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Technical SBC Timing Q

Discussion in 'The Hokey Ass Message Board' started by nosdan, May 4, 2016.

  1. nosdan
    Joined: Mar 27, 2009
    Posts: 228

    nosdan
    Member

    Quick question. So last night i fired up a new BluePrint 355ci roller motor. At TDC on 1, I dropped in the Accel HEI dizzy in w/ #1 terminal pointing to #1 cyl. With it running at idle, as i try to get it to 12* advance, the motor just takes off and I got the headers pretty hot. I shut it down quickly and let it cool. I tried it again, and same thing. It feels really good when at idle, the timing is 2* to 4* advance at idle.

    I came from the understanding that at idle, a SBC should be around 10*-14* advance at idle. I did not check out the total advance yet.

    Should I let it run at 0-4* where it "seems" better and check total timing from here? Thoughts?

    Thanks!
     
  2. jetnow1
    Joined: Jan 30, 2008
    Posts: 1,429

    jetnow1
    ALLIANCE MEMBER
    from CT
    1. A-D Truckers

    Two possible things to check- GM used several timing indicator and balancer combinations over the years- you could have a combination that gives you a weird reading. Also when you dropped in the distributor did you allow
    for the gear twisting the distributor? Sounds like it would be off about that amount.
     
  3. A lot of this is going to depend on knowing how much centrifugal advance your distributor has. This has to be added to your initial timing to determine your total timing. You may set your total timing and find that you have too much initial which can make the engine hard to start when hot. You may set your initial timing at idle and find that you have too much total timing which might cause detonation and possibly more serious engine damage.

    If you have a vacuum advance on the distributor, make sure it's disconnected while you're trying to sort out your timing.
     
  4. indyjps
    Joined: Feb 21, 2007
    Posts: 3,606

    indyjps
    Member

    Sounds like you're off a tooth. As stated above, can't trust timing cover pointer unless you assembled the engine. Verify TDC manually and mark TDC yourself for reference.
     
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  5. nosdan
    Joined: Mar 27, 2009
    Posts: 228

    nosdan
    Member

    When I purchased the motor, I also picked up the recommended balancer from Summit. I was also thinking, as I sit here at work, that perhaps the distributor could be a tooth off.
     
  6. nosdan
    Joined: Mar 27, 2009
    Posts: 228

    nosdan
    Member

    Vac advance is disconnected and plugged. I'll also manually check TDC just to be sure. Thanks guys!
     
  7. Blues4U
    Joined: Oct 1, 2015
    Posts: 4,340

    Blues4U
    Member
    from So Cal

    It doesn't matter if it's "a tooth off", the timing is still the timing. I.e. the ignition is still firing at the time indicated no matter which direction the distributor is pointing.

    But it is wise, as mentioned above, to verify TDC, not difficult to do with a tool to provide a positive stop for the #1 piston.
     
  8. Tooth off? Not a chance. The only time this comes into play is when you can't turn the distributor body far enough to get the timing where you need it. Think about what goes on here - "Timing" is when the spark fires in relation to the rotation of the engine. We explain this in degrees before Top Dead Center (Piston before it gets to the top of the stroke, and in rare instances After Top Dead Center) and it is measured in crank degrees on the harmonic balancer to a pointer. The engine doesn't care what "Tooth" the distributor is put in, or even where the plug wires are in the cap for that matter, as long as you can get the rotor to point to the proper terminal at the proper amount of degrees of timing and the firing order is right. If you are able to get 12° advance using your timing light, Resetting the distributor shaft by a tooth, only means that you will also have to twist the distributor body the same amount to keep that same 12° and you will still have the same problem. So it's time to go back to square one - use a piston dead stop and find absolute TDC. Compare that to what your balancer to timing pointer is showing. My guess, without being there is you will find this to be off. Confirm this before doing ANYTHING else. You have to know if your baseline is correct.
    Also you say that the motor "Takes Off" when you set it to 12° - by that I assume you mean the idle speed went way up. Well, as you advance timing, the idle speed many times will go up if you were way too retarded to start with. Did you try lowering the idle speed when it did this?
     
  9. nosdan
    Joined: Mar 27, 2009
    Posts: 228

    nosdan
    Member

    What I meant by a tooth, was that it in terms of degrees, it seems like it's one full tooth too BTDC. Yes, I realize I can turn the dizzy a bunch.

    Unfortunately I can work on it until this evening but I appreciate all the advise and input. I initially went off the timing tab, but I need to verify the TBC, rotor and cap are lined up properly... redo my base line markings. Then go from there.
     
  10. DDDenny
    Joined: Feb 6, 2015
    Posts: 13,098

    DDDenny
    Member
    from oregon

    Don't assume Summit got the balancer/timing tab combination right, not only is there an early/late balancer difference of about 8 (?) degrees, there were a handful of different bolt on timing tags offered by GM. The early 302, 327 and 350's used a balancer with the keyway in alignment with the timing mark, and the tab was spot welded to the timing cover, the 69 (ish) and later version was offset about 8 degrees and (most) ? used the bolt on timing tab.
     
  11. Actually there are two times that a motor gets the headers poppin hot, one is that it is timed too slow and the other is that it is screaming at idle.

    Now there are some things that can go wrong that we have no control over, one is the control mod in the distributer. You didn't say what you are using for a distributer so I am making the assumption that you are using electronic here and yes I have dropped in brand new electronic distributers that the engine would run but the control mod was goofy.

    now given than you don't have a problem with the control mod the next thing to assess is this which way are you turning the distributer and are you letting your engine run wild at idle. If you are turning your distributer counter clockwise then you are in fact advancing the timing. if you are letting it run wild and not slowing your idle down then one can assume that your headers are getting hot because of example number two of when headers get hot.

    There actually is a 3rd reason that I have not touched on, if your exhaust valves are too tight. But that is rare.
     
  12. Larry T
    Joined: Nov 24, 2004
    Posts: 7,679

    Larry T
    Member

    I'd leave everything where it seems to run the best until I broke in the cam (unless it's a roller). Then start troubleshooting it.

    I never understood the point the rotor at number one deal. Yes, you point the rotor at the number one tower of the distributor cap, but not the cylinder . I had a customer get mad at me the other day, because I reset his distributor to the way it came from the factory (63 327). He told me that it wasn't right.

    OK, I reread the original post and see that it is a roller motor, so you can start troubleshooting it. As everyone else has pointed out, verify TDC on the pointer to start with.
     
    Last edited: May 4, 2016
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  13. olscrounger
    Joined: Feb 23, 2008
    Posts: 3,613

    olscrounger
    ALLIANCE MEMBER

    Lots of good info here. Hot headers are a clue. Timing marks with various balancers and tabs can be a mess. Try a vacuum gauge-it will tell you a lot. Had this issue recently with a ZZ mtr. Found true TDC and made new marks-prior to that I set it with a vacuum gauge-with new timing marks and vacuum gauge they both ended up very close to 12 degrees. Factory settings can be goofy too. My 57FI BelAir manual said 6 degrees before tdc--wouldn't run well--liked 14 degrees!
     
  14. Some manuals show the number one cylinder to be off center by one tower from the front of the motor. I like them centered up personally but a lot of manuals show them differently than what I was taught.

    I think that the main thing is that the rotor is pointing at the number one plug wire tower whatever one that may be.
     
  15. DDDenny
    Joined: Feb 6, 2015
    Posts: 13,098

    DDDenny
    Member
    from oregon

    Speaking strictly on an oem application/component basis; here is (one) example of things needing to be exactly where they need to be. The vac. advance cannister limits distributor adjustment and has only a minimal amount of space between the intake manifold and the fresh air tube. These particular parts are all oem hi perf. related so not all applications will have these limitations. The crimped spot on the fresh air tube is factory applied.


    20160504_130547.jpg
     
  16. Larry T
    Joined: Nov 24, 2004
    Posts: 7,679

    Larry T
    Member

    Point style distributor, HEI is a little different.
    I set all of them I do exactly the same. You can almost hit the timing right before you ever hang a light on the engine
     

    Attached Files:

  17. Yea but that would be more of a hitting the proper tooth on the cam shaft/distributer gear more than which tower was used for number one wouldn't it?
     
  18. When I go to take an engine apart and re-use all the components, I bring it up on #1 and mark everything. Distributor body to the intake, rotor to the distributor body. Wards off some of the evils when it goes back together.
     
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  19. nosdan
    Joined: Mar 27, 2009
    Posts: 228

    nosdan
    Member

    Update - was too retarded on the timing. In my fast attempt to get this motor in the car, I must have dropped in the dizzy and slid just one tooth off. I say a "tooth off" because after doing so many, I always setup my dizzy's the same way and know visually where the vac advance should approximately be. Once I turned it counter clockwise a fair amount, it responded like normal. Yes, this is an Accel HEI dizzy as mentioned in my original post @porknbeaner.

    So a quick, pull and re-drop of the distributor (since the vac advance was almost touching the intake manifold), It's all good. Rushing to get keep the exhaust appointment on time... Thanks everyone.
     
  20. olscrounger
    Joined: Feb 23, 2008
    Posts: 3,613

    olscrounger
    ALLIANCE MEMBER

    Glad to hear all is well
     

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