48" Skag has 4 year old engine. Runs great for half hour or so, then starts to run ragged and loses power. Getting fuel and spark ok. Dealer thinks has been sucking in dust and dirt, and the intake valve is damaged. I hope its something less serious. Any thoughts? BTW can someone tell me the correct plug? I checked an Intertec manual and it says Champion CJ8; no way! It's the FC420V. Thanks for any input.