I’m new to this system and still trying to get the rules beneath me. Should I let the players know what the difficulty rating is before they roll or is that GM only knowledge.
So if my players need to jump a gap between two rivers and I decide it’s a D3 test should I tell them they need three successes
Yes, you should tell them. Otherwise it is very unfair, because players can spend Momentum to add extra d20s to improve their chances of success. If they don’t know the Difficulty beforehand, that makes it really not fun.
Knowing the Difficulty of a particular task is basically character knowledge. The character knows if jumping a gap is easy (Difficulty 0) or daunting (Difficulty 3).
As a GM, you still have secrets from the players. For example, you can keep the size of your Doom pool to yourself, so players tend to be cautious in adding more Doom.
I think that in all 2d20 games the GM is supposed to tell the difficulty to the players, so they can choose if they want to buy extra dice or spend Momentum. The whole Momentum/ Fortune/ Doom thing must be based on knowledge or it will not work out as planned.
It´s different.
I have got an issue: Players used Momentum and generate more Momentum after them. In this way it was too easy to win all fights and checks, in particular against npc of Minion level. At first it sounds well for players, but after a short time, they were bored. Because of them I decided, that they have to reduce the Momentum in amount of set Momentum before.
In some cases, they shouldn´t see the results or strenght of NPC, when they are in struggle with them.
Otherwise my group members loose tention.
If you’re having issues with Momentum “recycling” being too easy, I suggest a change we made in Star Trek Adventures - the first die you buy costs 1, the second costs 2, and the third costs 3, regardless of whether you’re buying them with Momentum or Threat. This limits how easy it is to get back all the Momentum you spent on a test, and makes buying extra dice more of a decision.
And does this apply to the Game Master in Star Trek? I don’t know the equivalent of Doom there.
That applies to the GM as well - the Doom equivalent is simply called Threat there, but it works in essentially the same way.
Does this apply only to the d20s or it also includes the damage dice that can be purchased with Momentum Spends?
I also have this issue where after 2k xp the group is generating huge amount of Momentum in their core skills and the pool is more or less full constantly. Reducing by 1 at end of each round is not really a problem.
It only applies to d20s. There’s diminishing returns to buying up extra damage dice already (because there’s a limit to how many wounds you can deal in one hit, no matter how much damage you roll).
Where is this mentioned in Conan? I presume you talk about the rule where you get 2 wounds if the damage is 5 after armor reduction and if brings the vigor to 0 at the same time?
Yes. Outside of special rules, that’s the most Wounds you can inflict in one hit, so once you’ve done that, there’s no point buying more damage.
Thanks for the clarifications! I will try this house rule next sessions or two and see how it goes. 
This would be to a single target right? When fighting a mob you could in theory do more than 2 wounds. So for example if I somehow do 15 points of damage to a mob (5 Vigor each) I would do 2 wounds to the first enemy (reduced to 0 vigor and more than 5 damage in a hit), 1 wound to the second (reduced to 0), and 1 wound to the third (reduced to 0). Correct? I really just want to be sure I am playing this correctly.
Yes, to a single target.
Worth making sure you don’t miss any of the existing rules that would soak momentum–depletion each round, escalating cost for defence and reducing momentum by difficulty, even in struggles.