I have a Sears Garden Tractor w/ a Kohler 25 hp engine. Recently,we had the fuel solenoid replaced (this problem was correctly diagnosed by NEWAUKUDOM! Thanks!) and since we have it gotten back, the engine burns black smoke (rightside seated), especially under load. Checked the plugs, heavy oil and new plugs turn black in short order. The mechanic who fixed the solenoid, said it was probably the rings or valves, a $700 rebuild should fix it. A buddy of mine who knows engines tore the whole thing down. Rings were fine, the cylinder heads and valves had heavy carbon deposits, which we cleaned. Put it back together and we restart the tractor and here's what occurs.. it starts fine under medium choke, go fast to full throttle and you get black smoke and some chugging... back it down and go SLOW to full throttle... no smoke and it seems to run fine. And a check of the plugs, show good clean, normal burn. It seems the fuel/air mixture at the thottle is at least one culprit... any help would be much appreciated... Thanks!

I am glad my previous advice worked. You are right black smoke is an air/fuel mixture problem. The fuel is delivered to the combustion chamber (cylinder) through a carefully sized nozzle in the carb, the only way for this to be wrong is if the mechanic who replaced the solenoid tore to far into the carb and lost the main nozzle and decided to replace it with one that fits. Not a likley scenario but given his diagnostics on needing a rebuild it would not shock me.

The other issue could be not enough air, remove the air filter and try it.

Lastly remove a spark plug wire while it is running and see if the plug is not firing. A twin cylinder engine will run relatively smooth with a cylinder not firing and it is hard to tell.

CHECK THE OIL LEVEL! Too much oil will cause the problem under high RPMs also.

Don

Don... did the checks above, the nozzle is the original (according to the Kohler site), it does the same with the air filter off and we did the spark plug test. The oil level is fine, but while cleaning the carb, we noticed that the diaphragm on the accelerator pump looks like a small dried up mushroom, very wrinkled. It's still pliable and it does not have any tears (light coming through it), but it does not look like the one on the Kohler site. My question is could these be the cause of the black smoke once the engine is at full throttle and/or under load (blades engaged)? Thanks, Chris

Private message me with a phone number and time I can contact you, let me go through this with a little more detail and solve the problem.