It will be one of 2 things piston is scratched or the diaphragm and needle need to be replaced either way u will have to take carb off take look at piston to see it's condition and dissect carb. If the diaphragm is not flat and is warped it is no good and will need to get a kit to replace the carb part's
Question edited for a lot more clarity, add maker, fix typos, change into what I think you were saying. Manuel is the name of a Spanish gentleman, 'manual' is the word you need.
Manuals in the link, plus the maker. This link show 'M Series' belt.
https://www.manualslib.com/manual/1865101/Lawn-Boy-M-Series.html?page=153
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