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Excessive blow back like that indicates the reed petals are not closing properly and they may have some dirt lodged in them or one may be broken. Being a down draft style carb doesn’t help much keeping crud out either as the screen on top will let small stuff into the carb throat and reeds.
I’d suggest pulling the carb and the plate under it to gain access to the reed block. Once it’s in your hand, you should be able to inspect it more closely. Generally the reeds can be open no more than 0.010″ but I like to see them just closed as this seems to improve idle quality. A small amount of spit back is normal on most two stroke motors and can be expected.
As for the motor sneezing, that could be caused by worn crank seals allowing air to enter the crankcase and leaning out the mixture. Here’s how I check for leaky top seals.