How to Diagnose and Fix Oil Consumption Issues in the FX600V 20 HP Engine?

It sounds like your engine is experiencing oil consumption issues, which can often be traced back to oil leaking past the valve guides into the cylinders. A common sign of this is fouling on the spark plugs, which you've already noticed. Since you've replaced the intake manifold gaskets and are still facing this problem, it's likely that the valve guides may need attention. If one of your spark plugs is fouled, it’s a good indication that the engine might require an overhaul. While you can attempt this repair yourself, it typically involves special tools and a fair bit of mechanical knowledge. If you're not comfortable with that, taking it to a small engine repair shop that specializes in lawn mowers would be a wise choice. They can assess the situation and perform the necessary repairs. Regarding the valve to rocker arm clearance specifications, you might find that information in the repair manual, which can sometimes be hard to locate online. If you can't find a downloadable version, I recommend checking with local dealers or parts suppliers who may have access to it. Once your engine is rebuilt, checking the valve clearances is a good practice to ensure everything is functioning optimally. If you have any more questions or need further assistance down the line, feel free to reach out.