Mark Haycock on the nuances of this necessity.
Ihad a question for the Q&A section from a reader recently where the subject was ignition timing, specifically how to check it on a bike with no timing marks, so I thought it might be an idea to look at the whole subject in a bit more depth. Firstly, a bit of background: we all know that a bike engine works by burning a mixture of petrol and air and if (like me) you have encouraged a feeble bonfire to perform adequately by using petrol, you will appreciate that the burning can be very fast indeed. It did work, by the way, but I didn’t repeat the experiment! You might think that the petrol/air mixture explodes in the engine cylinder: that is not quite the case. Under some circumstances the mixture can detonate, but this is very harmful to an engine and needs to be prevented. The burning process does normally take a finite time, measured in milliseconds (i.e. a few thousandths of a second). If the spark, and thus the start of combustion, were to occur at top dead centre (TDC), by the time the pressure had increased significantly, the piston would already be descending and some of the potential power output would have been lost. So the spark is set to be before TDC, so that the maximum pressure reached occurs a few degrees after TDC. One thing that complicates matters is that in contrast to an industrial engine, a vehicle engine’s speed varies and the timing setting needs to accommodate this by increasing the advance at higher revs. On top of that, the air/fuel ratio varies in operation and it has been found that richer mixtures burn quicker than weak ones, so they need less advance. Apart from those things, throttle position (and thus engine output), atmospheric pressure, temperature and humidity level, fuel composition and God knows what else have their own effects, so it is not a straightforward issue. In days of yore, ignition timing could be set by a handlebar-mounted lever. It would be normal to retard the timing to prevent
any possibility of the engine ‘kicking back’ i.e. firing too soon and suddenly reversing, violently forcing the kick-start upwards. The rider would then advance the timing for normal use but could make fine adjustments depending on how steep hills
were, the quality of fuel and other things. So, it was a great advance (ahem) when an automatic advance system was invented. This relied on a couple of weights flung out against spring pressure by centrifugal force, with greater rotational speed producing more advance, which was just what was needed. Photo 1 shows a typical auto-advance unit. The timing changes for changes in mixture strength could be accommodated by the use of an arrangement called a vacuum-advance unit. A small pipe from the inlet tract led to a diaphragm, which was connected to a lever that could alter the timing. During cruising when a weaker mixture was used to promote fuel economy, the throttle was scarcely opened, so the inlet vacuum was high and the timing was further advanced. This set-up was fairly rare on bikes, probably because carburettors were not set so weak for cruising. Of course, later the mechanical devices were replaced by electronic equivalents that did not wear and thus were more accurate. Later still, the control came from an engine management system, which incorporated fuel injection and ignition production and control. So how do you know what is the best ignition timing setting to use? Whatever system you have, you can be sure that Japanese manufacturers will have spent many hours testing engines under a wide range of conditions to come up with the best compromise between power output, economy, rideabilty, engine life, reliable starting and other factors. Consequently, it is usually the case that it is best to stick to their recommended setting. However, if your engine, carburettors or exhaust system have been modified you ought to carry out your own experiments to find out the new correct settings. Unfortunately, most of us don’t have a fully-equipped
laboratory with dynamometer in the back garden, so you will need to contract this work out. However, experiments can sometimes be carried out on a more informal basis and I do remember reading a report in an American magazine in the early Seventies about a series of tests they carried out on a CB750. It gradually increased the ignition advance in steps and, after each adjustment, measured the standing quarter-mile times as a simple way to gauge at least an idea of power output. The magazine found that the times did improve, up to a certain point, but the fuel consumption increased correspondingly. I would not recommend this idea though, as the increase in advance is accompanied by an increase in maximum gas pressure in the cylinder. Eventually, the mixture will start to detonate and produce a noise usually called pinking or knocking. This hammers the top of the piston and will eventually
break it, with expensive results. The danger is that it is not always possible to hear the pinking, particularly at high engine speed and high power output – just when maximum damage is occurring. This is especially crucial on high-performance two-strokes. Modern engine management systems often incorporate a special sensor that, in effect, continuously picks up the sound of combustion and therefore detects the start of pinking. It gradually retards the timing to just prevent it, which generally provides the best performance. But you don’t have anything like that on your old RD350 do you? That is why you need to take special care to get the timing correct. So now I hope you know what is meant by ignition timing and why the setting is important, so next time I shall move on to the workshop to check how the setting can be checked using two techniques on a couple of rather different bikes. cmm
The vacuum advance is powered by the silver diaphragm unit.
The Eighties saw solid state ignition units: this from a VT500.
Auto-advance unit: 1970s.
It’s all in the books you chucked out years ago.
You could try increasing the advance on your old CB750…
Modern(ish) and digital.