7747-4cyl experiment

[Shannen Durphey]

rr wrote:

> >
> > There is now some question as to what 747 base timing is.. "0" with d5
> > unplugged,( more logical, imho)  or "6" with eprom out? I never moved
> > distributor timing throughout test...
> 
> This doesn't make sense to me either.

Base timing is set for the 747 with the bypass line disconnected.  The timing
calcs work with the base timing value in the calibration, which should match the
set distributor position in most cases. Set the base timing to agree with the
747 bin using the procedure for the 747.

If the timing is showing six degrees with the eprom out and the bypass line
disconnected then you have a problem.  The problem may be in your timing light,
or in the distributor, or somewhere else.  But the ignition module should not be
using anything but internal timing with the bypass line disconnected.

The only exception to the "you have a problem" scenario is if the idle speed is
significantly increased with the eprom removed.  If the idle speed is high
enough for the module to begin to advance timing, that may be a valid reason to
measure six degrees of timing.  But the rpm would have to be fairly high for
this to happen.

You may have stmbled on an interesting side note.  Some set timing procedures
require putting the ecm in "fixed timing mode."  The ecm may be controlling
timing rather than simply allowing the distributor to remain in bypass mode. 
Consider that four cylinder engines experience fairly large changes in camshaft
rotational speed and an ecm controlled set timing mode may actually be more
appropriate.


Shannen
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