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Any leakage at the throttle shafts will have a substantial impact on economy -- they will go way rich under load if you have them set to compensate for the leakage at idle and/or for drivability at small throttle openings.
How is the timing advance? A B18B can *maybe* use as much as 34 BTDC at high RPM, and anything over that will reduce economy. That should come out to 10 or 12 BTDC at idle if the advance mechanism is working correctly. It shouldn't be anywhere close to pinging running premium gas (if premium is 91 octane where you are, I assume you're some distance above sea level -- the higher you are, the less octane rating is needed).
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