I have a Scotts John Deere 17.5hp riding mower. When I start it up it runs fine until it warms up and then the idle starts to sound as if someone is reving the engine. When I step on the gas and am moving forward or in reverse and the engine is under load the problem goes away. When I come to a stop or am going down hill and the engine is no longer under a load the idle problem picks back up again. I have changed the sparkplug, fuel filter, drained the tank and put in fresh fuel and blew out the fuel line. Any ideas where to go from here. I'd much rather fix this myself than take it to a repair shop.
Don't know about this specific unit, but possibly a low float setting problem, or perhaps trash in the bowl? Speeding up often is connected with getting too lean. Think of a weedeater or chain saw just as it runs out of fuel- gets real fast for about two seconds.
If it's worth doing, it's worth doing right. If you wouldn't put your name on it, it ain't done right!