Kohler 20 hp engine cranks way too fast and will

how much carbon is in this units top end sounds like you have a valve stuck open. most people use regular unleaded gas in these units and run them for long periods of time usually never under full load . pull the heads see if you can free the valves up to get them to move freely( always replace the head gaskets when accomplishing this task and use torque specs on heads). Recommend using Marval Mystery oil both in the oil and fuel tank it really helps to keep particles in suspension and cleans residue carbon off of vavles. I know this sounds old school but it works. If your unsure about pulling the heads take it to the shop and tell them you need a carbon and valve job how much carbon is in this units top end sounds like you have a valve stuck open. most people use regular unleaded gas in these units and run them for long periods of time usually never under full load . pull the heads see if you can free the valves up to get them to move freely( always replace the head gaskets when accomplishing this task and use torque specs on heads). Recommend using Marval Mystery oil both in the oil and fuel tank it really helps to keep particles in suspension and cleans residue carbon off of vavles. I know this sounds old school but it works. If your unsure about pulling the heads take it to the shop and tell them you need a carbon and valve job