I often see the forum speaking about "pop off's" - what is it

a watercraft don't have opend carbs circuit and the pop of presure is the presure nesesary to open the needle and seat valve

Airborne, from a practical standpoint it is something you (may) need to adjust on your carbs. And, in my opinion, just one of the things that makes tuning these carbs such a pain in the arse!

If you do anything to effect the airflow in the intake side, you will effect your pop-off. A restrictive air filter/intake creates a large negative pressure in the carb from the engine trying to pull air through it. In this case you need a high "pop-off". If pop-off is too low fuel will come in before it is ready and you will be rich, a too high pop will make it lean.

If you change air filters etc to a less restrictive style, the negative pressure in the intake will be less. With the stock, high, pop-off you won't be able to generate enough neg pressure to pop open the needle valve so you will get less fuel and a lean condition. In this case (aftermarket air filters etc) you will have to lower your pop off.

Pop off is adjusted by changing the combinations of needle sizes (bigger=lower pop off pressure) and spring strength (weaker springs= lower pop off).

Group-k has a pretty good section (very good actually) on carbs and explains some of this.

As far as I can tell, if you have stock airfilter/intake you don't have to change pop-off (although other motor changes that increase overall airflow (pipes etc) MAY require a change in your pop-off)