karlson

My 10-12 year old Snapper I 722 runs like a champ but I noticed that I was not so self propelled anymore in the lowest gear (a good gear to use with the deep, wet snow last weekend here in Minnesota). 
Anyway, more recently, it seems I'm losing power on the second to the lowest gear.  I can get it to engage if I nurse it but it's not reliable.   I'm detecting a theme and assume it is just a matter of time until
third goes.  I've done oil changes, lubes, plug and muffler replacements but have never opened the chassis.  Is this a case of needing to adjust something?  Replace a belt or two? 
Can a DIY guy do this or should it go to a shop?