There is another issue that can cause your problem. Behind the belt pulley that drives the friction drive plate is a hub.
That hub needs to be able to slide freely. The shaft for the drive plate runs through bearings that are mounted inside
the hub. That whole unit moves forwards and backwards to engage and disengage the drive system.
(pictures below)
Dirt, grime and rust can cause the hub to bind up inside the sleeve. When you depress the drive handle the hub moves
backwards and the friction plate comes in contact with the friction wheel. If the hub can't move freely you can have problems
with the drive not engaging smoothly. And not disengaging when the handle is released.
Picture showing hub with blower housing and pulley removed hub is fully extended (drive disengaged)
Drive engaged postion
It's easiest to get to, to really clean and lube the hub assembly by pulling the blower housing. Then removing the large drive pulley.
It's a lot of work.
But I've also been successful curing this problem removing just the belt cover and working from the top.
It 's a tight place to work with the pulley and belts right there. But less work than
pulling the blower housing and pulley.
So I would just you try spraying some WD40 etc...Some fluid that is thin not thick like oil. On the top of the sliding hub and in the wide slots on both sides of
the hub sleeve. Then working the handle up and down. Tilt the blower back a little so the fluid will run
down into the sleeve. (Don't spray to much at one time) Also take a large screwdriver or pry bar and make sure the hub is fully extending out of the sleeve.
It may be hung up / stuck not fully extended. You can pry on the fork shown in the photo above. Then lube it and work the handle.
Also check to make sure that the tube the the fork is weld to is not stuck or binding up on the shaft that runs through it.
Everything needs to move freely. Or the spring tension won't be enough to release/disengage the drive.
So you don't get confused, this photo is of an old ST724 I have sitting around. There's no engine on it so you not seeing any
belts. The blower housing is just the shell so your also not seeing the pulley on the end of the impeller shaft.
Yours will look different with all those items in place. The hub is behind the large belt pulley in the photo.
