The hsn should have a little o ring seal remove the screw an check it has one, I've never known a nitro to leak fuel from there unless it's missing the seal or it's split . You only need a bit of sealer round the carb not near the base just keep it away from lower part of carb save any going in the bottom end crack case. Same when sealing the back plate, just make sure when removing the back plate the piston s a tdc some pistons sit in the back plate slot so it will break the lower part of the piston if it's that set up on your engine. I know hpi did this on there 25 I think it was. You can also put a bit of sealer on the carb cotter pin ie it's the pin that holds cab to crank case. I've never sealed the hsn or Lsn just made sure there clean. After all you will need to adjust them so usually the seal will break as you adjust them but it's each to there own. Only used blue as that's all I used & had in my top box but I'm talking around 8 yrs ago lol.