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Technical do a isky cam make low vacuum ?

Discussion in 'The Hokey Ass Message Board' started by oldsfrench, May 31, 2019.

  1. oldsfrench
    Joined: Jan 26, 2018
    Posts: 150

    oldsfrench
    Member
    from France

    32 degrees is with the vacuum disconnected. ...
    I think now the problem is with the power break unit. Éven with the 32 degrees advance and 18 vacuum break pédale si hard.
    Il took a manuel vacuum pump
    Set it at 22 vacuum connected to the power break hose and still have hard pedal. 20190603_180920.jpg

    Next saturday i Will remove the power break unit and see what's happend.
     
  2. sunbeam
    Joined: Oct 22, 2010
    Posts: 4,206

    sunbeam
    Member

    My suggestion plug off all vacuum ports to get a base line number.
     
  3. partssaloon
    Joined: Jan 28, 2009
    Posts: 366

    partssaloon
    Member

    That is normal for any Olds with a cam. They had a hard time making good vacuum in the stock form.
     
  4. dan c
    Joined: Jan 30, 2012
    Posts: 1,885

    dan c
    ALLIANCE MEMBER

    i inherited a 350 nova that was a drag car. previous owner went nuts because power brakes wouldn't work. pulled the cam; it was an erson with 108* lobe separation. great for race but no vacuum. new cam with 111* helped immensely. new factory engines run a lot more lobe separation than that!
     
    Deuces likes this.
  5. I don't understand something. Usually a hard pedal means that you have good brakes. Maybe we're missing something in the lingo.
     
  6. oldsfrench
    Joined: Jan 26, 2018
    Posts: 150

    oldsfrench
    Member
    from France

    hard pedal is because vacuum power brake don't work..
    sorry if my explainations are not good ...( i am french , my english langage is poor ... )
    now with the new advance ignition timing , valves lash setting i am at 17/18 vacuum.
    there is a lot of vacuum at the vacuum port at the rear of the carb.( it want to eat my finger when i put it at this port )
    i have to check if the hose who come from the check valve and goes to the master cylinder is not "colapsed" inside
    or something in the master cylinder is broken .
    i first though the power brake don't work as i had very low vacuum...
    but now vacuum is good , so i have to check the master cylinder and the vacuum hose
    thanks guys for your answers , i keep you informed..for sure
     
    Deuces likes this.
  7. A warmed up camshaft will definitely cost you some manifold vacuum, at idle especially. Mostly due to the later intake valve closing if I remember correctly. But there are some mods you can make that will improve the situation. You just need to accept that making one change in your tune-up will probably require making other changes as well for the best effect.

    For example, as mentioned, it will help if you can increase your initial timing. But after a certain point you can easily end up with too much total timing so you might have to limit the rate and/or the amount of mechanical timing in the distributor. This might also be a good time to make sure your distributor has a good working vacuum advance. This can help keep the vacuum up at idle if connected to a full manifold vacuum source but without having so much initial timing that it becomes difficult to crank the engine over when it's hot. And again, you'll still need to make sure the total timing isn't excessive.

    Changing valve lash will also have an effect on manifold vacuum. Remember that most factory specs assume the engine is at operating temperature. You are sure you have solid lifters, correct? There is nothing wrong with setting them cold, but if you're going for .020" lash you should probably adjust them cold a couple thousandths looser. As the engine warms up you'll lose a couple thousandths due to heat expansion, especially on the exhaust valves.

    Running slightly more lash will open the valve a bit later and close it slightly sooner which should pick up the vacuum a bit. As you noted though it does make for more valve-train noise. But as the engine comes up to temperature it should quiet down a bit. If the noise bothers you, you might try adjusting only the intake valves a little looser than stock.
     

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