It depends on which MS261C he has but since he has decompression then it is either a C-Q or a C-QZ
(5) MS 261 C-BE, (6) MS 261 C-Q, (7) MS 261 C-Q Z
#6 & #7 uses the 1141 020 1200 PNC with decompression valve hole.
#5 use the 1141 020 1202 PNC without decompression valve hole.`
Plus if he has MS261C-QZ he will have a spark arrester in the muffler. These do clog with carbon causing running problems too.