far as I know,there is no b20 or bio available up here in Calgary.....any other cleaner or additive that might help?
The more i think about it, the more i agree that it must be either a fuel or IP related problem. If something in the top or bottom end let go,it would not clear itself or at least get better with higher rpm.

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1980 mercedes euro 300D 232000 miles(died,now parts car)
2004 2500 ram cummins diesel
1980 harley shovelhead
1988 anniversery corvette zr1(engine)
1978 240D 4spd 240000 miles (new benz toy)
1946 Cessna 140, Continental 0-200