YAMAHA DT175 MX
Ralph Ferrand is on with part nine: timing!
First up, as promised last month, is the ignition timing. This bike came with CDI (Capacitor Discharge Ignition), which was a massive improvement on the traditional points system of the time. The timing of the spark was previously triggered by a cam opening and closing what was effectively a switch. Points were very simple and easy to fix, but were vulnerable to the contact points of the ‘switch’ (points) getting burned by the arcing of the current as the circuit was constantly made and broken, many thousands of times a minute. The heel of the points, which was being pushed by the cam to break the circuit, would get worn, so the gap would change and moisture from rain or even a misty morning would often prevent a bike from starting. Also the mechanical wear in the system would alter the ignition timing.
Understanding CDI is not that heinous and I have drawn a simple diagram illustrating how it works. Basically under the magnetic flywheel is a lighting coil to generate power to charge the battery and run services such as lights, a charge coil to generate power for the ignition system and a pulse coil to trigger the spark at the correct time. When the magnets pass the coils an EMF (Electromotive Force) or current is produced in the coil. This is an alternating current (AC) i.e. its polarity changes depending on whether it is the north or south pole of the magnet passing the coil. A diode is like a one-way valve for a fluid allowing the current to flow in one direction only. To extend this analogy you could think of alternating current as being a flow of water that is going one way and then the other and Direct Current (DC) as being water flowing in only one direction. I have drawn two graphs to represent the voltage positive and negative against crankshaft rotation, for both AC and AC after it has gone through a diode. The flywheel has two magnets and so the graph represents one complete rotation i.e. two cycles per revolution. Every time there is a positive current passing through the diode it charges up Capacitor 1, which you could think of as a little reservoir, rather like a battery, that can’t store much, but can release its payload very quickly. The thyristor is like a valve that holds the voltage in the capacitor until it has a current applied to its ‘gate’ connection, which effectively opens the valve. Like the charge coil, the pulse coil generates an alternating current when the magnets pass it, which is what is needed to trigger the gate connection of the thyristor, though this too has to be converted to DC for this purpose by a diode. When the magnet passes the pulse coil, it generates a current which is rectified by the diode to DC, which
triggers the gate on the thyristor to allow the current from the large electrolytic capacitor (capacitor 1 in the diagram) to energise the primary winding of the ignition coil which causes a very high voltage to be produced in the secondary winding of the coil, which jumps the spark plug gap to return to ground, thereby igniting the fuel/air charge. Because there is more than one magnet in the flywheel, there is a second spark which is a ‘wasted’ spark, just before BDC (Bottom Dead Centre). It only takes a pulse to cause the thyristor to pass current and it will remain like that until the main load current i.e. the charge to the coil drops. This happens quickly as the charge in Capacitor 1 is depleted fast by using its energy to make the spark with the ignition coil, so effectively it is all reset very quickly. The beauty of this system is that there are no moving parts to wear and the only arcing is where you want it – in the spark plug. With points-based systems, the advance and retard is usually achieved with a mechanical centrifugal weight system, often with a couple of bob weights that move out by centrifugal force as the engine speed increases and using a form of leverage turn the ignition cam on the end of the crank shaft. The amount of cam movement (advance) is dictated by the strength of the springs opposing the centrifugal force. As with any moving parts, wear is inevitable and springs lose some of their strength with age (don’t we all). The Yamaha CDI system, on this bike, has an extra capacitor (capacitor 2 on the diagram) after the diode fed by the pulse coil, which charges up and releases the current from the diode. Shown in the second graph, the half-wave rectified DC current, after the diode, drops to zero every cycle (red line). The green line on the graph shows the effect of adding a capacitor to the circuit. As the capacitor charges up on the first part of the cycle, it then releases its charge on the second half of the cycle. There is a time against cycle relationship with the wave form created in this method and coupled with an increase of pulse coil voltage with the increased speed, clever Mr Yamaha has carefully chosen a value of capacitor and the resistor in parallel with it, that gives a steady level of advance of the trigger to the gate of the thyristor as the engine speed increases. The beauty of this system is, that once set up, there is never a reason to alter any of it because with no moving parts there is nothing to wear. Modern ignition systems are hugely complicated being integrated with fuel injection systems and more sensors than you could shake a stick at.
That’s the theory part covered. Had I got the ignition system from the same model of engine, it would have been a simple task of lining up some marks and screwing the stator plate in place. As it was, it gave me the opportunity to play with my genuine Yamaha DTI (Dial Test Indicator) set, which I had never used on a Yamaha before. Firstly, I turned the engine over until it was at TDC (Top Dead Centre). Then I screwed in the spark plug adapter into the plug hole and fitted one of the long extensions to the DTI plunger. I dropped the DTI through the adapter until the plunger touched the piston crown and then continued pushing it down until the little dial, the one that reads whole millimetres, read three and the long needle read close to zero and then locked it in place with the thumb screw. To ensure that it was accurately at TDC I rocked the flywheel until the long needle read its highest, and then turned the bezel to set the pointer at zero. With most four-strokes, which I am more used to working on, timing, both valve and ignition is determined by degrees before or after TDC, but on this little stroker the measurement is determined by the piston position, which I suppose makes sense given it is a piston-ported engine. I then turned the flywheel anti-clockwise until the DTI determined that the piston had gone down the bore by exactly 1.8mm. According to Mr Yamaha, if you’re using new engine cases, you punch a mark on the engine casing in line with the mark on the flywheel. Although mine wasn’t new, the cases were for a later CDI system than the one I’m using, so I marked the engine casing with this position. I then removed the flywheel with my puller and lined the new mark on the crankcase with the mark on the CDI stator plate. I then replaced the flywheel, went through the process with the DTI again and as you can see in the photo everything lined up. At this point I had to put blind faith in the instructions from Mr Yamaha, as much work had to be done, before it would be time to start prodding the kick-starter with my size tens. I buttered the intake face of the barrel with Wellseal and fitted the gasket. The reed block had both its mating faces brushed with Wellseal and this was then installed, followed by the rubber carburettor stub mount, which was held in place by four M6 set screws and plain washers, not forgetting to attach the pipe clip under the bottom right screw head. I then fitted the all-important oil pump in the front of the right-hand engine cover, and connected up the pipes that come from the two small manifolds in the top of the cover. You can’t be too careful with this pump – if it fails when you’re riding, the chances are you’ll be replacing a lot of engine parts and possibly your front teeth! On a day when SWMBO was out I took all the rubber bits into the kitchen for a good old bath in the kitchen sink. A nice soak in hot water with a kiss of Fairy was just the ticket. As always, I then scrubbed the parts with an Asda value toothbrush and dried them with a towel from the workshop, honest I didn’t use a kitchen towel! The huge deep finning on these engines, unfettered, makes a horrendous ringing, so Mr Yamaha jammed a series of little damper rubbers between them, to reduce the noise. They look so much like the liquorice drums from Liquorice Allsorts, it was hard not to give them a quick suck, just to be sure. I gently encouraged them in with a shot weighted, nylon faced hammer. Next month I’ll start by sorting out the carb!