It makes sense that when you blew through the fuel pressure line that there wasn't a smooth flow on the carb side. The tank actually builds up a substantial amount of pressure (that's why it's sealed). You would have to blow significantly to build up tank pressure to get fuel flowing freely. It wouldn't like just blowing soda out of a straw so to speak as one would think. So, that's probably working as designed...

So, you are doing really good so far with good troubleshooting. You troubleshot the fuel side well as a cause of running lean and it sounds ok. However, there is another huge factor that causes lean performance- air leaks! An engine can run lean either by fuel starvation, or by excess air getting sucked in somewhere. If the engine gets air from anywhere besides the carb, the air-fuel ratio will be off and it will run too lean. I have had an engine run too lean because of a cracked carburator, but that was the POS composite carbs on a Traxxas 2.5.

You shouldn't have this problem and this could be a manufacturing issue. You have to decide whether you want to fix it on your own or get Team Associated involved and maybe RMA the engine and/or truck. I am happy to walk you through the process further, but since the truck is so new and under warranty, you might just want to talk with associated and send the engine/truck in (whatever they require.) Air leaks aren't easy to find and the engine would have to come out and be pulled apart. If it was an older truck, we could do it, sure.

Talk with Associated and see what they say. Granted new trucks can be challenging, but maybe not this challenging yet...