Honda Outboards BF40A/BF50A Service Manual, Page 287Get this manual
BF40A-BF50A speed does not Increase: Faulty Automatic SE valveI I
Replace: P5-15 and 21
-.j Throttle cable misadjusted-.j j -.j
Propeller mismatched
Readjust (see base shop manual)
Replace with the correct propeller
Spark plug faulty