Troubleshooting stumble at idle in gear

I just installed the carb last weekend. I haven't looked at the plugs, but I do have an AFR gauge in the dash. It shows around 13 at speed and around 12 at idle. When I first installed the new carb I set it to 14 at idle. When I noticed this intermittent stumbling problem I set it richer but that didn't help. Speaking of plugs, I'm running cooler plugs (NGK heat range 7) that I installed when I was trying to solve a long-standing ping. Now that the ping is gone I could try hotter plugs, but I don't know if there's any reason to think that would help.