Bob Chilcoat wrote:

Another technique that works is to use a propane torch (unlit). Direct the
gas from the torch into the carb and pull the starter cord. The engine will
start on the propane and you can see if it will then run on the gas in the
tank by just removing the torch. I wouldn't try this on two-stroke engines
because there is no oil in the propane, but it works really well for
four-strokes.

....

The time it's running on a little non-mixed fuel will be unlikely to
make much difference at all...keeping a can of starter fluid on hand for
such useage is always worthwhile..