This is a post that belongs to another thread but I had email trouble so I post this reply late: Alan: > start and run the engine with -5 degrees of advance? Yes you can but you will need a very good starting arrangement, like compressed air or a gas generator or a strong spring or such. Forget ticking it over by hand. It will backfire on you every time and start backwards given a chance (2 stroke). Set up so you can set 0 to +3 degrees advance (3 deg. after TDC!) while starting and you can move it back to -5 or more after starting (an extra adjustment beyond throttle and mixture needle). >To account for RPM there are several dozen ways. With only one sensor you >can measure the 'width' of the pulse from the Hall. >So you propose a 'spark firing delay' based on the length of the pulse >from the Hall sensor - this would mean a fixed, known position for the >Hall sensor - would this be at TDC? As Alan Pearce said, it needs to be earlier than the earliest advance you plan to use. Modern ignitions often use two sensors to detect both speed and direction. The device refuses to work if the axle turns backwards at any time. This is a very good idea. See below. >Build an ignition circuit that won't issue a last >deadly spark when you turn it off (since it is on the trailing edge). >What do you mean here? If your circuit is built improperly it will produce an extra unwanted (and untimed) spark either when turning on the ignition or when turning it off. Normal circuits with points & coil can and do that. If the distributor rests in the closed points position when you turn the ignition off you will have an extra spark. Normally this will do nothing but with an older model engine it may cause a big booming backfire, unless the automatic advance moves the firing point slightly beyond TDC with the motor at rest (vacuum driven ones usually do this). You might remember old b/w movie scenes with Oliver & Hardy and old automobiles that made big booms when being started/stopped. The scenes were exagerated but relied on real facts. It is your responsability to plan for this sort of thing. A backfire on a fueled engine will start a carb fire in 3 out of 5 tries and may destroy the carb, not to mention send it flying off and burning you to a crisp ;-). Hint: small fires are as painful as big fires. >PS: BTW did you calculate the ignition power at 20,000 rpm and 20mJ/spark >assuming single spark ;-). Because I did and I will try to run the engine >very slowly ... >Where did you get the figure for the spark energy? Do you have access to >some information on spark ignition or is this just an experienced >estimate? I have access to several old engineering books that cover such things as combustion and ignition among other things. 20mJ is the input energy in some small spark based igniters, like in a battery powered gas lighter or very small gas engine. A car coil is more like described at: http://www.vtr.org/maintain/ignition.html and has some 500mJ per spark. High energy ones go to 6-8J per spark (disregard what they say in that link about the voltage in high energy coils - it has nothing to do with voltage in this case) and require special plugs and special everything. The power required to run the ignition increases very fast with rpm and spark primary energy. 20000rpm and 20mJ on a single cylinder 2-stroke require some 7W. This is a lot of power for a small engine. A untuned car ignition with electronic switching (aka controlled saturation) requires some 80W (4cyl 4 stroke 1 coil 5000rpm). Non-electronic ignitions consume much much more power, especially at low speed. sorry for the long post, but I felt that I owed an answer, Peter -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.