When it comes to motorcycle repair, most of us prefer to stay away from electrical problems — and for good reason. Without the benefit of proper diagnostic tools and experience, tracing electrical shorts and other electrical equipment problems can be difficult.
But that shouldn't stop you from at least trying to suss out some electrical problems, because every now and then you get off easy, like we did recently with a friend's 1972 Honda CB750. The owner had been trying, without success, to track down the source of an intermittent running problem — typically the worst kind of problem to track because, well, it's intermittent. In this case the bike would mostly run perfectly, but then it would misfire, acting like it had one or maybe two cylinders cutting out.
This is actually a fairly common problem on older bikes where the spark plug cap is screwed into the plug lead. The connection between the plug cap and the wire strands in the plug wire lead is made by a sharply threaded brass probe in the cap, screwing the two together. Years of vibration can stress the metal strands in the lead, and pulling on the leads (you should always remove plug wires by pulling on the cap) to remove the plug wires further stresses the wire, often with the kind of results we witnessed.
Many bikes have replaceable plug wires, but most Seventies Hondas use coils with permanently attached wires, meaning you have to replace the coil to replace the plug lead. Fortunately, this is a pretty easy and affordable fix.
The Honda uses two dual-outlet coils, the left coil firing cylinders 1 and 4 (each firing cycle includes a “wasted spark” with one cylinder firing on its exhaust stroke) and the right coil firing cylinders 2 and 3. The critical specs for the coils on our Honda are a 100mm center-to-center mounting pattern with a 3 ohm resistance rating when using the stock ignition points.
|Our subject bike, a mildly caféd 1972 Honda CB750, was faltering thanks to bad coil leads. Replacing the coils and their integral leads got it back on the road.
Replacing the coils is simple and well within the scope of the average weekend warrior, and barring any unforeseen issues shouldn't take more than an hour or so. Replacement coils typically sell in the $35-$40 (each) range. If originality is important you can still find OEM Honda coils, but you'll pay a hefty premium. Used coils are another option, but they're usually headed the same direction as the bad one you already have. You'll also want a set of spark plug resistor caps, which run about $2.50-$5 each. The basic process lined out here applies to just about every Seventies Honda Four built, from 350s to 750s. As ever, we suggest having a good shop manual at the ready to help aid identification of critical parts.
Similar to just about every UJM (Universal Japanese Motorcycle) ever built, the ignition coils on our CB750 Honda are mounted on the frame under the tank, out of harm's way.
To remove the coils simply unplug the color-coded electrical connections to the coil, disconnect the spark plug leads, and remove the two Phillips securing screws at the front and rear of the coil. The photo above shows why we weren't getting spark: the inner wire strands had fatigued and broken.
Here's what the inner wire strands should look like. The main photo on page 76 shows a replacement coil (at right) with a stock coil. Note the long leads on the replacement coil: Wait to trim these to length until the coils are installed on the frame.
You also want to replace the spark plug caps, which have a resistor built into them for noise suppression. Most motorcycle shops stock them, but you should make sure you have the correct type (straight or angled) before you start this project.
Our replacement coils bolted straight onto the stock brackets. Note that we underslung the coil on the bracket instead of mounting it on top like the original. We mounted it this way simply so the plug wires would be up higher under the gas tank.
The only modification required for our replacement coils was splicing in the original female bullet connectors for the ignition coil trigger wires. The original right coil trigger wire is color-coded yellow, so we cut and spliced its yellow female bullet connector to the new coil's trigger wire, which is blue.
We did the same thing for the left coil. The stock wiring harness has the left coil's trigger wire coded blue, the same as on our replacement coils: The left side ended up looking a little tidier. The splices were made using a standard crimpable butt splice, which we then sealed using heat-shrink tubing.
Here's the left or no. 1 cylinder plug lead trimmed and with its new resistor cap screwed on. We left the plug leads a little longer than stock so they can be trimmed back if ever necessary.
With all the spark plug leads trimmed and the appropriate spark plug caps installed, it's a simple matter of making sure the leads go to the correct spark plugs so your bike will run the way it's supposed to — on all four cylinders. You're done!