Alta has it right .. your ignition is firing too "early" as the piston is still coming up. Is it a magneto ignition, or electronic?

To work correctly, a gas engine has to have ignition timing that advances with RPM. Let me try to explain ... At low RPM, e.g. when starting, you want the spark to fire very close to the point where the piston is at "top dead center" (TDC). But as the engine speeds up, you need to have the spark go off before the piston gets to top dead center .. typically 26-28 degrees (almost 10% of the arc of a circle!), this is due to the fact that the combustion of the gas/air mix is not instantaneous, and this advanced timing lets the explosion of the fuel/air fully develop as the piston gets to the top, this gives maximum power.

If the ignition spark timing is advanced when starting, it will fire on the upswing of the piston, resulting in the kickback.

In a magneto engine, the designer compromises on an advance (the experts here can tell you how much) that is set a little before TDC .. so it is not too hard to start, and develops a reasonable amount of power, but not as much as it could with a more advanced spark timing. It's a compromise... Many magneto engines (e.g. the Zenoahs) use a spring starter to make this more user friendly.

The electronic ignition units work by having the spark advanced just a few degrees at low RPM, and smoothly advance to 26 or 28 degrees at top RPM, giving the best of both worlds. But they can malfunction, and advance the timing at low RPM and you'll get kickback. I have heard that some brands of ignition do this when the battery is low... or the unit might be defective. Or the sensor might be placed at the wrong point .. they often can slide back and forth a little for tweaking.

Hope this is helpful... and gives you a place to start troubleshooting.

Dave

(edit .. looked up the SPE motor .. it's electronic ignition)