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By early 1925 the horsepower of Hercules and Economy engines had been increased simply by upping the rpm about ten percent. At this same time the Hercules color was changed from dark green to a lighter green color. These changes are likely well known among Hercules engine enthusiasts.
Not so well known was the introduction of 1 and 2 HP model SK throttling governed Hercules engines at that same time. The Hercules block was slightly modified with additional holes in the base for the throttle rod to go from the governor to the inside above the fuel tank and out the front to the fuel mixer.
A water-cooled head is used and where the normal mixer screwed in, it was modified to provide a flat place for the two fuel mixers to bolt on. This mixer with its starting gas reservoir is quite similar to the mixer on the later and more common XK and JK kerosene engines. Since the water-cooled head is thicker on the 1 HP than the hit and miss engine head, a longer side rod is required.
Shown at right is a page from the manual with the special instructions for these engines.
How many of these 1 and 2 HP SK Hercules engines were built and currently exist? It is likely that fewer than 1,000 of the 1 HP size were built. Probably only a few hundred of the 2 HP size were built. Here is a listing of those currently known.