vacuum advance

Discussion in 'The Garage' started by Big GMC truck, Jan 15, 2006.

  1. Big GMC truck

    Big GMC truck Registered Member

    Aug 23, 2005
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    Could someone tell me what vacuum advance is? If the vacuum advance isn't working, could it cause off the line hesitation, but still allow for decent mid and top range performance? Thanks
  2. diesel4me

    diesel4me 1 ton status Premium Member

    Jul 24, 2003
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    Its a long discussion..but to answer your questions,yes,if its not working at all,or is hooked to the wrong source of vacuum,it can cause hesitation,fast idle speed,pinging,and a bunch of other things..

    I simply advances the spark IN ADDITION to the centrifical weights and springs mounted inside the distributor ,at the proper time,to maximize power,economy,and emission can hook it to "manifold" vacuum,(a vacuum port located UNDER the throttle butterfly )..that has 18-21" of vacuum at idle,and falls off at higher throttle openings..

    Or "Ported" vacuum,which is ABOVE the throttle butterfly,and is "dead" at idle,and increases during throttle opening (to a degree anyway)..both ways lose ALL vacuum during wide open throttle,thats then the centrifical weights "kick in" and control the advance yes,its very possible to get good "wide open" performance with a "dead" vacuum adcvance...because it does nothing then anyway..but you will notice the "lag" getting there without the extra advance supplied by it at part throttle..

    Most Chevies use "ported" vacuum,but I've had a few small blocks run better on manifold vacuum,especially those with aftermarket carbs like Edelbrocks..try both ways if your 86 305 would ping like crazy on ported vacuum when timed to specs..
    I tried manifold vacuum and it no longer pings,and runs much smoother at idle,and is peppier!..who knows how GM had it set up originally,with a "ESC spark module box" and "electric Q-Jet" carb..both were ditched in favor of an Edelbrock and an older HEI with vacuum advance..seems to get a few more mpg on manifold vacuum too..:crazy:

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