Small Engines (Lawn Mowers, etc.): Weedeater Riding Mower, briggs and stratton, spark plug wire

Question
Hi, A friend of mine is trying to sell me a weed eater riding mower model HD3K4E9. It has a Briggs and Stratton 14 and 1/2 hp engine, model-287707, type-0255 01, code-9602194A. I saw the mower running recently, but now it won't crank. He said he changed the flywheel to one that didn't have a plastic ring gear and the starter that didn't have a plastic gear. He said that after changing these things that it ran for a few seconds and quit and has not been able to get it started since. I have an inline spark tester and we hooked it up between the spark plug wire and the spark plug. It showed it was firing, but to me the light looked yellowish. I always thought the light should be blue. I many be confusing this with the fire in between electrodes of the spark plug. I tried spraying starting fluid in the carb and it did pop a couple of times and that is all. He sat the old flywheel directly on top of the new one he installed and it looked like the key and the magnet lined up. I suggested he take the metal ring gear off the new one and put it on his old flywheel and put the old flywheel band on that previously  had the plastic ring gear. He left for vacation and this is where we stand right now. I do not know what could be keeping the engine from running at all. Since it is not mine, I don't want to experiment with it at this time. He is offering to sell me the mower for $100.00 plus what it would cost to get it fixed. The mower repair shop that sold him the replacement flywheel said it would work perfectly. I don't know what could have happened to the engine. So, I'm asking you if you might know what could be wrong? The mower looks to be in good shape cosmetically. Any help you could advise me with will be greatly appreciated.

Answer
The yellowish spark should be fine...various elements in the air will cause the spark to typically produce yellow/orange or blue spark.  

Since it popped when you tried using starting fluid I would suspect the flywheel key based on the flywheel being replaced.  

I am concerned with the starter pinion gear as most of these gears are plastic; I have never seen a metal Briggs pinion gear.

The mower is probably a 1997 model, as the engine was made in 1996.  Unless the mower has extremely low hours a $100 may sound like a good deal but how is the transmission, belts, blade quill assemblies, blades, battery and tires?  I only say this based on experience.  I have bought a lot of "bargin" mowers over the years only to have them suck money out of my wallet with everything eventually needing to be replaced to include engines, transmissions, all the belts, blades and quills.  These parts get real expensive real quick.  I have found it cheaper to buy late model mowers that require very little repair.

Are you mechanically inclined and can fix you own equipment?

Eric