What exactly do you mean it still runs like a six? Same speed, same WOT RPM?
Manufactures used lots of small changes (economical) to get different horsepower ratings out of motors using the same block. Smaller carbs, restrictor plates, lower compression pistons and heads, port alignment on sleeved motors, and exhaust systems to name a few. Any of these will affect the motors maximum RPM’s. The 8 horse may also have a different gear ratio or pitched propeller than the six horse. Without parts diagrams of each motor for comparison, it’s a guessing game.
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