Today's problem is..

Today I was adjusting the idle circuits on my quadrajet and noticed while tuning the numbers bounce around rapidly sometimes up to a full point like from 13.8-14.8 at idle, is this normal for tuning with a wideband, seems difficult to get a good accurate picture of what is going on. The car is a 78 with long tube headers and the sensor is in the driver side collector. Also, should I tune for the highest vacuum and see what air/fuel ratio I am at, or tune to a 14.7 ratio at idle. I know it needs to be leaner off idle under cruise conditions, what is a good mixture for cruising?

Thanks for any info.

Chris

BigBlockk

The idle needs to be a little rich. Something like 13.5 to 14. The A/F signal jumping could be a header flange leak or a vacuum leak at the intake to head flange. Your cruise mixture should be something like 15.5 to 16.5.

BigBlockk

Later.....

8T8T2

Have had recent experience with this. In process of tuning with LM-1 on my 80 quad, M4ME if I remember right. She seems to idle best between 13-14 afr and this is usualy where she has the best vacume. In my case 13-14" and this is with vac advance hooked to manifold vac.
I would see up to a .5 erratic reading on the afr.

zwede

The bigger the cam, the more you'll see it bouncing around at idle. Usually it's best to set the idle mix to best vacuum and note the wideband reading for reference.

For cruise you should tune it as lean as it will go without surging or missing. Again it depends on the cam. Many engines will run fine at 15.5:1 at cruise.