Take this a little further......I robbed the coil and vent and carb off a strong running 660 to fix a 660 for a customer in a pinch. His wj-69 had failed (just got a kit for it the other day) so I ordered a zama clone from Dave to put on mine. Also put on a new OEM vent. The coil came off his and I just assumed it was good and put it back on mine. There are both composite flywheels and I've got good spark.
Here's where the problem arises. Idles sporadic, high/low like an air leak. Throttles really rich and bogs after a few seconds. Smokes as well from muffler with noticeable flooding. Upon killing it, it will puff and back flow fuel out of the carb. Enough that you can feel it on your arm.
Ignition issues.......that's my guess. I've got a couple of coils on the way.