81 911SC surging

I have an 81 911 sc and when at a constant rpm, in gear, the engine surges. Its like a small acceleration then deceleration. I have done a major service including wires and O2 sensor. What do ya think? Any ideas?

Try resetting your CO and see what happens.

Do you mean the mixture adjustment?

My car tends to do this also, and right now I am still making minor adjustments in the mixture and the timing. The mixture is about dead-on, but I have the timing retarded ever so slightly. Next step is to get the timing to 5 degrees BTDC, reset the mixture if needed, and see if performance improves. The car also can hesitate when I blip the throtle immediately after startup. While this is not a problem it didn't do it before and, properly adjusted, will stop. I removed the distributor and replaced things like injectors.

I will post my experiences at various mixture/timing settings, but I suspect that with an oxygen sensor duty cycle that is right at 50%,and ignition timing that is right at 5 degrees BTDC, the car will run perfectly.

Let me know if you need an explanation of how to set the CO, if you don't already have one. Oops. Wait a minute. Your car may not have and oxygen sensor. I may not know how to set CO in your car.

these things are critical though. CO (mixture) and ignitiion timing are critical. Anything out of spec will run poorly.

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'83 SC