I have a mbx5t with a .21 tema orion engine when it is started the engine revs up really fast and then goes down when i hit the brakes. when i adjust the idle should the engine be running? also i have not touched the low speed needle

1, adjust the LSN and your linkage is worng. Take the air filter off and see where the carb slide is at idle, then brake. You need to set it so it is the same at both idle and brake.

ok so when i hit the brake at idle the carb should not move right? thankyou very much for the info

Yes, while it is running set the throttle trim on your remote so that it idles good- I.E. low enough that the car does not want to move on it's own.
Shut it down, pull the air cleaner boot off and turn on the radio.
Start turning the idle screw in until the slide no longer moves when you hit the brake.

I would get the idle issue fixed before adjusting the LSN...
After setting the idle like I described you should have a 1mm - 2mm gap.
Your manual should tell you the idle gap, prolly 1mm.

ok i think i got it pretty close but the engine seems to sound spotty so to say like little tiny pops? also iam running 20% right now should i switch to 30% to race or just keep the same?