Driving me nuts
 

My 122's original K301 was getting tired and using a fair amount of oil. Planning to pull it for a rebuild, I picked up another to use in the meantime. The original's PTO started slipping last week so I went ahead and swapped engines. The replacement runs well and the PTO works fine, until it is warmed up and under load. It will be running along fine and as the governor kicks in for the load it will stumble, belch black smoke, and then die, pushing black smoke back through the carb as it does so. If I cut the throttle to idle it will stay running and I can throttle it back up. If it does stall it will start back up. I first thought that it might be a carb problem, but after adjusting it and then swapping to the one off the good running original, there is no change. Is this a sign that the cylinder is full of carbon, a valve issue, something else? I'm not sure when I will get a chance to rebuild the original so I'd like to sort this out. Any help would be great. Thanks.

Sounds to me like it may be a timing issue. I am sure you can find a thread here on how to properly set the points.

Slapping my forehead
 

I never even thought to check the points since the replacement engine started right up. Worn/out of adjustment points would lead to a weak or mistimed spark which would lead to an over rich mixture which would show as black smoke. Looks like new points tomorrow. Thanks. I hope the handprint on my forehead fades before I go to work in the morning.

We all over look things from time to time, that is why I love this forum. People here are great and I read through archives until I find the answer I am looking for. OR I will ask the question and soon will have an answer.

If points don't fix the problem, I have seen a bad head gasket cause those symptoms. Usually you will see some black residue on the side of the block under the head where there is a leaking head gasket.