Ignition timing setting

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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Ignition timing setting

Postby BruceB » Thu Apr 20, 2017 11:21 am

I set the ignition timing on my 59 650 as per the instructions in the manual. I would normally set it dynamically using a timing gun, however although I have the setting for full advance of 12 degrees from the dizzy I cannot find the I tial starting setting to work out the overall advance.
I reckon it's possible to do dynamically timing with the oil pump cover off, or has this been tried and it's practicable?
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Re: Ignition timing setting

Postby clive » Thu Apr 20, 2017 11:51 am

BruceB wrote:..... however although I have the setting for full advance of 12 degrees from the dizzy I cannot find the I tial starting setting to work out the overall advance................


??
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Re: Ignition timing setting

Postby G15 Roy » Thu Apr 20, 2017 11:55 am

Try one old fashion Way Works ever time.

500cc and 600cc - 3/8 inch, 39 degrees, before top dead centre.
650cc - 11/32 inch, 35 degrees, before top dead centre.
A timing disc can be used, and may be more accurate, but it involves removing the outer primary chain case which is always a pain. Therefore, the details are solely for the spoke through the spark plug hole method.


Setting piston in correct position:
Put the bike on centre stand so that the rear wheel is off the ground, select top gear.
(Only if the engine uses a magneto). Remove timing side cover and release the gearwheel from the magneto taper using special extractor. Then do up the nut holding the wheel on to the taper finger tight.
Remove the spark plugs. Rotate the engine using the back wheel until compression can be felt, using your thumb over the spark plug hole, on the right hand cylinder.
Take an old spoke, bend it at one end so that it will not drop into the cylinder, holding it as vertical as possible, and first rotating the rear wheel forward and then just rocking the wheel set the piston at its peak height in the cylinder (top dead centre).
Make a mark with a spirit based pen on the spoke that lines up with a feature on the cylinder head, like the spark plug hole.
Remove the spoke and scratch a permanent mark on the pen mark using the edge of a file.
Lightly scratch a second mark the required distance from the first away from the end that went into the cylinder. Check the distance and correct it by adjusting the direction of the file cut.
Check the top dead centre position using your marked spoke and then turn the rear wheel backwards, clockwise from the left side, until the second mark on the spoke drops just below the feature on the head, then turn the rear wheel clockwise (this takes up any backlash in the gear train) until the second mark lines up with the feature on the cylinder head.

Magneto models:
Set the advance and retard control on fully advanced: The cam ring should have rotated to its extreme anticlockwise position.
Remove magneto end cap. Insert clean 0.0015 inch feeler gauge between the contact breaker points. Traditionally a cigarette paper is used instead of a feeler, a modern equivalent is aluminium cooking foil.
Rotate the magneto armature (contact breaker assembly) clockwise, looking from the left side of the machine, until the contact breaker is about to be opening on the bottom ramp of the cam ring. Continue very slowly until the feeler gauge is released.
Tighten magneto gear wheel retaining nut.
Check the timing by turning the engine until the contact-breaker is in position given in 3. above. Insert the feeler gauge and slowly rotate the engine forwards until the gauge is released. The second spoke line should be in the same position.
Replace timing side cover (using a non-setting gasket cement such as Hylomar or Golden Hermetite), magneto end cap and spark plugs.
The ignition lead from the rear magnet pickup should go to the right hand cylinder head.
N.B. There should be no need to remove the contact breaker assembly from the magneto armature. It is driven through a taper and is located by a fixed key in that taper. This, and the mating keyway can easily be damaged.
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Re: Ignition timing setting

Postby BruceB » Thu Apr 20, 2017 12:10 pm

The timing is set, I did it the old fashioned way by finding tdc and setting 11/32 before tdc as described. The figure of 35 before tdc seems excessive, are you sure it is correct, would be expecting this at full advance?
The 12 degree figure I quoted comes from the markings on the advance stop inside the dizzy
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Re: Ignition timing setting

Postby zwarts » Thu Apr 20, 2017 3:09 pm

The ignition timing for the 650's is as advised - 11/32" BTDC (35 deg.) on FULL ADVANCE. The figure you are quoting of 12 deg (= 24 deg @ the crank) is the RANGE from full retard to full advance permitted by the distributor bob-weights and stop mechanism. So, if you set the timing at 35 deg on full advance, the retarded setting ( for starting/tick-over) would be 35 - 24 = 11 deg BTDC. As you are using a distributor it is very easy to set this precisely and by moving the distributor body in very small stages to also make minor adjustments and optimise the timing to suit the fuel in use. Figures in old handbooks etc. are a good starting point but engines vary e.g. with changes in comp. ratio, carb settings and currently available fuels are vastly different to those available in 2000, let alone the late 1950's.

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Re: Ignition timing setting

Postby BruceB » Thu Apr 20, 2017 11:02 pm

That makes more sense, 35 degrees BTDC did seem excessive. The 12 degrees I quoted is the mechanical advance I mentioned from the stop inside the dizzy.
I ended up setting mine at 11/32 btdc when at full retard as per the manual guidance. The manual does not say that this should at full advance and does not mention anything about setting at full advance using this 11/32 figure. This would be very difficult to do statically anyway unless you can manually advance the weights against the stop and lock in position.

So just to clarify this, if I set my timing at 35 degrees btdc (11/32 btdc) using a timing gun at say 3500rpm (ie full advance) then it would be correct.

Otherwise if done statically it should at 11 degrees btdc with the advance mechanism fully retarded (ie stationary) so when fully advanced the timing would be 35 degrees btdc

This is not what any of the manuals I have say but makes more sense as far as the actual btdc numbers mean in normal use on any other vehicle I have had.
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Re: Ignition timing setting

Postby zwarts » Fri Apr 21, 2017 9:16 am

Just to clarify - yes, 35° BTDC is the correct setting at full advance. It is not excessive. I have run AMC twin engines with 39 - 40° of full advance back in the days of leaded 4 star fuel.
If you set your timing to 11/32" (35°) on full retard then it would be at 35 + 24 = 59° when fully advanced. That would be excessive. I don't know which manual you have but some confusion seems to have crept into the process. If you set the timing to the correct fully retarded figure of 11° BTDC then it is very easy to check that the fully advanced figure is also correct. Set the crank to 35° BTDC (11/32"). Then simply move the points cam against the bob weight spring force using the rotor arm - the points should just be opening.
All this just gives you a setting to a figure from a book. Be prepared to make fine adjustments once it is all up and running - depending on your level of skill and mechanical sympathy with how the engine is running.
Should you wish to check with a timing strobe light you will need to make some timing marks. With the 18D2 distributor and the original bob weight/spring combination, full advance would probably be reached somewhat earlier than 3500rpm. Personally I wouldn't bother, these engines are not super-critical to slight changes in ignition advance and it's a job best done by 'feel' and experience.
One other thing to check with the 18D2 is that you have the same timing on both cylinders. Don't assume this to be the case - the cam spindle is of small dia. and often damaged through rough handling by previous owners. The centrifugal A/R mechanism can become 'sticky' (lack of maintenance) and is very prone to wear. The earlier Lucas DKX series are much more substantial items but they were never fitted to AMC machines (although there's no reason why a DKX2 could not be retro-fitted).

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Re: Ignition timing setting

Postby BruceB » Fri Apr 21, 2017 9:37 pm

Something just doesn't add up for m here. How does 11 degrees plus 12 degrees advance add up to 59 degrees? I may be missing something but this is 23 degrees btdc in my eyes?
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Re: Ignition timing setting

Postby Duncan » Fri Apr 21, 2017 9:52 pm

Bruce you may be overthinking this set it at full advance with light pressure on the rotor arm to advance it, and then go for a ride, it will be okay.

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Re: Ignition timing setting

Postby BruceB » Fri Apr 21, 2017 10:51 pm

I printed off the instructions from two different manuals and followed them exactly, the 11/32 btdc setting used was not mentioned in either manual about it being at full advance when setting and the bike didn't run very smoothly last week when tried, hence the questions to confirm the actual settings.
I am happy enough doing it at full advance if that is the setting but there just seems to be a lot of differing opinions out there as usual and the manuals are poor.
I assume that the difference between the 12 and 24 degrees is the half speed of the dizzy compared to the crankshaft? although my car experience would have marked it as 24 degrees on the dizzy as the actual advance on the crank rather than 12 degrees advance on the points cam, with a doubling to allow for the gearing down to the crankshaft. Another quirk to be aware of.
I guess I am happier setting at an actual degree setting than the 11/32" btdc, it just fits better with my classic car experience. The setting was only disturbed as part of my complete rewire and the fitment of electronic ignition, which incidentally was defective and is having to be returned right out the box!
I need to sort my tank first now so have a little time to get it sorted out properly. I am thinking ahead as I will probably put actual marks under the timing cover so I can dynamically time next time when the electronic ignition module is refitted once I statically time using the points initially.

I am quite happy doing all this and have built many a BSA bike or MG car engine as well as fully maintaining modern bikes and cars, its just getting the marque specific info and quirks for Matchless twins sorted, each manufacturer is slightly different and I am still getting to grips with this particular bike and its idiosyncrasies.
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