Ignition timing - 92 Metro

[302Z28]

I recently set the ignition timing on my '92 Metro. I set it @ 5 BTDC with the jumper wire installed on the plug by the firewall @ 900 RPM. Upon removing the jumper wire, the timing jumped to about 15 BTDC and the idle speed increased. I adjusted the idle to 900 RPM. Now at the slightest load, it spark knocks. Even at 900 RPM, the timing is still around 15 BTDC. It would appear that the computer thinks it needs to be set that far advanced. My thought is that the computer is trying to increase the idle speed through advancing the timing. Perhaps a combination of various sensor signals are causing this. I have an idea, but need opinions as to whether or not anyone has ever tried this: adjust the set screw on the bottom of the throttle body to open the butterflies slightly further. This should increase the vacuum signal as well as the idle speed. Then reduce the idle speed using the screw at the top of the throttle body. Perhaps this will eliminate any condition causing the computer to advance the timing. Any idea what this would cause? Any ideas would be greatly appreciated. Thanks

[RossT]

With the geo idling at 850-900 ground the terminal. When I do this, the idle drops some and the timing retards to the it's base state. Now adjust the timing to 5. When you remove the the ground the timing should advance past were you had set it, and the idle should go back up to 900. (at least this is what my geo does) If your idle is now above 900 adjust it down. If you can't adjust with adjusting scew, scew all the way in, then you have a vacuum leak or someone has messed with the capped screw (the one that your not suppose to adjust).

I wonder if your timing belt is off a tooth?

Are you adjusting the idle with the system grounded before you check the timing? I don't. I only adjust with the system ungrounded.

I may be wrong on all of this, but it works for my geo's.

[GM Line Rat]

Complete Thread on how to properly set the "Ignition" timing on 89-94 Geo's with electronic Timing (Inc Pics)

TeamSwift 1.0 G10 Ignition Timing

You should NOT have to adjust the idle speed on the car, the onboard ECM (Computer) controls that!.

[91Caprice9c1]

Originally Posted by 302Z28

Now at the slightest load, it spark knocks.

Perform a compression test. What we are looking for here are excessive carbon deposits. If you get results in the, say, 220psi and above range, then you have carbon deposits heavy enough to raise your compression ratio to the point that it is accelerating the flame front quicker than is accomodated by the factory setting of 5° base timing.

If you don't find high compression figures, the next suggestion I'd give would have to do with a non-functioning EGR system. This isn't my initial estimate, however, because even with entirely blocked passages, the engine should tolerate a little more than 'the slightest' load before pinging. Run the engine up to operating temperature, then with the engine idling, manually open the diaphram on the EGR valve with a screw driver, the car should stall within 5 seconds.

-MechanicMatt