It could be an injector or pump issue, but let’s eliminate the cheaper things first. With the engine up to temp, take off the oil fill cap, you should see a constant stream of blow by which is totally normal. Slowly raise the throttle up above idle and you should see the blow by disappear as rpms increase. If it does not, then your cdr is bad which can cause the smoke on decel. If it clears up, check your light load advance arm for free movement. You can do this with the engine off by operating the throttle back and forth on the passenger side of the pump, while looking at the drivers side. Directly across from the throttle arm on the drivers side of the pump you will see a small face plate or cam that has a ramp on it, an arm rides on this ramp and actuates on a pin on the bottom of the pump. Make sure the roller on the end of the arm moves freely with the movement of the ramp, and that everything is smooth with throttle movement. If that looks good, I would give your fuel system a good cleaning with your favorite fuel additive. I really like “hotshots diesel extreme” for this kind of work.
