The Cause of Misfire Problems: "Even Though We Knew the Cause, We Couldn't Fix It"—Here's Why

HONDA Trail CT110 Hunter Cub 1981 Semi-Restoration Vol. 16

The CT125 Hunter Cub is a popular model that everyone now agrees is a bestseller. Why has a model in the Class 2 moped category garnered so much attention!? While it’s naturally an attractive model, modern traffic conditions likely play a role—conditions that can only be fully enjoyed because it’s a Class 2 moped.The ancestor of that CT125, and the model that has finally started running, is the early domestic CT110 currently undergoing restoration. However, due to fundamental issues with the ignition system, it had been unable to achieve stable performance. “I want to replace the parts,” but “I can’t find replacement parts…” I’ve been stuck in this situation for the past few months. Just then, I discovered compatible new parts on an overseas online store!! Finally, the real work begins.

Finally found!! CDI ignition exciter coil + magnet rotor　

The final 6V model of the CT110 had already been converted to CDI. However, unlike later-model CDI systems, it used "mechanical ignition advance" rather than automatic advance. I finally found a brand-new stator + rotor set for that specific type on an overseas online store!!

At first, I blamed the "ignition coil" for the problem, but

When it comes to the cause of misfiring issues, people often assume it must be the ignition coil itself or the capacitor, but even after replacing each part with a new one, the symptoms did not improve.

The top shows the genuine CT110 IG coil and capacitor, and the bottom shows the Super Cub-style Flumag (AC) coil and capacitor I replaced them with. I tried several other sets of parts as well, but the symptoms didn’t improve.

When the ignition coil or capacitor malfunctions, cold starts are often completely problem-free, and the idle is stable. However, once the engine warms up, it starts misfiring erratically, like an irregular heartbeat.

When misfiring—meaning the engine isn’t firing properly—you can hear backfiring sounds inside the muffler, and the car loses power. I noticed that these symptoms are different from those caused by faulty ignition coils or capacitors.

Purchased an ignition coil and CDI unit compatible with the CDI ignition rotor　

A friend who’s knowledgeable about overseas shopping found a brand-new set of New Zealand-spec rotor and stator components in Australia. It wasn’t cheap, but I bought it. The box was beat up, but the parts were unused!!

I was able to get the IG coil for the CDI ignition and the fixed-advance CDI unit itself from a fellow biker. Is the green connector a distinctive feature of the fixed-advance CDI unit? I’m not sure.

The CDI ignition timing is an older system with mechanical advance　

If you remove the cam end cover from the points ignition era, the covers for the CDI ignition can be installed bolt-on. I was able to get a used pickup assembly, but...

I mounted the cam end cover for CDI ignition onto the cylinder head and installed the mechanical advance governor and magnet rotor, but since there wasn’t one available for the CT110, I adapted one from the TL125 Ehatobu.

When installing the mechanical advance governor and magnet rotor into the CT110, the position of the pickup that sends the ignition signal doesn’t line up at all. It looks like I’ll need to re-machine the mating groove on the governor’s iron core.

When setting the maximum advance to 30–32 degrees, I decided to re-machine the fixed groove on the governor core by eye, estimating roughly where it should be for the CT110 engine. The orange marking indicates that position.

If the spark becomes stable, it will make a difference in the carburetor settings　

I built the CDI ignition wiring using completely custom-made wires. I disconnected the exciter coil’s rising edge (black/red wires) from the connector and built a custom ignition harness from the primary input. Don’t forget to add a ground wire!!

Now that the ignition system is stable, the engine is running perfectly. I can now make detailed carburetor adjustments using a fuel mixture kit. Since ignition system issues and carburetor settings are closely linked, this is crucial.

The domestic CT110 model has undergone a major evolution from the stock magneto ignition to "CDI ignition" without significantly altering its appearance. Both starting performance and riding feel are now overwhelmingly stable.

This CT110 domestic model started out as a beat-up bike meant for restoration. Six months after it was brought into the garage, it had reached its current style. But the road ahead was long... When it first returned to the road, it ran smoothly, but eventually its condition began to deteriorate, and so-called "misfire symptoms" started occurring frequently.If it had been bad all the time, I could have just given up, but there were times when nothing happened and it would just run.Those symptoms gradually became the norm, and at speeds above 50 km/h, the RPM wouldn’t climb accordingly. In other words, the symptoms seemed less likely to appear when riding slowly, so it felt a bit different from the typical issues caused by an ignition coil or capacitor.

I had a feeling relatively early on that the problem lay with the exciter coil. However, I couldn’t find a replacement part…To begin with, there are very few domestic CT110 models and export variants from that era, and I couldn’t find any other models whose parts could be adapted. On a whim, I bought used parts from models that might be compatible and even tested installing them, but I couldn’t find any parts that could be adapted with just minor modifications.So, I also tested implementing a 12V battery ignition system, but it was still unstable due to insufficient power generation. At that point, I realized my only option was to find compatible parts, so I consulted a motorcycle friend who is knowledgeable about the overseas market.

The very next day, he found brand-new parts for a CDI ignition model—the same 6V electrical system as the early CT110 series.He found genuine parts for a New Zealand-spec CT110 (a model from around 1985?) in Australia—and they were brand-new, unused parts still in their original boxes. When I explained the situation to the owner, he replied, “If you can buy them, please do,” so I placed an order for them online.

The internet is amazing, isn’t it? I was reminded of that once again. It’s a world of difference from the old days of overseas shopping, when I had to spend hours poring over Japanese-English dictionaries. After waiting 10 days, the parts arrived, and I immediately began the swap.I connected a multimeter to the black/red leads from the CDI exciter coil and gave the kickstarter a good kick—the AC voltage shot up powerfully. After confirming that, I built a custom harness specifically for the CDI ignition. I laid out the other related parts and started the engine. It fired up effortlessly with a single kick and revved happily.The idle also sounds pleasant to the ear.

I immediately put on my helmet, let it warm up thoroughly, and then rode full throttle down the nearby farm roads!! I stopped to refuel along the way, keeping an eye on the engine, and confirmed that it ran smoothly even after a full warm-up.After repeated test runs and checking the spark plug condition, I finished the detailed carburetor tuning!! It looks like this bike will finally be fun to ride.