Sorry for the late response. I was referring to Tillotson carb'ed motors. The ones in the pictures are Walbro's (WMC's).
If I remember correctly, the 25hp Tillotson's have designations like BC1A, BC1B and BC1C. The 18hp has BC2A, etc. My (POS) Clymer's manual for this engine lists different timing at idle. At WOT (wide open throttle), I think they are all the same. Regardless of the HP and the carb maker.
Also, on the Tillotson's, there are two lines on the throttle cam. One is for the 18hp and the other is for the 25hp. Remember that the Walbro's are different.
My Tillotson carb(s) has a chainsaw float needle installed and it has the material for the tip that withstands the scourge of ethanol in our gas. The original tip swelled up so much from the ethanol that no fuel could get into the carb even though the bowl (plastic and not ethanol resistant either) was bone dry.
My Frankenmotor has a 25hp carb on when used to have the 18hp one. Same the throttle cam/linkage was adjusted for the 25hp setting. The idle timing was set something like 5-BTDC whereas the 18 was something like 2-ATDC. My boat (Abalone, a 14-ft Duranautic), is now wicked fast and I like to race dual-motor Regulators on the CT river. Scary and I always lose, but it is a lot of fun.
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