a factory manual is allways a good investment, you got to clean the crap out,
no additives will help, you should clean out the carb(s) too, new filter, flush the lines and the tank.
the carb has small parts, you would need to remove the carb, the float bowl,
the needle and float,... ect
mite be diafram carb (s) on thare, similar but dif'.
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