I have seen some racers with more experience than me do some carb adjustments at the start of a race while lining up.
What are they doing exactly, and what is the reasoning behind this exactly?
Are they leaning the low, and subsequently changing it back? Too much fuel / little fuel for the start?
It will vary depending on the engine, but generally with most 2-stroke engines I’ve run, when you’re doing this, you’re leaning the low-speed needle down on the pace lap to keep the engine from loading up with fuel and flooding when you go to accelerate. You’re looking for crisper take-off. Some engines it’s more effective than others. On a Leopard/X30 I never did it much, though sometimes it helped. Same with the KA. On the KA it seems it’s pretty easy to get it to starve for fuel on the low, so there isn’t much leaning you can do for the most part.
On the Komet/KPV engines the carb basically needed to be about 2 turns open on the low-speed needle when running at race pace, but you couldn’t get off the grid at that setting. So you would start it at about 1 turn open on the low, do the warm-up lap and open it up a bit to get going, and then close it back down halfway through the warm-up lap to clear the excess fuel back out and let it accelerate crisply. Then you’d open it back to 2 turns open as you accelerated for the start.
Rotax we used to pinch the fuel line a bit to achieve the same thing since its fixed jet.
You can also get a little feel for how the carb is tuned on the out-lap, so sometimes you can tweak the needles a bit to adjust your race pace settings too before getting lined up for the start. If you feel a dead spot somewhere in the rev range, you might lean or richen one needle or the other on the warm-up lap.
Generally, I have my hand on the carb during every start, just in case it doesn’t get rolling as good as it needs to and I need to make a quick tuning adjustment as we accelerate.
This is exactly the kind of information I’m looking for… it’s an X30 btw.
I think I should start a TJ appreciation thread also. 
