self propelled lawnmower has lost almost all of it's pulling power.

I have a self propelled Toro, 6.5hp,variable speed, front wheel driven lawn mower. The last time I used it,(months ago) I had just cut some high weeds. I noticed the lack of power when on my driveway and then when I go into the grass, it stops completely. I don't know if this mower has a gear box, a static clutch or a transmission. the belt and pulleys are on top. I've cleaned out all the grass and weeds from the axle and even took the wheel off to see if any gears were worn .
That just seem to make it worse, if nothing else.
So anyway, I am open to suggestion, I would've just taken it to a lawnmower
repair shop, but I don't have a truck to get it there.

 

Does the self propel cable have an adjustment wheel on it? If so, adjust it tighter.