to know. Gordon.

The genuine BSA extended bolt is slimmer than the pattern ones which are available. The solution? Take the genuine one from your BSA twin and put it on your Dominator, then fit the fat pattern one to your BSA. It worked for me!

Internal combustion petrol engines need the ignition retarded from factory recommendations if the fuel octane is lower than they list or if higher compression pistons are used. This is a basic fact with car and bike engines so I'm with Gordon on this. Back in the 1960s I had 9.25:1 pistons in my Dommie 99 and it would pink if I used less than 100 Octane petrol. (I normally used "101")

Ignition timing is by trial end error. Start with a 'standard' base setting and once the engine is warmed up, overload it in top gear to see if you hear it knocking. If it does, retard the ignition a bit and try it again. Setting the ignition too far advanced for the conditions will destroy it. Old car manuals used to tell you about the trial and error method of finding the right ignition timing. I would recommend no more advanced than 30deg if using unleaded and with HC pistons you'll probably need 28deg or 'lower'.

If you've ever seen the damage it can do you would know it makes sense!