Spitting out of the mixer indicates that pressure is coming from the cylinder. The most likely path for that is past the intake valve. If the intake valve isn't seating properly, it will leak compression and combustion gases into the mixer. If the intake valve is sluggish in closing due to either a weak spring or a gummy stem, the valve will still be partially open when the piston begins the compression stroke forcing air back past the valve and out the mixer. If the valve is leaking (as opposed to sticking open), it can usually be heard as a slight wheeze or hiss coming from the mixer when the engine is rolled up against compression.And that would cause it to spit out of the carb ?