With the advent of electronic/CDI and transistor controlled ignition systems, the standard “old school” points and condenser components are hardly recognized by today’s young bike mechanics. Motorcycle ignitions evolved from vintage automotive ignition systems, which generally used distributors, driven off of the camshaft to send sparks to specific spark plugs, as the engine rotates from compression stroke on one cylinder to the next. Distributor caps had anywhere from two spark plug towers (plus one for the coil wire connection) to eight, depending upon the number of cylinders in the engine.
Many early motorcycle engines were of the single-cylinder design, so crude magneto ignitions running from the flywheel were often sufficient for low-speed operation. Early Harley-Davidson V-twins had a single set of points firing a dual tower coil, which was called a “dual fire” or wasted-spark system. Honda adopted the same type of ignition style for the 360-degree firing Dream engines. Timed sparks come a few degrees before TDC on both cylinders, however only one side is on the “firing cycle” at a time. At the next revolution the compression stroke occurs on the opposite cylinder, then they revert back to the starting point again, taking turns each revolution.
There is no black magic in regards to setting up the ignition point contacts on the points/condenser ignition system. The points are merely a “variable switch” that turns the ignition coils on and off, depending upon the position of the point cam’s lobe orientation. The point gap needs to be set first, at approximately .012” – .016”. This sets up the amount of time that the points are closed, which is measured in “dwell” time. The longer the points are closed, the more “dwell” time is available for the ignition coil to saturate the coil windings with electrical EMF (electro-motive force). To a point, the more dwell time the coil can experience the more energy it can release back through the spark plug wire when the field collapses; and that happens when the point contact set is forced open with the point cam lobe. The cam lobe is just a ramp added to a “base circle” of the cam’s profile. For Honda twins, with dual point systems, the point cam has only one lobe which triggers one set of points at a time, in sequence, as it rotates at half-speed to the crankshaft revolutions.
For Honda Dreams, the point cam has two lobes to trigger one set of points once for each crankshaft rotation. The single, dual tower ignition coil fires once each rotation, through the twin spark plug leads to each spark plug. The dual tower coil secondary side is connected to both spark plugs, which discharge energy back to ground again. One side of the coil secondary is positive grounded through one spark plug and the other side is connected to ground through the opposite spark plug. This creates a spark which jumps from the center electrode to ground, while the energy seeks the other end of the coil to complete the circuit, so the spark jumps from ground to the center electrode in a “reverse polarity” function. You really can’t see this happening unless you put a pencil lead in between the electrode gap. If this is done, you can see the spark jumping in either direction, depending upon the polarity of that spark plug.
Ignition point contacts can carry a few amps of current without burning up the contact faces. When you open the points, to collapse the coil field, there is a strong surge of current which tries to jump across the point faces as the gap opens up. A condenser, which stores energy for a split-second, will capture the energy trying to jump the gap and release it back when the points close again. This allows the point contacts to switch the current on and off, with little or no arcing across the point faces. When a condenser fails, generally the indication is a strong arc across the point faces each time they open and close. Most Honda twin models have condensers attached near-by the ignition coils in order to reduce the amount of travel time for the energy to jump back and forth during the coil’s cycles. The one exception is the 250-305cc Honda Dreams, which have their condensers mounted atop the cylinder head cover with two screws. Twin leads come back from the condenser, matching up to the ignition point contact set and the 12v power coming from the ignition coil’s negative polarity side. When the condenser fails on a Honda Dream twin, then the two repair options are: 1) Remove the exhaust system on both sides, loosen the engine mounting bolts, removing all but the bottom rear set, then allow the engine to pivot downwards giving access to the top of the cylinder head. The two screws may be very difficult to remove due to their proximity to heat and moisture through the years. The second option 2) is to snip the condenser lead to the points and add on an additional new condenser in a convenient location and attach the point wires back together again with the new condenser.
Ignition coils come in varying resistance values for both the primary and secondary windings. Most original Honda point/condenser type coils have 4.5 to 5 ohm resistance values for the primary side. The resistance value reduces the amount of current flowing through the contact faces when the points are closed. When ignition coils fail internally, generally they will “short” windings together so the resistance value drops below specifications. The coil will be abnormally hot during operation and the performance suffers due to the drop in high-voltage outputs under load. The coil will often fire sufficiently to allow the engine to fire and run at idle, but when the throttle is opened up the coil output breaks down as it is required to produce higher voltages in response to the increased resistance across the plug gap when the fuel mixture ratio changes from rich to lean.
To measure the primary resistance, you must disconnect the wiring connected to it, to isolate the coil windings from the rest of the circuit. Using a low-scale ohmmeter check the small primary wires with each meter probe. Check the meter leads by touching each other together to verify that the meter will “zero” out first and then check the coil wiring. If the coil readings are supposed to be in the 4.5 range, any meter error will throw off the readings and perhaps create a misinterpretation of the test results. Checking a 1 ohm coil will be particularly trying when the meter isn’t accurate.
A Honda four-cylinder bike uses a basic Dream ignition system, doubled-up, so that two sets of points fire two double-ended coils in sequence. One set of coils fires the 1-4 cylinders and the other coil fires the 2-3 cylinders, again using the “wasted-spark” system, where one plug is firing a mixture and the opposite plug is firing at the end of the exhaust stroke which has no effect on power output.
As motorcycle ignition systems progressed into CDI (capacitive discharge ignition) and transistor/electronic ignitions, the controls for timing became simplified and the coils used to fire the spark plugs used resistance values down in the .8 to 1.5 ohm range. If a 1.5 ohm coil is used with points and condenser ignition controls, the current increase will cause the coil to overheat and fail quickly. Not only that, the ignition point contact faces will suffer from excessive current draw and either melt the contact faces or both.
Aftermarket coils can bridge the gap, at times. The “Dyna”-brand ignition coils, used for many of the Japanese fours, can be had in either 5 ohm or 3 ohm resistances and provide reliable performance. Solid-core spark plug wire is recommended for high-performance applications, but use of 5k ohm spark plug caps helps to minimize “radio interference” signals generated from these high-cycling ignition systems.
Honda’s 250-305cc twins were built with metal-case ignition coils, up until the mid-1960s. As the production methods improved and technology made materials more useful, Honda switched from the built-up metal coils to molded-plastic coil sets. The coils were riveted to the brackets, making fewer parts to produce and assemble. The coil wires are also molded into the ends of the coil assemblies, so when the wires are damaged or become too brittle to use, the coil has to be scrapped, in most cases. Sometimes, creative use of a Dremel tool around the wire/coil connection can free up the old wire and reveal the sharp tip ends where the wires are attached at the factory. Putting a new wire on the coil connection tip then backfilling the end with JB-weld or RTV/epoxy and allowed to set-up and reused once again.
With the Honda 350-450s coming up on 40-50 years of service, the heat and age the coils have endured is taking their toll on the original factory units, from current observations. In several cases, bikes coming through my shop have needed coil replacements due to failed coil/wire connections and/or plug wires which have become too brittle to use or too short to make the connection to the plugs when the ends are trimmed to improve plug cap connections. In some cases, the resistance values plunged to 2-3 ohms which created the idle-only spark conditions. Replacing the coil with a proper 4.5 ohm coil cured the backfiring and run-on-one cylinder symptoms.
In the end, coils are coils and all you have to do is to match up the resistance values of what you need and mount up whatever fits. A recent CB350F coming in for misfiring issues required a replacement ignition coil. A trip to the local motorcycle salvage yard yielded ZERO Honda twin lead coils, but there were several Suzuki GS750 four-cylinder coils which had the correct resistance values and fit the brackets perfectly. For a price less than half of new, the bike was put back on the road again, firing on all four cylinders, to the pleasure of the owner.
It isn’t rocket science, but you DO have to match up the right coil with the right application if you want to restore the bike to normal performance again. Make sure you get all the ohms you need for best results. If you get a coil that is “ohms poor” then the bike won’t run.