More Spark by Brian Winch
In the December issue we began our look at converting glow engines to spark ignition the next stop in our investigation being to understand the need for timing adjustment.
Now during my younger days, when I was hopping on and off of every type of motorcycle I could obtain by buying, exchanging or borrowing, I somehow ended up with a large, old Harley Davidson. A great old bone-shaker that ran quite well, but you had to be right on the button when starting it. The kick-start had a bicycle pedal instead of a bar for pushing down with your foot, and there was a lever on the handlebars to control the ignition - you could advance or retard it as required; for starting it had to be in the full retard position otherwise that kick-start would bounce right back like a rabbit trap and imprint a message into your shin. The message was -'you bonehead, you forgot to retard the ignition'. Well, I gotta tell you, that pedal really hit hard and I still bear the scar. I was a fast learner, though, as it took only one shin bashing to remind me - always - to retard the ignition lever and hold it retarded whilst kick-starting the beast.
A couple of my early cars had the same type of lever on the steering column, and you had to remember to set it in the retard position whilst crank-starting the car (by hand). The spark lever on spark ignition model engines of the time was adjusted in similar manner, or you'd cop a goodly whack on the fingers when the prop flicked back rapidly due to the ignition being too far advanced for starting. Experience taught you to fine tune the ignition to suit the propeller load. When using a very large propeller you'd trim the ignition towards the retarded position, and gauge the result by ear - you could hear the engine running smoothly when the adjustment was correct.
Back to the cars and motorcycles. The cars of that era weren't overly generous with horsepower, and this coupled with the oft-horrendous roads and steep hills meant that you needed to maintain control over the ignition, using full advance for smooth, low-load touring, bringing the lever back to the retarded position when a steep hill was encountered (particularly when you had the entire family aboard). It was a boon when the automatic advance system was incorporated in car ignition. In one respect the changes in modern vehicles are very similar, except that all the changing and movement is now done by jelly beans (jelly beans - my overall description of the small and often coloured components on a circuit board of an electronic device).
As mentioned above, tuning early spark ignition model engines required attention to the timing lever - retarded to start, then advanced to the premium setting according to the propeller load. Well, the good news is that we can - almost - dispense with this when using modern CDI units, as they take care of that need. Generally speaking, the range of ignition control is well within the range of propeller sizes you'd normally use on the engine, however, even this can be varied, and this is why I said 'almost' in the previous sentence.
A little information on a (sadly no longer manufactured) all-British engine - the Merco (this is related to our subject matter, so bear with me!) Merco produced quite a range of engines with several innovations that were quite good and, probably, a little ahead of the times. Such things as a three-needle carburettor, two glow plugs and, even before it became essentially fashionable, effective exhaust mufflers. The engines were available in all configurations: diesel, glow and spark ignition, and the 10cc (.60) version of the latter had a feature that would certainly appeal to some modellers these days - a very small electronic ignition system. The recommended propeller for initial running was a 13 x 6", which is at the large end of the range for 10cc if it was glow ignition, but not a problem for this petrol engine due to the automatic advance/retard ignition control. Just behind the carburettor was a small micro switch and, on the throttle rotor, a plastic cam plate. If you wanted to run 16" or larger propellers (remember, this engine was only 10cc) you adjusted the cam plate so that it closed the micro switch, and this action changed the timing of the engine so the ignition range was retarded to cope with the increased load and to prevent the engine overheating and / or pre-igniting (pinging). So here we are with a 10cc engine capable of swinging 16" props and larger - 20 to 25cc engine props - just by simply retarding the ignition timing. For checking purposes I used an 18 x 4" narrow prop on the engine, and it ran quite well without overheating or any signs of distress.
Now for my aforementioned 'almost' - this will be to our advantage. When we get to setting up the sensor for the conversion, I'll show you a method or two to allow you to have this broad ignition control if you want to use larger-than-normal propellers on your engine.