[newshirt]

The spark lever must be all the way up when starting, as mentioned by Mitch.

When I first read your description, it sounded like the spark was advanced. (As if you had pulled down on the spark lever or the engine wasn't timed right.)

When the spark is advanced (spark lever pulled down), and a cylinder fires, the engine compression works against the starter motor. The starter may halt or even kick back. Here's why: The piston is being pushed down by the engine compression while the starter is trying to push it up.

But hey... you may also have a weak battery.