Reply to "Idle Vaccuum"

Adjusting the idle mixture screws by 1/2 turn will not have a noticeable effect on idle vacuum unless the adjustment was close too being to lean or massively too rich.

1 1/2 turns out sounds about right.

I agree with the suggestions of looking for vacuum leaks and changing initial timing.


A vacuum leak will increase idle speed if the idle mixture is made richer as it will act like opening the throttle blades with the idle speed screw on the linkage.

Many high performance cams have lowish idle vacuum eg: 10" but a bit of clever tuning can usually get them up to 15".



Good idle vacuum is a mix of:

compression,

ignition advance,

valve overlap,

idle speed,

throttle blade setting,

Fuel and

bypass air eg: air bleeding in from vacuum powered devices like brake boosters, vacuum operated heater-A/C controls, pollution controls and even leaky vac advance canisters on the distributor.

Or even deliberately bleeding air in below the throttle blades to get more idle air into the engine without opening the throttle blades further, as in some cases that can start mixture flowing through the carburettor's transfer slots making the idle mixture adjustment screws unable to control the idle mixture properly.



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