Reddit mentions of Thexton THE404 Spark Tester

I would suggest diagnosing it, there are a lot of things that could be wrong. First I would check for spark and injector pulse, and I would jump out the codes like in that link I gave, to be certain that the 51 code is a powertrain code and not an ABS code. Because if you do have a 51, you probably need an ECM.

Between the ICM and the ECM, The ICM is the much more common failure point. It is where your 3 ignition coils sit. It's the more common failure point, but it is also often misdiagnosed when the culprit is a bad crank sensor as another poster suggested.

Start with testing spark and injector pulse. If you don't know how you need a Spark Tester and a Noid Light.

Side note, I much prefer this sort of spark tester because it has more utility, being able to measure actual kV can be very handy on some cars, but the cheaper one will work fine for you. If you are going to work on your own vehicles, a Spark Tester and a Noid Light are both must haves.

I'm getting spark but it is so weak that the plugs have to be perfectly dry in order for it to fire. When I use my spark tester it can't even spark over a 1mm gap when it should go 4mm or so. Same symptom before and after stator replacement, regulator/rectifier replacement, CDI refurbish and testing. Could it have a weak flywheel? Ground seems good, resistance from negative post on ebox to negative on battery is 0.0 ohms. New stator tests good as well.

I'm stumped. I thought it was a carb issue but I've fiddled with the carb forever and tried rebuilding, tuning, replacing the carb to no avail. I think the spark tester is accurate because I tried it on my old moped and the thing threw a strong spark over a big gap.