The first two times I tried smoking on my 22" kettle using my Slow N Sear, I did have problems controlling my temperature. It was a constant battle of see sawing, up and down temperature swings and I found it virtually impossible to keep the temps below 250. Doing a little research on ABC Barbecue's web site, I found this little tip:
Tip: The Slow ‘N Sear is so efficient at burning fuel that any unintentional air leaks (e.g. an improperly sealed lid) can prevent the kettle’s indirect zone from getting below 250° F. An easy solution: use 2” binder clips to seal the lid. Space three clips around the kettle lid for an improved seal.
Ever since I incorporated the use of binder clips, I have no problems maintaining temps. My guess is you are having the same problem. The amount of unlit charcoal you use determines the length of the cook, not the temperatures during the cook. That is controlled by the vents. If your kettle lid is not sealing tight enough and is allowing to much air in, it will be difficult for you to control your temps.
Here is a link to the article from ABC Barbecue:
https://snsgrills.com/pages/slow-n-sear-lighting-instructionsHere is a great Youtube video from ABC Barbecue on using your SNS on a Weber kettle.
https://www.youtube.com/watch?v=qDWRW3QAdzcFollowing the instructions on the video, I now find using the SNS on a 22" kettle as easy as using my WSM. Temps easily dial in and stay between 225-250. As per the video, I lite 12 Kingsford blue briquettes and dump them in the corner of the SNS. Then I dump a full chimney of unlit briquettes, filling the rest of the SNS. Use binder clips and follow the suggested lid and bowl vent settings in the video and you should have no problems. I cooked a rack of baby backs this past Saturday and it ran for 5 hours between 230- 245 with very little adjustment. I had enough charcoal left to probably go another two hours. I use a Thermoworks Smoke to monitor my grate temperature.
@KettleSmoke , let us know how it works out on your next attempt.