Several gas guys using those felt clunks have complained about having fibers in the walbro internal screens. I have 4 saito 120s and run them all inverted but I use 10% nitro fuel with 18% synth/castor mix. I use a 3 line dual clunk tank setup. No prime to start, just verify no hydro lock and hit it with the starter. I also slot the cowl from the head back to make sure there is enough cooling.
How old is the engine? One of my high time engines (~ 200 hours) needed some carbon cleanup after about 70 hours run time. Which I consider normal since I use some castor in my fuel. That can give you some valve sealing problems.
Just some things to think about while digging around the bottom of the barrel.
Edwin