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Difference between revisions of "Ignition timing"
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The reason that this small amount of time is important is because the combustion process is not as much an explosion as it is a controlled burning. The time available for this controlled burning decreases as the rpm goes up. To compensate for this, the point where the spark occurs, to start the combustion process, must occur earlier. To accomplish this weights and springs are attached to the points cam. As the cam spins on its shaft, centrifugal force causes the weights to move and turn this cam on its shaft, advancing the timing of the spark, thus giving the combustion process more time to occur as the rpm goes up. Full advance is usually reached on most engines by around 3000 rpm. | The reason that this small amount of time is important is because the combustion process is not as much an explosion as it is a controlled burning. The time available for this controlled burning decreases as the rpm goes up. To compensate for this, the point where the spark occurs, to start the combustion process, must occur earlier. To accomplish this weights and springs are attached to the points cam. As the cam spins on its shaft, centrifugal force causes the weights to move and turn this cam on its shaft, advancing the timing of the spark, thus giving the combustion process more time to occur as the rpm goes up. Full advance is usually reached on most engines by around 3000 rpm. | ||
Most motorcycle engines with points have only a centrifugal advance. A few have a vacuum advance. Newer engines with electronic ignitions usually have an electronic advance. Some engines have fixed timing with no advance at all. The lower the compression ratio of the engine means more spark advance is needed. This is because rapid combustion is aided by a high compression pressure. Because the combustion process takes place quicker with a high compression ratio less time is needed, therefore less spark advance. Conversely lower compression pressure slows the combustion process, necessitating more spark advance. Now why all this about compression pressure and spark advance? Simply this, as an engine wears out it starts leaking compression past rings and valves. This lowers the compression pressure (ratio). What does this mean? You got it, we need to advance the ignition timing as an engine ages. | Most motorcycle engines with points have only a centrifugal advance. A few have a vacuum advance. Newer engines with electronic ignitions usually have an electronic advance. Some engines have fixed timing with no advance at all. The lower the [[compression ratio]] of the engine means more spark advance is needed. This is because rapid combustion is aided by a high compression pressure. Because the combustion process takes place quicker with a high [[compression ratio]] less time is needed, therefore less spark advance. Conversely lower compression pressure slows the combustion process, necessitating more spark advance. Now why all this about compression pressure and spark advance? Simply this, as an engine wears out it starts leaking compression past rings and valves. This lowers the compression pressure (ratio). What does this mean? You got it, we need to advance the ignition timing as an engine ages. | ||
OK...Great...Now What? Well, if the engine is worn, lots of miles or hard use, we need to advance the point at which the spark occurs. This works best on large 4-stroke engines although it can be done on small ones too, but it is harder to do. I don't do this to 2-strokes. They are just a different breed of engine. I don't do this with flywheel magnetos ( Honda 50's, 70's etc. ) either, because it is a lot of work with that type of ignition system. On many of the newer motorcycles you just can't do it without major modification, but on almost all points ignition machines and a lot of electronic ign. ones you can. I don't do this with engines that have real good compression because they don't need it. On these engines the stock timing marks will probably work best. This is for old, worn, tired, engines. However, I have heard of this method being used with heavily modified engines, to get a starting point in the tuning proses. | OK...Great...Now What? Well, if the engine is worn, lots of miles or hard use, we need to advance the point at which the spark occurs. This works best on large 4-stroke engines although it can be done on small ones too, but it is harder to do. I don't do this to 2-strokes. They are just a different breed of engine. I don't do this with flywheel magnetos ( Honda 50's, 70's etc. ) either, because it is a lot of work with that type of ignition system. On many of the newer motorcycles you just can't do it without major modification, but on almost all points ignition machines and a lot of electronic ign. ones you can. I don't do this with engines that have real good compression because they don't need it. On these engines the stock timing marks will probably work best. This is for old, worn, tired, engines. However, I have heard of this method being used with heavily modified engines, to get a starting point in the tuning proses. | ||