Briggs and stratton 6.4hp working but VERY hard to start?

so i just got the carburetor working like its supposed to... there is a spark.. the gas is clean.. its got brand new oil... i prime it.. but it does not start easily.. it takes about 10 minutes of non stop pulling to get it going... and when it does go the plastic piece on the carb that turns the valve.. moves back and forth slightly.. making an idle type sound.. and when i push the valve all the way open the motor hits a good speed like its supposed to... but if i hold it there too long the ending shuts off.... what should i do?