Small Engines (Lawn Mowers, etc.): Kohler Engine runs rough, hp gas engine, methyl hydrate

Question
Dear Paul, Thanks so much for providing this service - I have learned a lot just reading thru your site.  I have a Kohler V-twin 25 hp gas engine in a Simplicity Legacy lawn & garden tractor.  It suddenly started backfiring & missing while mowing one day a couple of weeks ago.  Sounded to me like water in the gas, although that was not possible, as it always kept inside when not in use.  Took the plugs out, they had about 70 hrs on them, one was covered in a whitish powder, the other a sooty black - more normal looking.  Cleaned them & re-installed, cleaned the air & fuel filter, added about 1 oz of methyl hydrate in case there was water in the gas.  Result was a very slight improvement, but still lacking power.  So I put in a new air filter, pre-filter, fuel filter, drained & replaced the gas & new plugs gapped to what the owner's manual said.  Still the same problems.  The missing is worse at higher throttle settings & when mower is engaged.  Have not touched the carburetor or coil, could this be the problem?  If so, are there any Kohler books that would detail how to do this that you would recommend, and if so where could I buy one.  I have talked to the dealer, and they said just to bring it in, but that is not easy as we are in a rural area, far from their shop and their hours are not very convenient.  I like to do my own work if possible to know it's done properly, and the dealer, although very good, would have the machine in there for about 2 weeks.  Thanks so much for your assistance.  I really aprreciate it and keep up the great work!!
- John Morgan
Argyle, MB Canada

Answer
Getting a manual is easy.

http://amazon.com/s/ref=nb_ss_gw/104-2741631-0761516?initialSearch=1&url=search-...

You have 2 different but very common problems.
Current engines run so lean that the white plug is normal.  The black sooty plug is the problem.  "Black sooty"  means rich (carb leaking) every time. EVERY TIME.  

The other problem is that both plugs should look the same.  The white plug side has a blown head gasket.

So, you need to, replace both head gaskets (the other side will blow soon) and rebuild the carb.

Difficulty level...Front disc brakes.