My 2.0L zetec (blackbird carbs / gems 3d ecu) pops a little at only a couple of ignition load sites (quite quiet not loud bangs, I probably shouldn't worry really)

2000 rpm @ 0% load to 2500 rpm @ 0% load. The LC1 wide band confirms that the mixture is slightly lean at these points (15-16AFR).

(0% = throttle fully closed)

advance at these sites is

2000rpm @ 0% 24.61 degrees
2400rpm @ 0% 26.02 degress

Apart from these two sites the car runs very well which is confirmed by the LC1 wideband showing good AF ratios everywhere else so I am loathed to adjust the fueling. So can I just reduce the advance at these two sites to stop the popping. I was thinking something
like

2000rpm @ 0% 20 degrees
2400rpm @ 0% 22 degrees

Or should I just enjoy the popping

Popping on overun is/can be an air leak on the exhuast side of things, most tuned cars "pop" on overun though, if the exhaust is short enogh nice flames come with it.

Thanks Jon, but the popping only happens at the specific load sites above. If the exhaust was leaking air wouldn't it pop in other places too ? Mixture is definitely slightly lean when it does it.

[Edited on 26/4/06 by givemethebighammer]

Id make it worse. A pop sounds good.

AS Jon said its usually an air leak.

Popping after letting off the gas comes from being lean. You hear it a lot on dyno pulls where a car is running out of fuel on the top end. If you were injected Id suggest fattening it up a little, but if the A/F is good everywhere else just leave it alone. Some engines just do that and its more noticable with louder exhaust. As long as its just a couple little pops its fine. Louder bangs are hard on valves .

And yes, an exhaust leak can introduce air into the gases making them lean as well.
Cheers.

thanks for the replies. One of the westy people suggested that I increase the advance at the sites in question. Two degrees of extra advance seems to have reduced the popping to a quiet burble. I guess this may change if the engine / exhaust gets really hot. But I'm happy with it.