Idle when timing 70z28 LT1

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

  1. 70lt1z28

    70lt1z28 Veteran Member Gold Member

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    All correct but one more thing I would add is due to the original goal of reducing emissions, the OEM vacuum advance may not be the best fit for performance, especially if there has been a cam change which results in an idle vacuum reduction.

    Here's a pretty good read on the subject. Very detailed:

    https://www.crankshaftcoalition.com/wiki/images/e/e4/Vacuum_Advance_Specs.pdf

    Excerpts from the .pdf:

    One of the key statements is the following: (VAC is short for vacuum advance canister)

    The basic rule for vacuum advance control (VAC) selection (henceforth referred to as THE RULE):THE VAC SHOULD PROVIDE FULL ADVANCE AT NOT LESS THAN 2" LESS THAN PREVAILING IDLE VACUUM AT NORMAL IDLE SPEED WITH APPROXIMATELY 24-32 DEGREES TOTAL IDLE TIMING.

    One other issue. Some think that ported vacuum advance is "correct", but it is NOT on pre-emission controlled engines.Ported vacuum advance is an emission control technique to increase EGT, which promotes oxidation reaction in the exhaust, but it also increases operating temperatures, increases the tendency to detonate and run-on at shutdown, and increases fuel consumption. With a handful of exceptions, all GM pre-emission engines equipped with vacuum advance used full time manifold vacuum.For some inexplicable reason, the '63 FI engine used ported vacuum advance- the first year vacuum advance was used on Duntov-cammed engines. Maybe GM thought that idle quality (always a problem on FI engines) would be better with ported vacuum advance, but it wasn't, and the '64-'64 FI engines got full manifold vacuum advance.L-72/71 have ported vacuum advance to meet CA emissions since there was only one version of this engine for all 50 states.If your experience with Corvette engines does not go back to pre-emission engines, then all you've ever seen is ported vacuum advance on emission controlled engines.Duke

    To do it right, you need a vacuum gauge and a Mityvac to test the canister, which many times is bad.
     
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  2. 70RSZ28CAMARO

    70RSZ28CAMARO Member

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    You guys have been very very helpful thank you for all the information. The other day I was trying to determine total timing which I believe should be around 36 on my set up which is 1970 Z 28 stock LT1 with hooker headers 2 1/2 inch exhaust and MSD street fire distributor. after setting initial timing to 8° BTDC I set the motor to 3500 RPM to check for total timing and it came in at 44 1/2. why would it be so high and what should I do to get it in the 36 range

    thank you
     
  3. K1ng0011

    K1ng0011 Veteran Member

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    Total timing is the timing you set at idle + the timing added from the springs/weights in your distributor at 3500RPM. Make sure your vacuum advance is disconnected when setting the idle timing and total timing. 8 degrees BTDC minus 44 degrees is 36 degrees of timing is being added between idle and 3500RPM. How are you checking total timing with timing tape or a timing light? Also might be a good idea to verify that nothing is wrong with the springs under the rotor if you pull it off and take a look. Not sure if this is the exact distributor that you are using but if it is it looks like stock it is set to provide 22 degrees by the time it reaches 4000RPM. To me adding 36 degrees with the springs and weights seems like a lot so it seems like there is something wrong. Spring/weight issue, how your checking, vacuum advance hooked up, or harmonic balancer slipped.

    Street Fire 8362 Distributor: https://www.holley.com/products/ignition/distributors/chevrolet/parts/8362

    upload_2019-6-11_17-7-29.png
     
    Last edited: Jun 11, 2019
  4. 70lt1z28

    70lt1z28 Veteran Member Gold Member

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    K1ng0011 likes this.
  5. 70RSZ28CAMARO

    70RSZ28CAMARO Member

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    I’m using a timing light with a dial. If I’m understanding you correctly if total timing is 44 degrees and initial timing is 8 degrees BTDC, And I subtract that from 44, i’m at 36 which I believe is correct (34 to 38) on this motor. Is that right ? I do have the 8362 street fire distributor. I also have the vacuum advance stop plate kit which I never installed. As I said previously my mechanic plugged the canister on the distributor and it ran great for awhile then I started having problems with warm starts and popping, not pinging, at WOT in 3rd and 4th gears only. That is intermittent and to me sounds like unburnt fuel getting into the exhaust. Thanks
     
  6. BC_Homegrown

    BC_Homegrown New Member

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    Total timing = initial + mechanical advance so if 8 is your initial and your total is 44 and you don't have vacuum advance connected, your distributor is pulling in 36 degrees of timing at 3500. If you haven't modified the advance stop bushing in the dist. then there is either something wrong with the distributor or possibly your timing light is giving a false reading (happens sometimes with msd and some lights).

    Some people swear by adjusting timing by sound. Advance till miss at WOT or hard to turn over when starting then back it off a bit. Sounds like its too far advanced if its popping or pinging.

    Is it still hard starting when warm at 8 BTDC or did you cure that problem? What was the initial set at by your mechanic before you changed it?

    Vac advance can be quite beneficial for a street car, not sure why your mechanic plugged it, maybe the vacuum pod is bad. Just wondering, did he plug the vacuum line from the manifold?
     
  7. 70RSZ28CAMARO

    70RSZ28CAMARO Member

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    Thanks for your replies. I advanced the initial timing from 8 to 7 BTDC. But I haven’t had time for a test drive yet. Do you think that will make a difference?
     
  8. K1ng0011

    K1ng0011 Veteran Member

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    Basically what we are trying to say is that there is likely something wrong with your distributor. Likely the stop bushing in the distributor failed or your mechanic messed something up when he was working on it. Likely they only way to fix it is to take out the distributor and inspect it to verify what is causing it to add 36 degrees of mechanical advance which is way too much.
     

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