Timing effects on AFR

[getZ]

This surprised me the other day playing around with the new carb so I was wondering if this was normal.

 

I installed the new 750 Hp and base timing is set to 20 degrees advance for base timing. I checked the AFR and it was reading about 11.5. I plugged the vacuum advance in and the AFR goes to about 12.5. I didn't go back to double check to see exactly how much timing the advance put into the timing, I would guess its about another ten degrees so a total of about thirty. Can 10 degrees of timing change the AFR that much?

[grumpyvette]

If youve ever wondered why the air/fuel ratio changes when the ignition timing changes at times,its not unusual,but

its NOT exactly the IGNITION TIMING CHANGE,

but the resulting differance in manifold vacuum readings ,that changed the air fuel ratio, because carbs flow fuel in direct responce to changes in the vacuum in the carbs venturies

 

http://www.centuryperformance.com/tuning-with-a-vacuum-gauge-spg-148.html

 

http://www.centuryperformance.com/ignition-and-timing-settings-spg-219.html

 

http://victorylibrary.com/mopar/cam-tech.htm

 

http://victorylibrary.com/mopar/intake-tech-c.htm

[Pyro]

I'm sure the idle speed went up when you plugged in the vacuum advance. and since you didn't adjust the throttle means more air into the carb from a higher vacuum with the same amount of fuel.

 

Next time, compare a/f using the same idle speeds. And readjust idle mixture screws for best idle (highest vacuum) with each timing change.

 

yes, a lot of timing at idle allows the engine to run leaner. low timing at idle is an emissions thing.

[getZ]

thanks guys, this is all good information. I always thought you tuned to an ideal AFR (usualy richer than 14.7, maybe around 11-13), but that isn't the case and it's not always the same for the entire rpm range.

 

I'm going to install a vacuum guage inside somewhere, so I can get a better idea of how the engine is performing.