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The article relates to the AJS 16M, 16MS, 18M, 18MS and Matchless G3, G3L , G3LS and most pre-1950's four-strokes.

This task is quite easy once you've done it once and no special tools are required with the exception of a small spanner (like an M5).

It is VERY important set your ignition timing right, so follow this carefully!

Step 1 - Align the piston Top Dead Centre (TDC)

Take the cap of the magneto points (just so you can see which way they rotate!). Put the bike in 4th gear whilst on the main stand. Remove the spark plug and the tappet cover (marked 'A' on the picture above). As you rotate the rear wheel in the normal forward direction, if you put your finger in the plug hole you will feel the compression build on the compression stroke. (make a note of which way the magneto arm turns while you are moving the rear wheel forward)

Once you have it roughly at the top of the compression stroke, take a drinking straw and poke it in the spark plug hole. Make sure it doesn't fall it, although it shouldn't if the engine is on it's compression stroke as the piston will be near the top. Push the straw in until it touches the piston and keep the straw as vertical as possible.

Rock the rear wheel back and forth until you can see when the piston pushes the straw as high as it will go. Once you are happy that the piston is Top Dead Center (TDC) move on to the next step for now.

Step 2 - Loosen the magneto drive sprocket

Click on the image above to enlarge. Remove the screws that hold the timing chain cover on. On the Matchless the timing case in a mirror image with the magneto behind the cylinder.

Once you have the case off you will see two sprockets as shown in blue. Undo the nut holding the top one on. You don't need to take the nut off, just undo it to allow 3-4mm in slack.

No pull off the sprocket. You may need a fine 3 point gear puller (or grind a puller down) or you can use 2 or 3 tyre levers at once. Be VERY careful not to break the back of the timing chain housing as it is very thin aluminium! Eventually it will pop off and be stopped by the nut you just slackened off.

The reason you do this is because that sprocket is linked to the crankshaft and the position of the piston. This enables you to rotate the magneto independently of the piston to set the timing.

Step 3 - Check the tappet gaps

Repeat the latter part of step one to get the piston at TDC as it may have moved while you were slackening the timing sprocket. Once you have it as accurately TDC as you can using the straw, you can check the tappets. When the engine is perfectly on TDC you should be able to rotate both push rods by hand but with no vertical movement. This is quite a fine adjustment if not.

If you can't rotate them by hand or they have vertical play (at all) then you need to adjust them but altering the two nuts on the tappets at the top of the push rods. You can do this with two small spanners. Again; set them so that you can rotate the rods but ZERO vertical play. (if there is vertical play you will get a tapping / tickling sound when the engine is running and it may burn your valves).

Step 4 - Position the piston Before Top Dead Centre (BTDC)

Now with the engine perfectly at TDC and the timing sprocket loose, mark your straw and a reference on the cylinder barrel. Now take out the straw and measure 1/2 Inch on the straw ABOVE the TDC mark. Make the mark a couple of mm less to allow for error. 1/2 Inch is standard for most vintage AMC bikes like the AJS 16M, 16MS, 18M, 18MS, Matchless G3, G3L, Norton 16H, BSA etc.

Now with the straw back in the cylinder, move the rear wheel BACKWARDS until the 1/2 inch mark is now level with your cylinder reference mark. The piston is now at the correct firing point of 1/2 in Before Top Dead Centre. (BTDC)

Step 5 - Set the contact breaker gap

Now set the contract breaker gap to to 12 thou (0.3mm) by using the adjustable nut. You need a very small magneto spanner or jewellers spanner. You can buy them off ebay here: Ebay Spanners.

The gap is important because a bigger gap has the same effect as advancing the timing, so making the spark occur earlier and the piston could be in the wrong place, causing loss of power or damage.

Vintage four stroke ignition timing

Step 6 - Set the contact breaker timing

Now is a good time for a cuppa and a chat with a passer-by who likes the look of your vintage motorcycle...Luckily you can tell him that you know what you are doing because you had the foresight to make a not of which way the magneto contact breaker rotates when the engine is turning forwards.

It often turns towards the advance / retard cable entry but if you weren't paying attention and forgot to write it down no worry because if you studied my picture above you will notice an arrow on the casing!.

If you look at the breaker you will see the timing disc behind the breaker. Notice the lump on it? that's what causes the points to open, which is when the spark occurs. Now set the advance / retard lever to FULLY ADVANCE.

People often wonder as I did how you tell which way is fully advance for the lever. Well the answer is to look which way the magneto rotor turns then move the lever and which ever way cause points to open earlier is advance. This can be seen by the lump on the timing disc moving opposite to the direction of rotation of the contact breaker.

Now rotate the breaker by hand until it opens and then inset the thinnest thing you can find like cigarette paper or film. Rotate the breaker until it closes and grips the paper. Now with the ignition lever in fully advance, VERY SLOWLY rotate the breaker rotor whilst gently pulling the paper until the point where it just lets go. That is the point the spark occurs and that is your timing point.

Step 6 - Tighten and check

Nip around to the timing sprocket side and with a big socket over the loose nut, give the magneto sprocket a gentle tap to put it home on the tapered magneto shaft. Tighten up the nut a little and RE-CHECK THE TIMING AGAIN. Using the socket helps minimise movement when pushing it on by hand as does the fact you didn't take the nut right off. As a tip to increase accuracy make sure the timing chain was tight in the direction of rotation before you pushed the sprocket back on.

Put the straw back in and the piece of film in the breaker and then rotate the back wheel while pulling the film / paper. Watch the straw and when you see the BTDC mark line up you should feel the paper let go. If they match, you are spot on and good to go.

Tighten up the sprocket nut, put the covers back on and start her up. The engine should now run perfectly (from an ignition perspective at least!).

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David Crook   12/03/2019 at 09:04:02
My M18 does not have manual advance/retard instead has bob weights in front of the mag drive sprocket. Question is do I use full advance to set the timing ?......Sunny Days.......David
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roger bryant   04/07/2018 at 11:43:08
very informative
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rob   22/04/2018 at 21:19:30
are the settings the same for a matchless g3c comp, tdc etc cheers
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nicholas gagola   24/12/2017 at 00:45:41
Brilliant article, just what I needed to read.
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MARTIN   06/04/2017 at 07:42:21
Hi Julian, Great and helpful article. I am a complete beginner on this big singles ,just a couple of points do you know where i can get a puller to fit this pinion also do you have to take the mag chain off to get the puller on. Also at TDC there is no tappet clearance on inlet valve and exhaust is quite slack do you know what size spanners are required and can you buy these off the shelf. Kind regards, Martin.
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k bennett   16/08/2016 at 21:03:45
Hi is it the same for a 500cc matchless g80 1953
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 - Julian   16/08/2016 at 21:08:14
Hi yes, as per the AJS 18M and MS which are both 500cc

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