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Username Post: Vacuum Advance- how is it supposed to work?
Posts: 291

Reg: 11-28-09
04-07-10 08:53 PM - Post#1894730    
    In response to NYH1

Missing: an engine with low vacuum, high overlap and high compression using full vacuum will have a tendency to hunt and stagger at certain combinations of throttle position and engine speed right above idle, making setting idle speed for auto trans very irritating.

Set idle speed to 700 with vacuum off.
Connect vacuum, extra advance raises speed to 1,000.
Set idle mix.
Scenario #1: if you touch the gas, the idle vacuum jumps, which spikes the vacuum canister and the RPM goes up way more than you intended.
You back off, but the vacuum goes higher momentarily, and the engine doesn't slow down. After 2-3 seconds, it does - and may stall.
Scenario #2: more throttle. Vacuum drops.
Can retards spark timing, which reduces vacuum, which retards spark timing, blah.
Result: soggy response in a narrow range of throttle motion above idle until RPM picks up.

Ported prevents the engine idle from being too "nervous" as to changes in idle vacuum. Yes, it generally requires more initial advance.

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