Small Engines (Lawn Mowers, etc.): Scotts by JD 21 inch walk behind, blown head gasket, john deere self

Question
I have this Scott's by John Deere self propelled 6.5 B&S mower.  Its been a pretty good mower except the way it directs clippings into the bag from a side discharge - gets clogged a lot.  But I have begun to have a few problems.  It is SP6211 1-speed - model THDWALK1SP.  Issue 1) Cord recently broke and after I replaced it (myself) when I start it up it revs to a hi RPM and then goes back to normal.  But then after a few uses it fouls the plug.  Its like the timing is off.  Issue 2) recently the drive belt for the self propel broke.  I just thought it was old, so I bought a belt from an auto parts store that seemed to be the same.  The width was a little wider, but still fit in the pulleys.  After installing it I engaged the drive and it kicked the belt off.  So I assumed the transmission is locked up.  But I have opened the transmission and it looks like everything is ok, it might just need  some new grease.  when I manually push or pull the mower (while off) the mechanisms on the rear axle, wheel gears and I guess the transmission sounds a little clanky.  I guess I cant bring myself to pay $100 bucks for a new transmission on a 5 year old mower. Could a slightly wrong size belt cause it to jump when engaged?  

Thanks for you help.

Answer
Hello Izzy:

The Required Belt is a 39.1 X .380 Belt. I Usually use a 39 X 3/8 Belt. The Belts Stretch During the First Use so the .1" is a Non Issue. Also, is the Belt Guides Properly Set? Is the Input Shaft Bushing Worn? If the Input Bushing is Worn, it can Cause the Shaft to Bind when Engaged and Throw the Belt. It Sounds like the Belt Guides are Not Set Properly. The Plug Fouling can be a Blown Head Gasket Between the Push Tube Galley and the Compression Chamber. This is an OHV Engine and this Happens a Lot. You can View a Breakdown/IPL of the Engine and Mower at this Site Addy, http://jdpc.deere.com/jdpc/servlet/com.deere.u90490.partscatalog.view.servlets.H... and Enter the Model Numbers SP6211 and Click Find. Then Select your Model from the Search Results Area. Then Select the Sectional File. Then Select the Section of the Mower you Wish to View Online. If you Hit a Snag or this Does Not Correct the Problem,  I am Here if You Require More Assistance. Hope this Helps. Let me Know What Happens, Please. May the All Mighty Bless You and Yours. Thanks.

Good Luck

Respectfully

John

PS: I am Allowed to Answer 10 Questions a Day. If you See I am Maxxed Out, then Try Submitting your Question at or Just After 8pm EST (US). My New Day Starts then. Thanks.

Respectfully

John