My 280 is a six plunger and am pretty sure the 250 is the same. Same proceedure as the 113 or at least that is what I have been doing all of these years with success. Only differences might be the firewall idle switch and firewall throttle shaft and where it attaches to the accelerator pedal. Minor differences though.
As a side note: I love the 250s and wish mine was one. I just bought a used wood instrument binnacle to convert my leather covered housing to the earlier wood housing. Can't wait.
