Timing jump at idle

[Bruce Plecan]

From: "Bob Valentine" <bob at tecmark.com>
> I'm trying to figure out why my timing varies +/- about 4 degrees.

Cause it's computer controlled and the ecm is always refreshing it's
thinking.
Timing erraticacis (?) due to the timing chain
Chains have tight spots on them.  While really difficult to usually find (is
that enough qualifiers), for a good example look  at a motorcycle's cahin,
when it's about worn out.  Some will get so bad that you can almost hold
them out straight, the links are so worn.

>  I've
> flattened the table for several cells around in gear idle, 625rpm.  MAP
> stays rock-steady at 35kpa.    Injector pw varies from 2.55-2.6ms.  The
> idle is reasonably smooth no surging or misfire, but would be smoother
with
> the timing stable.

There are so many other things going on I'd wonder how much smoother.
The O2 is going rich lean, rich lean, IAC might be moving, each charge is
going vary some in how it burns.

> With the timing connector pulled and a timing light on the crank, the mark
> stays even on 6deg.   The mark moves and follows the scantool with the EST
> hooked up.  I changed the distributor (and module) since the one in the
> motor had a slight bit of end play - no difference, same symptoms.   Plug
> wires are routed as far from the harness as possible.    Disconnected the
> alternator to rule out AC noise, no difference.
> Is this "normal" or is something going on?

How rough is it?.  Or is this just something that caught your eye, and now
has you thinking?.  If the engine is idling smooth then, I'd say your overly
concerned.
The XFire with the big cam, would have the timing really dancing, and when
in gear be really smooth for what was going on, inside the motor..
HTH
Bruce

> -> Bob Valentine
> -> bob at tecmark.com
>
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