The reason for changing from ported to manifold was one of idle emissions -
these are significantly reduced with manifold. If idle exhaust emissions
are part of any routine government testing you may have problems. Changing
from ported to manifold should not introduce any fast idle problems (what
were they, too much or not enough?), nor performance since the only
difference is at and just off idle. Backfiring on the overrun can be caused
by a faulty (or missing!) gulp valve which injects air into the inlet
manifold.
Balancing timing against octane is always empirical as every setup is
different - especially after 30 years of wear and changed parts! Even when
new the factory had to individually test every car and selectively pin the
vacuum advance plate on some cars to meet the regs.
----- Original Message -----
From: "Marc Siegel" <smarc@abs.net>
To: <mgs@autox.team.net>
Sent: Monday, July 14, 2003 7:15 PM
Subject: Lessons learned: vaccuum sources
> So, performance problem solved, fast idle problem solved, backfiring
problem
> seems solved!
>
> Other than empirical testing, is there a good rule-of-thumb for timing
> settings vs octane rating?
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