Sounds to me that the thread starter is having more difficulty dealing with his mouth gaurd that with breathing. With the whole spit thing, I find that usually happens with these conditions:
1) You are tired as hell and are breathing open-mouthed. Not a lot you can do about that except work on your cardio.
2) You are using a double mouth gaurd with those "breathing channels" in the front of the gaurd. Those things act like spit channels when you are gassing.
In the second case, there are a few things you can do. Mold or fit yourself with a real quality single piece. Make sure that it is fitted so that you can put it in and it will stay up on your teeth from the suction alone. This can help lessen the spit, and it's access to being shot out from your mouth.
Regardless of using a single or a double, one of the things you can do to get over adjusting your breathing habits and learning on how to deal with the excess salivation is simply to train with that thing in all the freaking time. You want that thing in when you are doing cardio, lifting, drills, everything. This includes all your training that doesn't involve sparring. After a while it will feel like second nature and you won't even mind the thing.
It doesn't matter if you agree or disagree
... As long as I don't bore you and I spark a moment of thought, my goal is achieved
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