Probably the differant head dropped your compression ratio. That would make the engine want to run at full speed.
By itself, bore doesn't affect compression ratio (CR).Are the 12.5hp & 14hp carbs the same? That 12hp carb may be getting overwhelmed by the bigger engine. I just checked the specs, The stroke on the 14hp & 17hp engines are the same, However the bore is larger on the 17hp, That would cause a compression decrease. - Brad
BRAD,Jerry, I fully understand about what was said above. I'm not trying to start anything here, But if he put a cylinder head from a larger engine that had a larger bore, The combustion chamber in the cyl head on the larger engine has been casted or machined to acommodate the lager bore. When it was installed on the smaller engine, The piston wouldn't be able to fill the cylinder with as much volume as before, Because of the larger head. Thus decreasing volumetric effeciency and compression. Its like putting a chevy 350 cyl head on a smaller bore 305, The compression ratio will drop significantly. - Brad