Re: Johnson 9.9 & 15 hp common parts
Search this message board, you'll find several threads on this. It sounds like the 15 uses a different carburetor (larger venturi size), a different reed plate and different exhaust parts. The basic engine block is the same as far as bore/stroke but intake and exhaust timing may be a bit different too, meaning that the intake and exhaust ports may be cut in the cylinder walls of a 9.9 differently than those of a 15 hp motor. If that is so, then the engine block itself is actually different too. I don't recall if the 15 runs higher compression than the 9.9, but if it does, that suggest different pistons as well. <br /><br />In a previous thread, someone suggested obtaining full parts lists for both motors (use parts catalogs for the 9.9 and 15 of the same year) and comparing pertinent parts numbers.
