Vacuum pressure

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Joel Chapman

Joel Chapman

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Just got done swapping the intake and carb on my stock 71 318. Went from a 2 barrel carb and cast iron intake to 600 cfm Holley 4 barrel and LD4B edelbrock intake. Holley says use a vacuum gauge while turning the idle mix screws. What is the goal reading I am looking for on the gauge while turning the screws?
 
Simple. SET TIMING FIRST. Get idle as close to what you desire RPM, then adjust carb screws alternatively for highest vacuum reading, and if idle goes up or down, readjust RPM. Do this until no improvement is made, and then "end up" with the idle screws "leaning" on the lean side of the peak. That is, just lean them both down a tiny bit the vacuum should not fall off much or at all. Engine must be FULLY warmed. If you have a tach/ dwell, highest vacuum should also coincide with highest idle RPM
 
IIRC you are looking for highest vacuum you can get at idle.

That means the butterfly's are as closed as much as possible, and the idle mixture screws are open or closed just right.

What 67dart273 said!
 
what 67Dart273 said. I just finished doing that exact set of steps and my Dart is running just fine. Took my dog for a boot, thoroughly fun.
 
Joel Chapman said:
Thank you so much!
Click to expand...
Also, make sure that both side idle screws are adjusted the same (ie same # of turns out. - you dont want one at 1.5 turns and another at 3.0).

You should have topped out around 15hg, what did you note?
 
Vacuum reading depends on several things, including altitude, "I think" compression ratio and certainly camshaft specs. It also depends on where your initial timing is set.
 
Considering you are, what, 3K ft? above sea level that is probably "not bad"
 
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