Hi mika,
Have you checked weather it chokes fully or whether the flywheel key is sheared?, If its good I would think about checking the exhaust port and muffler for blockages. If all else fails I would start to look at the amount of fuel that�s getting thought, as it may be a simple blocked jet ect. I would think that the spark would be sufficient because it does fire and run after 20 pulls. Hope this helps
Regards jay
