Re: [ST] Surging [was: Carbon Fiber Pipes and Helibars]

["Rick & Joyce" <rnjwes@xxxxxxxxx>]

> > <snip>  the engine is a little snatchy right off closed
> > throttle, which was not there with the standard tune, but
> > it's not a big deal.  Has anyone else noticed that?

Paul,
Low-RPM surging can be caused by improper idle fuel mixture.
Your '02 has a closed-loop idle trim control which is modifies
idle CO based on inputs from the oxygen sensor and others.  This
shouldn't affect running above 1500rpm.  But the O2 sensor only
'sees' the mixture from one cylinder, so a bad injector spray
pattern in another cylinder, for example, wouldn't be detected.
This was a problem in the automotive arena some years ago and
manufacturers relocated the sensor(s) downstream to the exhaust
collector area.  If the 'race pipe' tune were to call for a fuel
mixture which was dramatically richer than that of the O2
sensor-controlled idle trim, one might have an abrupt transition
to open loop around 1500 rpm.

On the pre-'02 ST/RS, loading a new tune is only part of the
procedure.  The other part is to check and adjust the idle CO,
which in the US except for CA-spec engines, should be in the
2.5 - 3.0% range.  Changing the tune will often affect the idle
CO, sometimes dramatically.  If a following rider sees black
smoke during hard acceleration, an over-rich mixture is
indicated.

Another factor is fuel quality.  My '01 surges horribly on
'winter-grade' oxygenated fuel (gasahol), particularly when
holding a steady throttle in the 4k rpm range.  When I'm out of
the PDX metro area and can fill up with the normal formula, the
transformation is nearly instantaneous.  Dirty injectors will
cause surging as well.  Once in awhile, I add injector cleaner
with a full tank.
HTH,
- -Rick in Oregon
'01 Sprint ST



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