Backfiring through the exhaust sounds like the fuel is too rich and/or the extreme oil consumption is burning in the hot exhaust after the ignition is terminated. The 2 stroke Detroit Diesels that I drove only used 2 qts in 350 miles, 8 - 10 hour trip, 80,000 lbs gross on the trip home.
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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