I doubt it is underpowered - sounds like something on the deck is binding - with the drive belts off check each pully & spindle for free rotation. You should find the problem. May be grass or twine or something wound around spindle underneath or may be bearings that were not greased or shaft bent ??
also check to see if the deck belt is to tight....beleive it or not, but if the belt is a little to small and forced onto the deck, the motor will have a hard time to spin it.......But i would also look at the post before mine....they have some great advice....good luck....let us know what you find.....Don
Well just as the season is coming to an end , I see your post ...
Anyhow I am thinking you are talking about kicking in the blades a minute or two after cold start, if so slide the choke up about three quarters just enough to not let it affect the engines running speed..Then kick in the pto(blades) it should continue idling well ,then slide choke off and go , Works for me...
There is also a spacer washer on some that allows the pulley to spin the blade freely without binding. If the washer is missing the pulley is not at the same height as the other one and can wear greatly on the belt. Good luck. Gearjammer