® Optimizing the Standard SOHC4 Ignition System
(Or, Way More Than You Wanted to Know About Honda's Point Ignition)

2014 update: This article was written almost fifteen years ago. In fact it was one of the articles I first built my website with in 1997, and was probably on my first Angelfire page before that. The concept and viability of this procedure is no less valid than it ever was, and in my mind even more important with so many people buying these bikes todsy. The only thing that has changed is the original equipment TEC and ND points are very hard to get now. Even the new ones Honda dealers sell under Honda part number are junk today. The original points are out there, they just have to be bought used. In case it is lost to anyone, the point of this article is the stock point system is not only plenty adequate, but is actually a very good performer once it is set up properly, equal in every respect to any Dyna S type system. But it needs to be set up. At the very least this means dialing things in as explained here. The next level up in optimizing would be to double-spring your points, and also fit GL1000 ignition coils (or good aftermarket ones).

Replacing the standard points plate with an electronically triggered setup is probably the most common Honda SOHC Four upgrade. This is probably due to the stock system's periodic maintenance requirement, which many feel is more trouble than it's worth, as well as the ignition's presumed inaccuracy. Well, much of what people think is untrue. The stock system is designed to be quite accurate, but due to uninformed service technique all too often does not function up to its full potential. Once it is set up correctly however, the system is a match for even a modified engine, and requires very little maintenance. This brief look at the SOHC Four ignition examines the correct set up of the system.

The stock battery/ points ignition has only one real flaw: its collapsing field design. Unlike most ignitions, the "Kettering" system (named after its inventor, Dr. Charles Kettering, the founder of Delco and father of the electric starter) goes through two stages before firing the spark plug. First it magnetically saturates its coil, then that magnetic field is made to collapse before the spark plug fires. Because of this longish process, some of the spark voltage bleeds off before the plug fires, resulting in a less intense spark. This is an inherent weakness of the collapsing field design. But, there is at least a partial way around it. Careful techs discovered long ago that setting the point gap to the minimum end of its range (or even a little under) increases the performance of the Kettering system. The time the points are closed ("dwell") is increased, and coil saturation time and output voltage are maximized. The effect of this one adjustment on an otherwise well-tuned engine will surprise you. The spark's energy is more intense, and combustion efficiency -- and thus engine torque -- rise.

But that's just the beginning. If you want to do the job right, here are more things to take note of when servicing this ignition. First, the points backing plate is a very loose fit in the crankcase casting, which results in timing which changes with engine temperature and dwell that changes with each adjustment to the timing. To deal with that, use only a stock points backing plate. Its thick steel design resists the heat warpage that is common with the thinner steel and less rigid aluminum aftermarket plates. It also fits best, but you can make it fit even better by lightly peening the crankcase bosses all around the plate to snug it up (see the illustration at the top of this page). It should be so tight that it will rotate only with a forceful nudge.

A word or two about the points assemblies themselves. Honda used only two brands of points on the SOHC Fours: TEC (Toyo Electrical Company) and ND (Nippon Denso). These points are best because they're made the best: the pivots have the least slop, the steel bracket is the stoutest and flattest, and the contacts are the most resistant to burning. There are of course several aftermarket point assemblies available to fit the SOHC Four, among them Mitsubishi and Diachi, but Honda never used these inferior products as original equipment on the Fours. Their poorly stamped, light gauge, rounded backs cause them to change timing when the screws are tightened, their springs are not as firm, and their insulating washers are flimsy. In a word, they're junk.

Trust me, it is essential to use a dwell meter, because it is the only way to get both points gapped exactly the same. But, instead of trying to interpret the cylinders/ angles schemes used by various dwell meter manufacturers, just set one gap manually one time, then note the meter reading for later reference. Having identical point dwell dramatically sharpens engine tune.

A strobelight is also essential, but for a different reason. This tool reveals the entire timing curve instead of just the timing at idle. The manual explains how to use it. The manual omits however any mention of the two most common problems with the SOHC Four's advance mechanism. First, very early CB750s have off-center advancer mounting studs. The hole in the crankshaft was drilled eccentric to the main journal, resulting in a wobbly point cam and subsequent ignition timing inaccuracy (specifically, "ghost effect"). The factory recommended truing the stud while turning the engine over at cranking speed. The other problem is common to all Honda mechanical advancers: they over-advance. In a mechanical advance assembly, the advance limit is determined by flyweights which bang against stop ears built into the unit. The ears gradually bend outward with the miles, permitting the timing to over-advance slightly, which results in pinging and flattened full throttle acceleration. When strobe-timing your system, first make sure the bike is idling at no more than 1100 rpm. Next, ensure that the idle timing is at the "F" mark at that rpm (or less). Once these are correct, rev the engine until all the advance is in. The timing should advance no further than dead on the second of the twin full advance marks. If the system advances farther than this, and the idle timing is correct, the advancer is in need of tweaking. Remove it and gently squeeze its ears in a vise. Gently. You may want to use a caliper. We're talking maybe 0.020". Put it back in, re-dwell and re-time, and check the advance again. Don't lubricate the advancer mechanism. Its hardened steel design completely eliminates the need for lubrication that was the hallmark (and downfall) of its pushrod British twin counterpart. Do however lubricate the point cam, using either Harley-Davidson or Delco point cam grease. Your points will return the favor: the stock points will wear very slowly if kept clean, dressed and lubed. It is acceptable to dress them, and under most conditions they will bear one or two dressings between replacements, which means that a set should last three tune-ups.

One more thing. The Kettering system is prone to what is often called "point bounce." Actually a misnomer, point bounce really refers to point float, because it is when engine rpm overcomes the points spring. Since a floating point cuts into the coil's saturation time, the result is reduced system voltage. Point bounce is even worse with aftermarket points. The solution to point bounce is simple and like these machines, vintage. It's called double-springing, and it's easy to do. Just grind the rivets off both a used set and a new one, and bolt up the springs, piggy-backed, using the screws from the old set in place of the rivets. The difference, particularly on bikes that have engine work in them (even if just exhaust, jetting and air filters) will surprise you.

Finally, there are high performance coils. Though many would have you believe this modification brings a big return performance-wise, it just isn't so. The truth is, if you follow the steps above carefully, you'll maximize your system to the extent that hyper coils will add very little. Moreover, aftermarket coils are somewhat at odds with the vintage character of these machines, are a bit expensive, and in many cases require considerable ingenuity to mount properly. But if you're dead set on doing it, at least do it right. Use the coils from a GL1000. These were a popular upgrade during the 70s and were actually recommended by electronic ignition companies such as Dyna. They're a lot easier to mount than aftermarket, and they're readily available in scores of salvage yards. Sort of a period modification that will have much less affect on the old bike value of your Four.

There you have it: the basic SOHC Four ignition blueprint recipe. To recap, we adjusted the point gap to the minimum for maximum coil dwell time and system power. We used only stock points and the stock steel backing plate, and used a dwell meter to get the gaps exactly the same. We also made the final timing adjustment with a strobe timing light to ensure that both idle and full advance timing were right, trued and blueprinted the advancer assembly to get rid of "ghosting" and over-advancing, lubed the point cam to maximize point life, and double-sprung the points to make the system rpm-compatible. Finally, we exchanged the coils for those from a 1975-1979 GL1000 to give the system the ultimate period, quasi-factory upgrade.

Don't buy an electronic ignition system. Carefully set up, the stock points system can go head to head with the best of the points replacement systems. And don't let all this detailed work scare you; two-thirds of this stuff needs to be done just once. From then on, maintenance is minimal and fairly standard. While giving your bike's ignition system all this attention won't prepare it for Daytona, these careful adjustments do offer genuine performance advantages and, just as importantly, result in as little impact on the bike's retro appearance and value as possible. They're also cheap (being mostly labor) and they're easy. Would more could you want? Enjoy.

Further Reading
Powersports Ignition Evolution
Resistor Plugs, Wires, and Related Issues
U-Gap and Splitfire Spark Plugs
High Performance Ignition Coils
Honda DOHC, CBX Ignition System Troubleshooting
Reading Spark Plugs
GL1000 Ignition Tech


Now this procedure is in booklet form! 35 pages of step-by-step instruction. Over 125 photos. More information here.
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