Honda HRB 475 Stalling when blade engaged?

Hi. I've been donated an HRB 475 which starts and pulls fine but when I pull the lever to engage the cutter the engine stalls as if something is seized. When not running I can turn the blade and can hear the engine turning over whilst the lever is pulled. I guess it's approx 10 years old. Your help would be gratefully received as my garden looks like a field!!! Thanks