I had a similar issue, I had to run it so damn rich, it was almost flush on the high side.

Fuel tank, mine was in the lid ... used the rubber ring in a new HPI primer tank I bought and now it's leak proof.

Here's the best way to test:

1. remove tank from chassis, with lines attached (empty)
2. plug/pinch the feed line (goes to carb)
3. take the return line (from exhaust) and blow into the end of it, listen for whistles

You could take dishwasher soap and dab a little around where you hear the whistle, you will see bubbles where the leak is. Mine was under the cap.