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Unlimited All-Stars

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C. Piston Port, McCulloch or US820 (West Bend) Two-Stroke Cycle Engine or engines, 170 pounds plus one pound for every cc or fraction of a cc. Multiple engines or cylinders are permissible. 210 cc maximum total displacement.

By far the most common piston port (valve) engine is the Yamaha KT-100 although almost every major kart engine manufacturer around the world makes a similar engine. Some of these engines fall under the classification of JICA engines. All of these are subject to this weight formula. Twin piston port engines have become a fairly popular engine choice in the UAS.
Piston valve engines are distinguished by the carburetor that is mounted on the lower front portion of the cylinder.

If the engine pictured measured 98.7cc the weight would be calculated as follows;

170 lbs (base weight) + 99 (displacement rounded up) = 269 lbs

Although not pictured, West Bend and McCulloch engines also fall into this category for weight purposes. They are reed valve engines but are fan cooled and have a blower housing and other cast or sheet metal shrouds.