I the HSN is moving by itself when the engine is running then the rubber seals in the jet need to be replaced. I had a new engine axial engine and when I was tuning it the LSN and the HSN would move from the vibrations. I bought a seal kit for the engine and it ran great after that. No more tuning issues. I fuel is spitting out the HSN on the screw end then the seals are needing to be replaced. As for the low compression you can check a few things. First make sure you are putting on the new copper seal for the plug. Make sure the head screws are snug. It is possible that the piston and sleeve are worn if the engine got to warm when it was leaning itself out. Glow plugs will burn out if the engine gets to hot.