Altitude affects all normally aspirated engines in the same way. Fuel to air ratios typically want to be roughly 14:1. With less air density at higher altitudes, the engines run rich (too much fuel for the amount of air available). Rejetting the carburetor with smaller jets can keep this ratio in the proper range but even if done so, the general rule of thumb is that there is a 3% loss in horsepower for every 1000 feet above sea level. The effects become more noticeable once you exceed 3,000 feet. If the engine is not jetted correctly for the specific altitude, this loss increases. Note: modern fuel injected engines automatically compensate for variations in air density and atmospheric pressure and maintain the correct fuel to air ratio. However, that being the case, they are still subject to the same 3% per 1000 feet loss.
