It just so happens, I've gone out to cut my lawn after writing the earlier message and found the same problem. My drive's gone! Found some better info though. You'll need the model and serial number of your mower. Mine is a 911 SP6 too and the model number is: 12B-658C611. You should find the serial number on the chassis somewhere. If not, use mine. It's: BG150V10252. Go to: http://manuals.mtdproducts.com/mtd/Public.do and enter those and there's a good few manuals on there.
Good Luck!
Eugene
May 2013
The support for this mower is disgraceful. I eventually found the manuals but I doubt it will help. It's pretty uninformative. I'd say that the belt has probably gone that goes from the engine to the back wheels/roller. If you look under the mower you'll see it under the cutter. Manual addy is: http://www.mtdeurope.com/docs/Manuals/769-05609C/EN.pdf
Eugene
May 2013
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