ECU and Timing Question

I've just adjusted the timing on my 95 Civic (SOHC 1.6L PGM-FI). I warmed it up to operating temp, jumped the 2P connector and checked the timing at 750 RPM. Then I made the necessary adjustment.
I've had no problems with setting the timing, but I noticed something. I had supposed that the jumper was there to stop the ECU from "messing" with the timing, but that it was not necessary for *reading* the timing. Of course I checked and adjusted with the jumper on just to be sure, but I found that when the jumper is not connected, timing at 750 RPM is retarded by a several degrees.
Is this normal behavior? Can anyone enlighten me as to why it does that?
Thanks in advance.
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sharx333 wrote:

are you sure it retarded, not advanced?

read this: http://www.ecutek.com/tuning/ignition

very punny.
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jim beam wrote:

Sorry about that. I just couldn't resist.
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jim beam wrote:

Jim, I checked again, and at 750 RPM, it's definitely *retarded*.
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It's necessary for BOTH.

Very simply, when you set the timing with the ECU locked out, you were setting the "base timing". "Base timing" is the datum point for the ECU. It's the point from which all other timing adjustments are measured.
Since the timing is affected by distributor position, the ECU has no idea where the timing really is when it does its adjustments. The ECU just has to assume you've set the datum to the right place.
Why did the ECU decide to retard the timing when you un-jumpered the connector? Couldn't tell you. But so long as the engine runs normally, I wouldn't worry about it.
In cars without distributors, the ECU knows exactly where the timing is at all times, so there's no need for the "base timing" adjustment.
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TeGGeR

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Thanks, TeGGer. Spot-on as always.
Jim: Yes, I think you're right, it's advanced, not retarded. Thanks.
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