I've got a Sears Craftsman 22" self-propelled lawn mower that's maybe +5 years old, and which has worked pretty well over the years. Recently, we've had trouble with the "self-propelled" feature of this self-propelled mower - in that when you retract the bar to engage the drive wheel pulley, the wheels seem to fail to engage and rotate.
I've pulled the drive wheel pulley cover, and absent any other immediately visual problem, I took a chance and replaced the v-belt with the stipulated replacement part pulley. That seemed to not help all that much. The problem as I see it, is an issue with the distance of the "throw", when the drive wheel pulley is tensioned and pivoted on its axis, creating friction with the drive belt and transferring torque from the drive pulley to the drive belt pulley. From when the drive belt is slack to when it is tensioned, the belt just doesn't seem to become tight enough to transfer torque to the drive wheel pulley.
By my diagnostic logic there are three or four solution/options, all of them with the goal of increasing the friction on the drive belt when it's tensioned to transfer torque to the drive wheel pulley: 1) increase the length of the "throw" of drive wheel pulley assembly 2) increase the diameter of either the drive pulley or the drive wheel pulley 3) decrease the length of the drive belt 4) buy a whole new lawn mower (my wife's favorite solution to most any problem where something is broken). Let's toss out #4 as being any kind of legitimate option we're going to consider (we'll just keep that a secret from my wife for right now . . .).
My examination of the drive wheel pulley assembly seems to show that there's no way to increase the amount of distance when the assembly is pivoted. You can't shorten the retraction spring (creating more tension), and it appears as though there's hard stop built into the assembly which limits the "travel" distance and the assembly looks like it's already at it's maximum throw distance when it's tensioned right now.
For #2, I'm pretty sure I can't replace either the drive wheel pulley or the drive pulley, since those are engineered pieces which would be hard to find suitable replacements for - and which had larger diameters. I guess I'd be open to the idea of "mechanically" increasing the diameter of the pulleys (by wrapping something durable around the drive surface(s). Not sure what that material would be which would be sufficiently durable to stand up to the drive forces on the pulley and which would provide enough surface friction to transfer torque from the drive belt when it turns. I guess there's always Duck Tape, right? The universal solutions to all problems around the home, except for maybe mending a broken heart (let's leave that last one as an "open question").
Lastly, for less than $20, I could purchase a drive belt that's shorter in length than the OEM belt size. This seems like the best, easiest and most long-term solution, but how much shorter does the replacement belt need to be? The current belt spec is 32.9528 inches. I've got to believe that the shorter length needs to be not more (less) than ¼ to ½ inch shorter - since I'm pretty sure nothing bad is going to happen (belt failure, etc.), if the belt is slightly
over-tensioned, when the drive wheels are engaged.
I think I'll stop there for now. I'll include a couple of pictures of the drive pulley, just so everyone can see what's (not) going on, when the drive belt is tensioned. I'll monitor this topic and I'm happy to respond to follow-up questions.