tips on setting idle mixture........

[Bill S.]

screws on my L72 with the 3247 carb. i set up a vacuum guage and found i peaked around 15 or 16 lbs. i have seen reference to 11 lbs. is this normal vacuum at idle? i find that it is constant through a wide range of adjustment of the idle screw. what is the right proceedure to set this up? thanks,bill

[Joe C.]

Re: tips on setting idle mixture........

Bill,

15-16 in-hg is high for L72. Are you certain that the camshaft is original, or spec replacement? Your lifters could be set too wide, which could also cause too much vacuum. 11-12 in-hg is where it should be, @ idle.

Best way to set idle mixture is to obtain highest steady vacuum reading, as you are doing. Both screws should be within 1/4 -1/2 turn of each other MAX.

Joe

[Bill S.]

Re: tips on setting idle mixture........

by set too wide you mean too loose? what are some sde effects of too much vacuum? the cam is supposed to be original spec.

[Joe C.]

Re: tips on setting idle mixture........

Bill,

By too wide, I mean too loose. If that is the case, then even if the camshaft is original, or original spec, then the effect of this is less lift, less duration and less effective overlap. This will cause the idle quality to improve, and sound more like a mild camshaft. This will also cause idle vac to increase, as with a milder cam, because of the smaller overlap.
More vacuum is NOT a bad thing, but, check the specs for the L72 engine. I suspect that the stock camshaft, with properly clearenced lifters, should produce no more than 11-12 in-hg @ an idle speed of about 750 RPM. As I said, more vac than this, AT APPROX 750 RPM IDLE, indicates either wide lifter setting or a milder camshaft.
Have someone give you a more accurate idle vacuum reading from a real L72, with properly adjusted lifters, AT A SET RPM!! ie. 750-800RPM.

Joe

[Bill S.]

Re: tips on setting idle mixture........

i am measuring at 850 rpm at idle with constant vacuum. i will read as factory set. i set the valves after cam break in after "normalizing" the engine as per the chassis manual. thanks,bill

[John L.]

Re: tips on setting idle mixture........

If i remember corectly, he has converted to a manifold vacuum source. What he is reading is fine, in my opinion, if he in fact is using a monaifold source.

[Bill S.]

Re: tips on setting idle mixture........

i am, thanks,bill