I bought a DC pressure washer kit to use on the boat this summer, and it works pretty well. I'm thinking I will try it with hot water and detergent this fall to clean out the leg in a motor I might work on -- lot of nastiness sometimes in a new project. Most of that residue you show was probably already there. If in doubt about what the stuff is, wipe some up an smell it. If it smells like gas, it's gas. Gear oil has a funky sulphur smell. I see in an earlier post that you did some gearcase work, so not impossible there is some gearcase leakage (unlikely). Need to pressure and vacuum test it anyway.
Think the main issue when running in a barrel is a tendency to over rev, but I'm a believer in barrel testing. Lot of guys seem to just run on muffs instead, but that's no good for adjustments or testing -- and popping it on the back of a boat for testing is unrealistic for all but a few of us. (Along with some hard water issues out on the lake when I'm messing with a winter project.)
I purposefully left a large section of driveway in gravel when I had a workshop/garage built, in part because I have this messy hobby. I use a barrel, keep the hose running, and let the overflow kill weeds in my parking area. I'm guessing your motor will run progressively cleaner, now that you have it largely dialed in. Have to wait to boat test for final settings, but can get it pretty well prepped, and ready for the restored hull. Clean up the plugs, wipe off the leg. Keep the hose running.
Good luck with the project. Nice rig.
