The Yamaha SZ's engine may be getting heated up and stopping for 20 minutes due to a number of factors. The engine may not be generating enough air or fuel, it may not have been adjusted properly, the spark plugs could be broken, the cooling system could be not working, or the engine could be operating at an excessive temperature. 

You have to tune the engine, change the spark plugs, examine the cooling system, and check the air and fuel filters to resolve this problem. To help keep the engine cool, you can also add water and/or a radiator fan. The Yamaha SZ engine shouldn't overheat and halt for 20 minutes if you follow these recommendations.