If you are getting some smoke it suggests the spark is making some attempt to burn the fuel. It might be worth fitting a new plug, is the one fitted the correct one and does it have the correct gap? I had a B&S engine with a lovely spark that would not start, a drop offuel down the plug hole and away it went and kept running, now starts with no problem. If it is a fuel issue it may well be a blocked jet in the carb. If it is blow them through with an air line, do not be tempted to poke a bit of wire through to clean them.