is the Latvian gambit sound and/or an acceptable opening to use in longer time control games than bl

it´s easier to play with black than with white.   I had some official classic games with 90% moves same than the computer,  this is the good thing about latvian ,  black plans are easy and natural. 

What i do not like about latvian gambit, is  the liquidations of pieces once you get ab equal game,  same happen in another openings like sicilian,    against a weak opponent you do not want to make a draw,  that is one of the reason i stopped to play latvian gambit in slow games, because there are more chances to win with more pieces over the board.  Another reason is you  can not make mistakes in the beginning or you are tecnically lost .  It depends of the variation of course,  , i´m talking about the critical lines  after Nxe5 leonhardt and main line or Nc3-Mlotkovski.  

 I give you an example in a slow game where i made the 95% moves as the computer.

It´s a risky opening,  probably you will lose games in 20  moves sometimes  , but also you wil get winning positions if your opponent start to play passive ( very common ) .    There is another reason why i do not like more to play gambits, is the variance.  Chess is  a game, and luck is always there, so if you play all-in all your games  (and latvian gambit is an all-in opening)  it´s normal to lose or win with players that you would never expected it. 

Agreed. White is close to winning with any sensible treatment -- heck, if he just plays sensible chess, he's almost winning.

I don't understand why anyone would want to play the Latvian.

Nope, not really. Black has nothing for the pawn, and basically has to grovel for counterplay by trying to get a knight to f4. Also, 3. Nc3! is an easy clear advantage.

I don't know... I play 3.Nc3 (and if fxe then of course take the e5 pawn) and from there just use calculation and intuition and I seem to get really good positions every time.

It may be important to know after 4.Nxe5 Nf6 the move 5.Ng4 is good.

Of course the opponents that I face may not be very booked up and are just experimenting.

I guess I do know this though: 1.e4 e5 2.Nf3 f5 3.Nc3 Nf6 4.exf Nc6 5.d4 e4 6.Nh4 which is sort of important I guess because the knight on h4 may look weird, but it's very safe.

Also, the main lines are easily busted. Tell me what you do after 3. Nxe5 Qf6 4. d4 d6 5. Nc4 fxe4 6. Nc3 Qg6 7. f3 exf3 8. Qxf3 and now 8... Nc6 9. Nb5! (NM ih8sens) or 8... Nf6 9. Ne3! (also NM ih8sens).

I always got into trouble in the mainlines and I didn't want to study, and 3.Nc3 helped me get into positions I was already familiar with from looking at different openings, so that's why I play not the best move

At our level Fenix, it can be very sound...lot of traps for White to fall into, if you learn them.  I'm learning it myself now and besides, it's fun!  In a blitz game, our level, it can be deadly. 

you did not read my thread about my novelty .    7...exf3?! is dubious  , so instead you play 7..Be7 and after 8.fxe4 Nh6!?  with good chances,     8...Nf6?! is refuted.  after 9.e5   10.Bd3, etc  

I give you more details in my thread  https://www.chess.com/forum/view/chess-openings/new-idea--the-quotkgb-variationquot-in-the-latvian-gambit-main-line-7f3

Anyway i wil explain all of this soon,  I´m producing a serie about latvian gambit ( 2 videos)   summering  up  the opening strategic ideas and themes  length 40-50 min.

How to play the latvian gambit properly

In what world? 9. g3 followed by Ne3 and Bc4+ is just winning. White has compensation for his extra pawn, as some English players like to say.

black has enough compensation ,   0-0   c6   Nd7 --<>     White has some tecnical problems to castle long     0.65+=  is not WINNING  .      Ng4 or Bg4, b5  some moves  , if the oponent plays Bc4-Rf1-Rxf8 , then Nxf8 , this knight goes to e6 and  it is easier to play with black .   

This line is an improvement  ,  still white keeps an opening advantage ( a pawn up ) but its position is restricted,  and queens are still in the board. 

Latvian Gambit is a dinamic opening,  with queens on the baord 0.65 advantage is a joke as it happens in the kings indian defense ore dutch defense with many complicated lines and the engine gives you that evaluation.

7.exf3 goes against the spirit of latvian gambit,   i can resign in the next move, because black best chance is to try trade queens in g4 ,  white can reject this exchange ,   it´s to play in the WHITE  FIELD , and this is a nightmare ,    in latvian gambit this is a suicide , you can not make more concessions being a pawn down or behind in development.

I don't think 3.Nc3 gives "very little margin for error." I think white's big advantage is very safe, and after poor play only gives black equality... but if black is inaccurate then it's game over right away.

Oh, I didn't see his 0.7 comment. Yeah, the latvian, when properly played, gives white way more than that.

Pfren showed you a loss by force. Those were ICCF games.

In tournaments you play humans, not stockfish. As for myself, a big advantage out of the opening doesn't make me feel pressured.

Let any strong GM prep, and I assume they'd win a won position vs an engine. Maybe not every game, but at least some of the games.

Wait, are we talking blitz or tournament play?

I play solid stuff in blitz, but I'll also play silly openings, some so bad they don't even have names.

My policy isn't to try to refute the Latvian. I content myself with what I feel is a stable big plus after 3.Nc3.

I don't try for engine moves or necessarily for theoretical moves, I try to reach a position where logical and intuitive moves are the good moves that preserve my advantage.

So for example in a position like 1.e4 e5 2.Nf3 f5 3.Nc3 Nf6 4.exf5 if black plays 4...e4 the engine wants me to play 5.Ng5 but I prefer 5.Nh4

after 4...d5 the engien wants 5.Nxe5 but I prefer 5.d4

I'm happy to keep the position calm and comprehensible while at the same time preserving my big plus. I don't need to crush my opponent immediately. 20 or 30 moves worth of +0.5 is enough to wear down most opponents I'm capable of beating.

play the gambit... you'll have more fun ;-)