Idle when timing 70z28 LT1

Discussion in 'Troubleshooting & Diagnosis' started by 70RSZ28CAMARO, Jun 2, 2019.

  1. dave@ztech

    dave@ztech Veteran Member Lifetime Gold Member

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    Did the mechanic lock your timing?
     
  2. 70RSZ28CAMARO

    70RSZ28CAMARO Member

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    Not sure what you mean by lock timing ? Are you referring to the distributor hold down bolt ?
     
  3. COPO

    COPO Veteran Member Lifetime Gold Member

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    Last edited: Jun 9, 2019
  4. dave@ztech

    dave@ztech Veteran Member Lifetime Gold Member

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    Sorry , what I meant, was he did some modifications to the dissy , and it no longer needs the vacuum advance, to do it's thing, some mechanics think that they are not needed, they don't understand how they work, and will plug them off, not a good idea, unless the advance is locked in, because now the vacuum advance is not doing it's thing, like kicking up the advance by 10* at idle and cruising , so you will need to do a little reading and investigating to see what your setup is.
     
  5. badazz81z28

    badazz81z28 Veteran Member

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    The distributer advances timing with centrifugal force as the distributer spins. Springs on the rotor control how much timing depending on RPM by how heavy the springs are. If it’s “locked out”, timing doesn’t change because springs are not there. What you need to do, is keep the vacuum line capped. Set your idle initial timing. Keep your timing light on it and rev the engine slowly and see if the timing advances. If it doesn’t, it’s locked. If it does advance, see how many RPMs it is until it stops advancing and what timing it stops at.
     
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  6. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    ^This (above) is what you need to do for a starting point. But if you have a stock harmonic balancer, it does not have degree marks, so you can't read total timing, which should be 34 ~ 36*. You can get timing tape to put around the balance that will give degree marks from just about any speed shop or parts supplier, just make sure you get the correct diameter. I "think" yours should be an 8" balancer, but check first.
     
  7. 70RSZ28CAMARO

    70RSZ28CAMARO Member

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    It does have the timing gauge I’m going to check for total timing today. I’ll let you know. Thanks for your help!
     
  8. FlaJunkie

    FlaJunkie Veteran Member Lifetime Gold Member

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    The timing specs for the original 1970 engine were based on several things that may or may not be important today.

    Fuel, performance envelope, pollution control, and insurance ratings come to mind.

    Mine had headers and I always set the timing at 8 BTDC at about 800 RPM. But the fuel was super back then and you didn't have to worry about pinging. As fuel octane degraded, the timing had to be adjusted to allow the motor to run as best as it could.

    In 1972, my car would out-accelerate anyone on the street I met in Denver or Monterey. The only adjustments I made off the recommended setup was leaning the main jets in Denver. The car was stock in Monterey at sea level.

    The last thing I would say is to keep in mind where you plan to shift if you decide to dial in any additional timing.
     
  9. G72Zed

    G72Zed Veteran Member

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    As COPO stated, could use more initial timing, the WOT "pop" in 3-4th gear may be a lack of fuel pressure/volume, or that new "Street Fire" MSD.

    Yes, vacuum advanced should be used whenever possible, even if limited advanced is used. But ask your mechanic to "explain" why he got you to permanently plug your vacuum canister, perhaps it may be due to old style iron heads with inefficient chambers, domed pistons, fuel availability, or a combination of all of them.
     
  10. K1ng0011

    K1ng0011 Veteran Member

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    Timing is one of those things that most people understand well enough to just get the engine running. The WOT and idle timing will vary depending on a lot of different factors. Like cylinder head design vortech vs 70s smog era heads. Or how rich/lean your running at idle or if your running the factory suggested timing which is mostly for emissions. When people disable or remove the vacuum advance canister from a distributor I believe it is because they don't really know what they are doing and how it functions. You only make vacuum at idle and part throttle positions like cruising down the highway at 60MPH. So at WOT down the drag strip/road the vacuum advance canister is not doing anything. As a rule of thumb leaner AFR mixtures require more advance than richer AFR mixtures. Your engine usually runs leaner at idle and at light throttle cruising down the road that is where the vacuum advance canister comes into play. For a typical 70s era SBC total timing will be around 36 degrees. Typical vacuum advance canisters add around 13-15 degrees of timing so it is not uncommon to see 45 degrees of timing cruising down the road. At idle what you do is you set your timing to somewhere around 8-10 degrees of initial advance and hook your vacuum canister to manifold vacuum NOT ported vacuum on the carb. The vacuum advance canister will add an additional 13-15 degrees of advance to the 8-10 degrees you set at idle bringing you to a total of around 25 degrees of advance at idle. Having this high timing at idle and cruise allows the lean air fuel mixture to burn more completely in the cylinder. This will also help to reduce engine temp at idle and cruising, too. The ported vacuum on a carb is nothing more than a primitive emissions device. All ported vacuum means is you get no vacuum at idle. So the vacuum advance canister on the distributor is not advancing the timing at idle when connected to ported vacuum. If you hook to manifold vacuum at idle your vacuum canister will work at idle. The engineers wanted to increase the engine temp at idle to help with emissions so that is why the ported vacuum came about but low timing at idle makes the engine less efficient and run hotter. I have a generic rebuilt auto parts store 350 in my car with garbage 70s era heads and small cam. I have ran this engine with a carb and EFI and I run around 25 degrees of timing at idle and 45 degrees of timing at cruise with no issues. People think high timing is just used with radical camshafts when it is not. Most people with radical cams will set the idle timing high around 18 degrees or something which can kickback the starter. Well if they set it to 10 degrees and hooked their distributors vacuum advance up to manifold vacuum they would have the advance they need without the kickback. Good luck.

    GM Smog Era Heads Total Timing: About 36 Degrees @ 3000-3500RPM
    Idle Timing/Initial Timing: 8-10 degrees + Vacuum Advance = 20 degrees or more
    Cruise Timing: Add timing at RPM + Vacuum Advance = Cruise Timing
    Ported Vs Manifold Vacuum: https://www.chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm
    Timing Tape: https://www.holley.com/products/acc...ing_cover_hardware_and_timing_tabs/parts/8985
    GM HEI Weight and Spring Kit: https://www.holley.com/products/ign...accessories/distributor_components/parts/8428
    GM HEI Weight and Spring Kit Advance Curves:
    upload_2019-6-11_9-35-2.png

    HEI Vacuum Canister Adjustment:
    [​IMG]
     

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