toro tractor will not maintain constant rpm's

Purchased a new B & S 17.5 intek motor for my wife's lawn tractor several years ago to replace one that went bad. I noticed from day one that rpm's was too low and after putting a tack on it, showed 2600. No way to adjust it except to bend throttle rod some to get to 3400 rpm. Still will not maintain constant rpm, 3400 with no load, 2800 under load. Took it to a dealer and he has no idea why the governor will not maintain rpm. Everything moves freely and with engine running can speed it up by moving governor arm. I hate to be defeated by an engine but this one has got me baffled. Any ideas would be appreciated! Thanks, Richard