I feel stupid for asking these...

Discussion in 'The Bench' started by racerxjj67, Jun 23, 2017.

  1. racerxjj67

    racerxjj67 Well-Known Member

    I read things on here and being the shy kid in class I'm too afraid to raise my hand, but I can't learn if I don't ask.

    So I need these dumb questions answered:
    What does bleed down mean?
    What does dwell mean and why is it important?
    What are the laymen's definition of mechanical advance, advanced timing, and initial curve?
    When referred to "ported vacuum" does that mean to the carb?
    When I reinstall my rockers to the heads do I need to measure 'lash' or is that for something else? I assume that's the space between the rocker and the pushrod. Yes or no?
    And when people talk about finding TDC (I know this acronym) I see a lot of mention about gauges, dials, etc. can't you simply (if heads are off) turn the crank until the #1 piston tops out?

    I'll have more in the coming week.

    Bonus question: What is the air-speed velocity of an unladen swallow?
     
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    First, they aren't dumb questions, and I can answer all of them, but more importantly, you can't work on your car unless you understand the basics. If I answer your questions, you'll just have more, or not understand my answers, because, you don't understand the basics. What you need is a book that explains all the basics. Do you have a copy of the Buick Chassis manual for your Buick? That is where I would start if I was you.

    Bleed down usually refers to hydraulic lifters. They run on engine oil pumped by the engine oil pump. Oil enters the lifter, and escapes past the plunger clearance as the lifter operates. If the clearances are excessive, or the lifter is worn, the lifter won't hold enough pressure to operate adequately, and it will make noise.

    Dwell usually refers to ignition points. Dwell angle is the amount of time (measured in degrees of distributor cam rotation) that the contact points remain closed. Dwell is the time period where the coil is charged, so it affects the spark intensity.

    Mechanical advance consists of 2 weights inside the distributor that pivot outward as the distributor spins. As they swing out due to centrifugal force, they advance the ignition timing. All this is covered in my Power Timing thread, have you read it? Advance curve is ignition advance vs. RPM plotted on a graph.

    Ported vacuum is one of 2 types of vacuum available from most carburetors. Manifold vacuum is the other. Ported vacuum originates above the throttle plate, so it is non existent at closed throttle. As the throttle opens, it rises rapidly. Manifold originates below the throttle plate, so it is always there when the engine is running at idle and part throttle.

    With a stock Buick engine, there is no adjustment for valve lash. Hydraulic lifters operate with preload, not lash, and in a stock engine, all you do is torque the rocker shaft down to the head, and every valve is adjusted. When you rebuild an engine and you use an aftermarket cam, all that goes out the window and you need adjust ability. Either adjustable push rods or roller rockers.

    TDC is Top Dead Center. In a running engine, the piston is at TDC 2 times in a full cycle for that cylinder, once on compression, and once on exhaust. It corresponds with the balancer mark lining up with the 0 on the timing tab. Actually, that isn't exact as the piston can dwell at TDC for a few degrees. You can find true TDC with a piston stop or a dial indicator.
     
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  3. sriley531

    sriley531 Excommunicado

    Is it an African or European swallow?? :D:p
     
    Last edited: Jun 23, 2017
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

  5. lemmy-67

    lemmy-67 Platinum Level Contributor

    Timing is measured in degrees before TDC...hence BTDC in a 4-stroke engine. Suck, squeeze, bang, blow.

    As RPMs increase, the piston reaches TDC at a faster rate, but the flame front from the spark plug igniting the fuel/air mixture reaches the piston crown at the same speed every time. Hence, the higher the RPM, the more advance is needed for the ignition timing.

    Hence...
     
  6. bhambulldog

    bhambulldog 1955 76-RoadmasterRiviera

    Unladen Swallow speeds
    Climbing ; 31 mph
    Level flight; 35 mph
    Diving ; 41 mph
     
    racerxjj67 likes this.
  7. bhambulldog

    bhambulldog 1955 76-RoadmasterRiviera

    This thread should be a 'sticky'
    Good answers to good questions
     
  8. OHC JOE

    OHC JOE Mullet Mafia since 2020

    No dumb questions asked on this board ever I have asked many questions when I first started on my pops GS and I have learned a bunch from all these dudes on here great place for great information. Have a great weekend guys
     
    bhambulldog and hwprouty like this.
  9. John Codman

    John Codman Platinum Level Contributor

    Great explanation Larry!
     
  10. GMB-GS1

    GMB-GS1 Silver Level contributor

    And thats why he is called "The Wizard". I would bet that Larry has helped the majority of us V8 board members either with a direct reply or with one of his many articles saved as Stickeys. Thanks for helping us all Larry!!
     
    70skylark350 and OHC JOE like this.
  11. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    You're welcome, I can't believe I have the top post count, by a lot,:D
     
  12. sriley531

    sriley531 Excommunicado

    I can... :cool:
     
  13. racerxjj67

    racerxjj67 Well-Known Member

    "With a stock Buick engine, there is no adjustment for valve lash. Hydraulic lifters operate with preload, not lash, and in a stock engine, all you do is torque the rocker shaft down to the head, and every valve is adjusted. When you rebuild an engine and you use an aftermarket cam, all that goes out the window and you need to adjust ability. Either adjustable push rods or roller rockers."

    I'm installing an aftermarket cam from TA. It says stock pushrods are ok. So can I still just torque the rocker shaft down and feel confident the valves will adjust properly, barring they aren't faulty?
     
  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    No, you have to check pushrod length anytime you change a cam. What cam are you planning on?
     
    Last edited: Jun 26, 2017
  15. racerxjj67

    racerxjj67 Well-Known Member

  16. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    You will need to check for the correct push rod length. You can use your stock rockers, which is what TA says.
     
  17. racerxjj67

    racerxjj67 Well-Known Member

    Oh crap, I totally read that wrong. I'll call TA. Dang, that adds to my ever growing budget.
     
  18. racerxjj67

    racerxjj67 Well-Known Member

    Adding another timing question: What is ignition timing? And when does it come into play as far as setting the timing?

    Is there a chronological order that needs to be followed when setting timing? Like you can't do one without already doing the other?
     
  19. John Codman

    John Codman Platinum Level Contributor

    Assuming a stock distributor with mechanical and vacuum advance, you must disconnect the vacuum line to the vacuum advance diaphragm and plug it (golf tees work great for this - I always keep a couple in my toolbox). The ignition timing, which is the point that the spark plug fires, is then set with a timing light which is basically a strobe triggered by the electrical pulse in the spark plug wire. There is usually an index and a pointer on the crank pulley or harmonic balancer. The distributer hold down clamp is loosened just enough so that you can twist the distributor by hand, but not so loose it's flopping around. You align the pointer with the proper timing mark on the balancer at the manufacturer's specified RPM, and tighten the distributor clamp. The timing light is attached to the #1 cylinder spark plug wire and the battery. There are a few oddball engines (some International V8s come to mind) that have the timing set to a cylinder other then #1, but Buicks are normal. I realize that this subject can be made vastly more complex, but I'll leave it at that.
     
  20. racerxjj67

    racerxjj67 Well-Known Member

    So generally speaking, when the mark on the balancer lines up with the zero mark on the index pointer that is what is referred to as the: Ignition time? So that should be the first step to setting the time? When you say; "...the manufacturers specified RPM..." the manufacturer of what? The cam, engine, distributor, etc?
     
    Last edited: Jun 28, 2017

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