The problem with using a dwell meter is that, unlike car with points and distributors, dwell affects timing on motorcycles. So you can have dwell exactly on spec and the timing might be way off. And timing is way more important than dwell.
Some background on terms and principles for those who don't know much about old-style points ignitions. Dwell is basically the percentage of time the coils are receiving 12v current, measured as some number of degrees of crank (or cam if the points are cam driven) rotation. The principle of a battery-coil ignition system is that the primary windings in the coils receive current for some time where they reach "saturation" and current flow stabilizes. Then the points open and the sudden collapse of the magnetic field causes a huge voltage surge in the secondary windings, firing the spark plug. The exact spot where the points open and the magnetic field collapses determines the timing.
With old cars with points ignition, dwell and timing were relatively independent. You use a dwell meter to set points gap and ensure the coils received the correct amount of current to reach saturation. (You could also just set points gap, but using a dwell meter is more accurate and easier.) Then you timed the engine by rotating the distributor. Points-gap/dwell and timing were independent adjustments.
Not so with our bikes. When you vary the points-gap/dwell on a SOHC Honda (and almost all motorcycles with points ignition), you're directly affecting the timing because there is no downstream distributor to control timing - the plugs fire when the points open. So while getting points gap within spec initially with a dwell meter is fine, this is just to get things roughly in spec - you have to check timing after setting points-gap/dwell.
Honda does provide some ability to independently adjust timing without affecting dwell by rotating the entire backing plate (with both sets of points), or on some models by providing either individual back plates for each set of points or having one set of points mount on an adjuster and the other be adjusted by the entire backing plate. If you rotate the entire backing plate, you're affecting timing on both sets of points and if you rotate the backing plate on one set of points only, it only affects the timing for that one side.
So there are a lot of things to fiddle with. My experience is that if you have high-quality OEM Honda points, you can simply do the following:
1. Rotate the backing plate(s) to the center of their adjustments.
2. Fit the points and set gap with feeler gauges. This will generally get the timing close enough to be able to start the engine.
3. Use a timing light to set idle timing by using the base plate(s), keeping in mind that the overall plate changes both sides. If the bike doesn't have a separate base plate for one side, then set one side with the base plate and the other with points-gap but be sure to do it in this order.
4. Check advanced timing.
5. Confirm points gaps are within spec.
Almost all the time, with the baseplates in the middle of their ranges, you can get points-gap/dwell well within spec with correct timing. But the key thing is that you always have to time the motor as the last step because changing points-gap/dwell affects timing.
I think inductive timing lights are the only way to go, but I'm tool junkie. A ohm-meter or continuity light will work for low-speed timing, but you can't check advanced timing. Eyeballing when points open/close? Maybe this will get the engine running acceptably to some, but to me this would be like using vice-grips to remove bolts because you're too cheap to buy a proper wrench.
- Mark
