Keep trying, and failing, to start engine - what am I doing wrong?

[Triacontakai]

I have no idea what i'm doing wrong with burns either, I get my burn going 66 oxy 33 plasma, get the cold loop going with plasma, do hot loop, ignite, open vent, pop hole in the burn chamber and rcd the space under the tanks. This is exactly what everyone usually says to do, but every time I do it, it get's up to 1 MW and then drops to none and frankly I have no idea what i'm doing wrong.

Start the burn chamber, and then after waiting like 4-5 min (less if you wanna get going quick, may risk engine failing later) open the valve leading through the burn chamber. Same thing happened to me and I had no idea why the engine was failing, and after I waited a bit it started working perfectly (aside from the crippling lag engine bug   )