FC420V sometimes doesn't start - weak spark?
I am looking for troubleshooting guidance for a walk-behind brush cutter (~15 year old DR) with a Kawasaki FC420V AS-22 engine. I'm not experienced with small engine repair or diagnostics, but I'm trying to learn. So I've probably checked some things that don't need checking, and could well be missing something. Symptoms:
- The engine has, for a few months, failed to start reliably. Oftentimes it would start right up, but once in a while it would crank and fail to turn over. Seemed random, but would start quickly 4 out of 5 tries.
- Sometimes this changes for a period of time (days to weeks), in which it consistently won't start.
With a spark plug tester hooked up during the times that it won't start, I see only a weak spark. Orange; not a hint of blue. I suspect, but do not know for sure, that a weak spark is why it's not starting. When it _does_ start, there's still no blue in the tester - just a stronger orange. Things I'm pretty sure of:
- The battery is good. The battery is about 18 months old. I initially thought this might be a battery problem, so took it in to the battery store who tested it and found strong CCA and voltage.
- Fuel delivery. I can see fuel moving through the filter when I try to start it, and the inside of the carb had fuel when I took it apart to check.
- It's not the kill switch. I've detached the kill switch from the ignition coil and see the same symptoms.
- It's not as simple as a bad spark plug. I proactively replaced a plug about a month ago when doing some general maintenance work, and then when it stopped starting completely got another plug just in case there was something wrong with the replacement. So I now have three spark plugs that all work the same, and show the same results in the spark plug tester.
I suspect that the problem is either a bad ignition coil or the flywheel. The flywheel is very rusty (see picture below), and I've read (conflicting) reports that that might be a problem. Or maybe it's the ignition coil? I have no reason to suspect that other than process of elimination. Is there any way I can test it? Other background:
Nothing in particular happened between the time it would mostly start and when it stopped starting at all. A couple of weeks before then, I did a bunch of general maintenance - new fuel filter, cleaned the air filter, thoroughly cleaned the exterior of the engine, sprayed some carb cleaner in the carb, changed the oil. One of the bolts on the case rocker in front of the head cylinder seems to have some thread damage, and was leaking some oil, so I added some thread sealant and the leak is much, much slower. I have checked and reset the air gap between the ignition coil and the flywheel to .012".
At this point, I'd love suggestions about what to try next. Ignition coil? Flywheel? Something else?