My gut tells me your initial ignition advance may be set too low. The PO may have lowered it so they could run regular fuel without knocking... This is just a guess though.

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-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250