Engine only smokes at start up?

As a first guess I'd suggest that you're getting a little "cylinder washdown", meaning that an injector or the carb is allowing too much fuel into one or more cylinders and it's picking up some of the oil.

That can be caused by a carb with a sticking float that allows fuel to dump into the intake after you turn off the engine... assuming that it's a carburated engine.

OR

an injector that's leaking a little and when you turn off the engine it's allowing the fuel to leak in much the same way.

It could be other things as well but this is a more likely cause and certainly a good place to start.

could be valve seals. oil left in the heads when engine is shut down gets by the seals when engine is stopped ,and leaks into cylinders. this is ONE thing that could cause it, I'm sure there are other causes, that are possible.