Small Engines (Lawn Mowers, etc.): Kohler K321 wont rev up,idles fine, spark tester, valve job

Question
Hi David; I am working on a Ford 145 Rider with a Kohler 14hp engine. K321AQS, Spec # 60244, Serial # 12139775, Compression is 125#. The motor seems to start and run fine at an idle, but when you raise the throttle to rev it up it spits and sputters sometimes backfires and wants to quit. As soon as you lower the throttle back down it will idle fine again. I have switched out the carburetor, coil, new coil wire, new condenser, new points(set @.018), new spark plug(set @.025). The spark plug gets kind of sooty black real fast like it is not burning the fuel. I have the points cover off so I can watch the spark and it doesn't seem to be interrupted when it is spitting & sputturing. I also put a spark tester on the plug and it shows no intermittent spark. I checked the engine ground. I guess my next step is to look at the valves. I am about at my wits end on this engine. I guess I need some help to point me in the right direction on this one. Any advice would surely be appreciated. Thank You--Jerry

Answer
I would do a valve job as you indicate.  When you switched out the carb, did you switch it out with a new one?  a known working one?  

Be careful spending to much money on repairing this engine, many parts are no longer available.

FWIW.. my dad had one of these things that did the same darn thing in the late 70's   we never did find a fix for it!