Smoking is usually cuased by too much oil in your gas. If it has been setting
for a while oil will separate from the gas and line the bottom of tank
thereby causing a rich mixture. That will foul your plug, causing it to die.
If that doesn't solve you problem the carb is the next logical step assuming
the correct plug is installed (preferably NGKs). By the way I have a complete
engine with new top end points condenser and flywheel 250 if interested.