It could be as simple as the idle speed set too high. A little more information would help, what transmission? air conditioning? idle speed in neutral? Does the engine run too hot prior to shut down?
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
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