I am going to add my 2 cents since I had the same problem and I fixed it. The problem with the spark rattling out is an issue with the jetting. I have the same issue and the only way I have been able to fix it is by jetting it rich. Once I jet it rich the problem goes away. My current set up is as follows.
421 Cub, 20cc domes, +4 timing, Hemi Head, Pingle, 34MM Pjs.
My Jetting is as follows 50 pilot, 165 mains, and DGK neddle. I use VP MS109 mixed 50/50 with 91 Pump.
Note at 160 main the plugs come out when I place the engine on a load (dunes or drag). At 165 I have no issue.
With this setup it boggs down a bit. I am thinking of changing the DGK needle to the CGL in order to lean it out at the 1/8 - 3/4 range to see what it does.