A couple more flights on the big Rascal and a small problem popped up. I noticed the engine wasn't running as smooth or consistent as it should, so I landed and started inspecting. I think the problem is fairly small - bubbles in the gas line. This could have been from the gas tank vibrating and the clunk picking up some air, or one of the fittings isn't tight and is letting in a little air.
To test the tank I filled it completely and watched the gas line and tank as we ran it up on the ground. The fuel in the tank was vibrating, but not enough to be the issue - and air bubbles were abundant in the line. So my issue is somewhere between the gas tank cap and the carb... Yay...
I figured the best option was to pull the tank and inspect each connection. Pulling the tank is a pain in the butt, as I have to pull the carb, muffler, prop, cowl, windshield, and dashboard. Once this was done and I had easy access to the full fuel system I found the cap on the Fiji bottle could be tightened a very small amount (probably not enough to have been my leak), and the fuel filter also had a little play in it. The filter is the more likely source of the bubbles as the O-ring was slightly bulging in one spot. The filter was replaced, the tank re-installed, and I put additional foam padding around the tank to hopefully keep the fuel a bit more isolated from vibration.
Another issue is shown in the pic below - a crack in the carb isolator. I knew the crack was there when I helicoiled the hole a while back. It's on the outside of the isolator and doesn't create an air leak into the air/fuel mixture, but it's something I need to watch. So far I haven't been able to find a replacement piece as this version of the XYZ (Turnigy?) engine is discontinued and the new version uses a rear-mounted carb. With the plug, carb, and muffler removed I was also able to inspect the piston, cylinder, and spark plug. I've run the engine a little over an hour since starting this project and the cylinder and piston (at least what's visible) appear to be in great condition. The plug is telling me it's running a little lean, so that'll be the first thing to check when the air bubble problem is solved.