Could be a bad coil too. The FE350 I just dropped in my DS was doing the exact same thing. Sometimes nice spark ran great. Sometimes a faint yellow spark, if I could get it to spark at all. This time, I caught it in a failed state and put a meter on it, had 13k ohms through the secondary coil (spec is around 8, and another 5k through the plug cap), but 2.something MEGAohms (basically no continuity) through the primary coil, which spec is around 1 ohms. Swapped it with a spare coil from another motor I have apart that tested in spec, and nice fat blue spark.
Funny thing is, I dropped the bad one on the floor a couple times, and suddenly it tested correct per spec.
Luckily I thought ahead... Before I put this engine in the cart I intentionally removed the 2 bottom 10mm bolts on the fan shroud... just in case. So I was able to swap the coil relatively easily without pulling the motor. It still fought me a bit, but less so than pulling the motor out