My 8N has started sputtering and almost stalling out when it has to pull an implement. Took out our single row cultivator today and it worked great for a few passes, then started losing power until it wouldn't pull at all without almost dying. I checked the easy things....cleaned sediment out of glass bowl and checked air intake. Seemed clean. I know I don't have my plug wires crossed, because it's been running fine. So: is this issue my carburator needing a rebuild, or possibly a fouled plug??? Help! :?