Have a JD lawn tractor that will not start, however it is trying and only result is a VERY loud 'back-fire'. Whats my problem? Thanks.

 The loud bang on the start when the engine is still cold is indicative of ignition timing problem. Your John Deere probably has a single cylinder Briggs & Stratton engine which is known to sheer its flywheel key which throws off the ignition timing and it fires when the exhaust valve is still open, that's why the bang. You can check the key but you will have to remove quite a few parts that are one the way: the blower housing, the starter cup, nut/washer, the fan and its backing plate. You should be able to see the key without actually taking off the flywheel but it is still a pretty difficult task to get to this point, so getting it to a professional shop may be a prudent option.