Quite possibly a bad spark plug. But, have you spun the engine over without the plug in and with the cylinder pointed downwards to see if the engine is flooded? Too much gas is just as bad as not enough. Also, try a 6 volt battery connected directly to the ignition box; bypassing any switches, etc.

The ignition timing should be set at 28 degrees "antes de punto superior". (BTDC)

I prefer to run 20:1 oil in the .90/15cc engines if using non-sythetic oil. A good synthetic oil should be fine at 32:1.

AV8TOR