Double check its air escaping from the carb and not a sucking sound.
Pull the reeds out and check that all the petals are intact and sitting snuggly down with no gaps its a ten minute job. You may have to replace the gaskets if they are old or damaged. If the reeds are sweet eliminate possible air leaks around the reed block and carb by replacing gaskets and cleaning mating surfaces.
A common problem after the 300 motor has had the oil injection removed people dont always do a good enough job blocking off the injection port on the cases so check this is sealed good.
The other thing out of interest 300s dont like to start unless the battery, starter motor and cables are in top condition. If everything is healthy the motor should crank fast even with the spark plug in.
