Workshop Guide to Flywheel Magneto Timing

Workshop Guide to Flywheel Magneto Timing
By Steve Cooper, VJMC Editor

There are a hundred and one ways to compromise a motorcycle’s performance but one of the most pernicious has to be timing. Getting that all-important spark to arrive at precisely the correct moment has a dramatic impact upon the bike’s power delivery. If you check out a manual for your bike, you will often see a figure quoted such as 1.4mm +/- 0.05mm BTDC. This means the plug has to fire 1.4mm before the piston reaches the top of its stroke, and our degree of variance either side of that figure is just 0.05 of a millimetre. There are various methods for getting the spark to the plug, but here we’re looking at one of the most basic and reliable systems; the flywheel magneto.

The Japanese took this method of ignition to their hearts because it is so simple, elegant and robust. It can be mass produced at minimal cost and takes very little in the way of clever equipment to set it up, with a surprising degree of accuracy. It’s only real limitation is the rotating part’s mass. Whilst perfectly okay on a small engine, operating at low to moderate revs the iron magnets within the rotor’s body may play merry hell with the crank, its seals and bearings if taken to high speeds. Large rotating masses on the ends of crankshafts are never a good idea. For this reason, the flywheel magneto is normally confined to smaller engines and singles. Honda C50s, Kawasaki & Yamaha trail bikes, Suzuki AP50s and Yamaha Fizzies all use the same basic system. In this instance we look at a typical example, in this case fitted to our recent restoration: Project Chappy.

We'll first need to remove the cover(s) surrounding the flywheel before we can check the timing. Some bikes have a cast outer engine cover, along with a pressed steel inspection panel and manuals often show just the latter removed. To make life easier and give unfettered access, we're going to take the entire cover off. We use an impact driver for any stiff or rusted case screws as this will generally give a positive result, whereas a worn-out Phillips driver is simply likely to further butcher the screw head, causing more grief and hassle. The spark plug is also removed so that we don’t have to fight with the piston’s resistance to compression.

Now we have the cover off, there are four holes in the flat face of the magneto; two large holes for access and two smaller holes that will allow us to grip or hold the flywheel, should we need to. To set the timing correctly we need to locate the static timing mark, which is normally fixed inside the flywheel on a peg or pointer that’s part of the condenser. Therefore, it’s imperative that we have either a genuine condenser or a pattern item of known quality with the same peg.

Making sure everything is clean and accessible, the main connector from the magneto is unplugged and connected to a battery (6v in this case). We’re using a 6v 21w indicator bulb in an old automotive holder on this job, but realistically anything which safely holds the test light away from an earth and is bright enough to see works. With the battery earthed to the engine case via a crocodile clip (or similar), the other end of our test circuit is attached to the flywheel magnetos live side. This is normally a black wire or a black wire with a coloured tracer.

Here we’ve got everything connected and the bright bulb confirms that the points are open. On this engine and many others there are two marks etched onto the rotor; one indicates where the points open and the other indicates where the points close. These are our two 'datum' or 'reference marks' and the latter is the key area for our purposes. A workshop manual will highlight what’s what and where’s where, but with our simple test lamp and battery arrangement it should be obvious which mark is which; if it isn’t it could be either having the wrong points installed or some very serious wear.

If everything is working as it should and the engine has stopped with points open (as is most likely) the light bulb should be like the picture above. Next we are going to turn the engine over in the normal direction of rotation and observe the intensity of the bulb. In theory it should go out but in practice never quite does.

The images above show that as the flywheel rotates and the points begin to close, the degree of illumination declines. Looking close up at the picture below, we can see that the pointer on the condenser lines up fairly closely with the mark on the flywheel. The perspective of the camera actually indicates a small discrepancy with the flywheel mark slightly behind the pointer, but viewed head-on they are spot on. It’s worth running through this scenario a few times just to ensure the bulb is at its dimmest. Carrying out this procedure in daylight can be difficult, particularly with a low capacity 6v battery, so either putting something up against the shed window to block out the light or pull the garage door down is always handy for us.

If the timing marks line up then the job’s done, so we can reconnect the loom, put the tools away and make a brew. If not we have to get into the bowels of the ignition. There are two methodologies of getting the timing correct dependant on the way the engine is designed but both address the same issue (i.e. wear on the points faces and abrasion of the fibre heel that bears on the cam mounted inside the flywheel). The first process in our example allows us to access the points inside flywheel (shown below), the adjuster locking screw that holds the points their back plate and the adjuster slot via the large window in the face of flywheel. By slackening off the screw and either opening or closing the point’s gap the timing will be either advanced or retarded. By making a note of the gap and the position of the points closed mark relative to the pointer, we can quickly see which way to adjust the gap; greater or smaller. Most workshop manuals will give a maximum gap and anything bigger normally indicates that the points are beyond their service limit and should therefore be changed. Given the low cost, we often grab a spare set at an auto jumble or bike show for such eventualities.

A second process is used when the points are essentially non-adjustable and here we have to move the back plate or stator around the crank, its key way and thereby the cam itself. Although a royal pain, the process is effectively the same but requires the removal of the magneto, adjustment of the back plate via the stator’s mounting screws and its slotted back plate.

To remove a flywheel magneto from its taper and keyway on the crank we need the appropriate puller… period! The fact that your mate’s brother can remove these devices with a spanner, two tyre levers, a crowbar and a hammer does not mean you should follow his lead. A flywheel magneto may have been cheap for the Japanese to install in 1974 but a replacement one is going to cost now, so spending £7-15 on a puller and not three figures on a new magneto makes more sense for us.

Here we have a typical puller that will provide years of home workshop service without breaking the bank. Genuine factory pullers turn up at shows for sale and some are still available from dealers, but unless rebuilding bikes for a living a generic version will be fine. Before cracking off the flywheel the centre nut needs to be removed and there are various options here to hold it solid whilst the nut is loosened. Canvas or rubber oil filter wrenches can be secured around the body (you can even use a chain wrench), but the surface is best protected with several layers of duct tape. Workshop manuals often detail a special forked tool with a long holding arm that fits snugly into the two smaller apertures of the magneto, and these are very effective at locking it. Although a little bit of a bodge in many people’s book, it’s often possible to screw a soft aluminium adapter into the spark plug hole that stops the piston reaching TDC and thus locks the engine. This is fine if you are 100% certain that the flywheel nut will undo easily and has not been fitted with Loctite or similar. Try this trick on a rusty or abused bike and there’s a good chance you may bend the conrod or damage the engine, so be warned. On our demo unit I’ve used my windy gun or air wrench; the high frequency percussive impacts imparted to the nut mean there’s no need to hold the flywheel and everything comes apart in a gentlemanly manner.

With the nut removed the puller is wound into the internal thread, remembering this is normally a left-handed thread. With everything in place, tighten the puller into the body of the flywheel magneto via the two flats on the puller body and then tighten the bolt. This will bear down on the end of the crank and pull the female taper off the end of the crank. If the unit proves to be reluctant to move, we apply just a little more torque and then hit the end of the bolt with a good sharp crack from a large hammer. This axial shock is pretty much guaranteed to loosen all but the most abused or rusted unit.

Inside we can plainly see the lighting and HT coils left and right, the condenser and integral pointer at the bottom and the points directly above the crank. Just above the condenser and directly below the crank is the lubricator; this is a little felt pad that retains a film of oil and lubricates the cam to prevent premature wear of the points cam. The points cam is integral with the inside of the flywheel and normally requires no attention unless obviously rusted or damaged. Sitting at 9 o’clock on the crank is the half moon key that locates the flywheel precisely to the crank taper; lose this and the engine will never run properly. We remove the half moon key and pop it into one of those very useful magnetic trays; it’s so much harder to lose a tray full of parts than one or two vital bits. On the perimeter of the stator plate at 1 o’clock and 7 o’clock are a pair of countersunk crosshead screws - these hold the plate to the crankcase. In our example this back plate has no adjustment but if we needed to remove the stator or alter the timing via slotted mounts, a quick crack with an impact driver should see the screws loosened. If these screws are damaged we would never be tempted to replace them with Allen bolts or hexagonal headed fittings. The screw heads need to lie below the surface of the plate otherwise they can potentially rub against the magnets or pole pieces within the flywheel causing serious damage.

While we’re in here, it’s worth checking that the wiring is properly routed and free from damage. The coils’ mounting should be free from cracks and should be firmly mounted with no rocking or movement. We also have a good look at the quality of the insulation of the coil’s windings. The odd crack or missing flake is probably just about acceptable but having two adjacent wires without insulation touching is asking for trouble. We would either replace or seek expert advice on whether it’s possible to get them rewound. Although Japanese wiring insulation is infinitely better than the shellac coated rubbish from Wipac or Joe Lucas, it is likely to be a quarter of century old and not in its first flush of youth. Also, we always check the lubricator pad has some oil.

Here we’re adding a few drops of sewing machine oil; not enough to run off the felt pad but enough to ensure the cam and point’s heel don’t wear unduly. Finally have a quick look at the crankshaft oil seal; if it’s leaking we will see oil around the stator and possibly a little weep running downwards. With care, these seals can often be changed in situ.

With everything set up and checked, the flywheel is carefully replaced on the taper, ensuring the half moon key is in place and the nut (and any washer) is fitted and retightened. There should never be any need to add any form of engineering adhesive to the taper. If used, it may make it impossible to later remove the magneto from the crank. Old fashioned engineers will probably add a dab of saliva to the taper for good measure; this promotes a controlled level of corrosion within the taper and ensures a pretty much perfect joint. We then check the motor runs okay, replace the engine covers and enjoy the inherent reliability of a very effective ignition system that doesn’t need the complications of a single silicon chip.

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The information contained in this blog post is based on sources that we believe are reliable and should be understood as general information only. It is not intended to be taken as advice with respect to any specific or individual situation and cannot be relied upon as such.