Check the idle speed, maybe too fast, I don't know what a stock 250 should idle at but somewhere in the range of 550 - 750 rpm. How about the heat range of the spark plugs? if they are extremely out of the specified heat rang it may be creating a hot spot in the chamber and "dieseling"
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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