Small Engines (Lawn Mowers, etc.): One confusing Kohler, black smoke billows, head gaskets

Question
Hello Ben. I need help diagnosing my CV730-17 Kohler 25hp on my 2003 (Sears) tractor. Quick history, would not start this spring. Repair shop replaced the fuel cut solenoid and it started right up. Over the phone, repairman said it was running rough, blowing black smoke and fouling both plugs with oil. I told him it had never done that before.  He suggested a rebuild ($700) to fix it. However, since that time, it will start, idle fine but as you increase throttle speed to full, it begins to chug and black smoke billows out of the front end (near exhaust). Under load (blades engaged),heavy blue/black smoke and it is really not operational. My friend who has some knowledge of engines volunteered to look at it. This is what we have done,to no avail. We tore the whole engine down and removed it from the tractor.  Cylinders, valves, oil reed and rings all looked fine and were cleaned. We replaced the accelerator pump kit (original diaphragm looked bad) and we replaced both head gaskets and bolts.  The carb has been totally disassembled and cleaned.  The float appeared OK and had no leaks,but we did not replace it.  Of course early on, we did the plugs, fresh fuel, filters, fuel lines and fuel pump. We retraced abit and removed the fuel solenoid (thinking that may be restricting the fuel mixture) but it continued to do the same chug/heavy smoke at full idle. It seems to be an air/fuel problem. We have adjusted the fuel mixture screw with it running, without success.  We have not replaced the breather/PVC filter above the left cylinder. Any assistance you could provide would be greatly appreciated.  Thank you for your time, Chris

Answer
Chris
Did the repair shop rebuild this engine? If they did Bring it back to them.
Your problem sounds like you may have valves adjusted wrong or may have a bent or bad valve,You should have ground down  the valves and the seats. If the intake valves leak or are out of time you are starving the engine of air while it is running causing the blue smoke and the chugging. You said that the "Cylinders, valves, oil reed and rings all looked fine" Looking at them doesn't tell you if there is a problem also the rings may be wore.