Low engine vacuum?

Discussion in 'Troubleshooting & Diagnosis' started by VortecZ, Oct 14, 2021.

  1. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

    Messages:
    11,430
    Likes Received:
    932
    Joined:
    Nov 4, 2004
    Location:
    Ringgold, GA
    Let's take a re-look at the vacuum. When you say "I cannot get this engine to make more than 14-15 in of vac", I was assuming that included idle and cruise conditions. Another reason I keyed in on a vacuum leak is that you've tried several carburetors and they all seem to have idle and off idle issues. The cam you have is pretty mild, but depending on timing, 14-15" at idle could be acceptable.
     
  2. G72Zed

    G72Zed Veteran Member

    Messages:
    3,317
    Likes Received:
    1,759
    Joined:
    Sep 8, 2015
    Location:
    Canada
    I have timed a lot of these "crate" engines, sometimes the tin timing tabs/markings are off, not much is needed to make them 2-4* off, and that makes a difference. Once the timing and carb tune is done, the idle vacuum will be what it will be, 14-15" is not that bad actually. I'm sure with a correctly tuned and balanced carb/idle circuit, you could pick up a point or 2.

    We also install lock out plates and run 32-36* right at idle (pending tab location) on the "602" crates, almost the same as the SP357, except a flat tappet cam.

    I will tell you, 12* at idle with these 9:1 CR engines with a slow centrifugal ramp to 4k, are off idle duds, ok for grandpa in his early 80's smog Impala LOL.

    From sandlapper's link, here's GM wording:

    "Set spark timing at 32º before top dead center (BTDC) at 4000 RPM with the vacuum advance line to the distributor disconnected and plugged. This setting will produce 32º of total advance at wide open throttle (WOT). The HEI vacuum advance canister should remain disconnected. This engine is designed to operate using only the internal centrifugal advance to achieve the correct timing curve."

    So, if the tab/marking are off, and your cruising at 2,500 rpm in high gear, no vacuum advanced being used, what's the timing at cruise? most likely way below the 32* they ask for. And if the carb is well tuned for good lean cruise AFR's at that rpm, combined with low timing and no VA, well, that's a crappy combo. Why does GM have a VA canister with the HEI and they tell you not to use it. This is not a race engine.

    FWIW, I cruise at 2,300 rpm, all my timing is all in at 36* by that rpm and the vacuum advance gives me another full 8-10* as well, so total of 44-46* cruising down the road with a 14.3/5 AFR (10% Ethanol) and I get over 20 mpg's, not bad for a 350 none roller engine making an additional 180 HP then the GM SP357.

    To me, GM is being very over cautious protecting there warranty a$$ with the timing recommendations, unless these engines are mismatched and are detonation/pinging monsters at only 9 to 1 CR. JMO



     
    Aml383, 1320feet and MChamp like this.
  3. COPO

    COPO Veteran Member Lifetime Gold Member

    Messages:
    21,195
    Likes Received:
    2,671
    Joined:
    Sep 15, 1999
    Location:
    Ontario, Canada
    Setting the valves the EOIC method may get you 1-2 extra in of vacuum plus I would not go more than 1/2 turn from 0 lash. Just my opinion that worked for me. If she doesn’t ping at 12* BTDC initial then shoot for 14*.
     
    Aml383 likes this.
  4. rchydzik

    rchydzik Veteran Member Lifetime Gold Member

    Messages:
    909
    Likes Received:
    43
    Joined:
    Nov 13, 1999
    Location:
    Colbert, WA USA
    14-15 inches of vacuum at idle seems ok on a cam with 215 degrees duration and 108 LSA? My comp 268 HE (218/110) idles at 750 rpm and 16 inches vacuum.
     
  5. sandlapper

    sandlapper Veteran Member

    Messages:
    920
    Likes Received:
    367
    Joined:
    Oct 9, 2020
    Location:
    SE CSA
    OP
    Imperative: Find a real good dist & carb tuner.
    I usually agree w/ GM directives, but here I'm at odds w/ No vac advance... agree w/ G72Zed on need for good PCV & timing; a good dist & carb tuner can include vacuum advance & PCV. Me, as well, thinks GM dumbed it down & playing it safe for warranty.
     
  6. MChamp

    MChamp Veteran Member

    Messages:
    400
    Likes Received:
    110
    Joined:
    Jun 6, 2021
    Location:
    north carolina
    I'm constantly just in awe at the knowledge and experience this site/members have to offer. Thank you..
     
  7. sandlapper

    sandlapper Veteran Member

    Messages:
    920
    Likes Received:
    367
    Joined:
    Oct 9, 2020
    Location:
    SE CSA
    I must also note ... look at several GM crate sbc motor install instructions ... it's clear they simply cut n paste from other GM instruction sheets ... why not ... but they continue to repeat the same ol' misspelling typo ... when they instruct you to prime the motor's oil pump ... they continue writing "dill motor" ... instead of drill motor. Soooo ... I wonder, when instructing "no vac advance"... is GM mistakenly quoting a GM instruction source which May not apply in all cases ??? Dunno ... but I do KNOW GM is repeating one typo while cut n paste.

    yea, I know; anal. Did I mention I made a career of authoring & editing process control docs in a AS/QS/ISO 9000 & TS 16949 environment?
     
    cadillac_al likes this.
  8. MChamp

    MChamp Veteran Member

    Messages:
    400
    Likes Received:
    110
    Joined:
    Jun 6, 2021
    Location:
    north carolina
    ^ great point sandlapper. it was an old habit when I took over as a lab supervisor seeing the same. the SOP's/QC reporting were a nightmare. dealing with state & federal jurisdictions I sure as hell wouldn't sign off on them though.
     
  9. sandlapper

    sandlapper Veteran Member

    Messages:
    920
    Likes Received:
    367
    Joined:
    Oct 9, 2020
    Location:
    SE CSA
    ? .... per CLIA '88 ?
     
    MChamp likes this.
  10. Cardinal

    Cardinal Veteran Member Lifetime Gold Member

    Messages:
    8,342
    Likes Received:
    66
    Joined:
    Jun 22, 2003
    Location:
    Endicott, NY
    Two things come to mind.

    One is float height and drop! Edelbrock carbs are NOTORIOUS for being shipped with incorrect float height and drop! Go to the Edelbrock website and get the float height and drop. They have directions on how to set them. And there's always YouTube!

    Having said that, your intake with "spread bore" pattern and a your "square bore" carburetor probably are leaking vacuum around the gasket between the intake and carb. When we installed a Wieand Action+ intake along with an Edelbrock 1405 carb on our youngest son's '83 S10 with a 350 SBC, the engine ran OK but NOT as well as it did with a Q-Jet. Hard starting even with a choke. Just didn't run quite right.

    So one day I said to him, WE are going to troubleshoot this MF'er until we either kill or fix it. We tried a known 650 Holley DP I had which ran great on my 292 SBC in my '87 Drag truck. No difference. Hmm. I happened to look at the pattern on the gasket and say a VERY narrow area on the area between the primaries and secondary's on both outside edges! Ah ha I says!

    I found a Q-jet gasket (for the intake side) and a square bore gasket for the carb side. Fabricate a "goof" plate out of a piece of scrap 1/16" aluminum plate and installed that between both gaskets.

    Voila! Problem solved! Idle good now. Easy to start up. Ran like it used to with the Q-jet. No after run.

    Summit sells a "goof plate set and it's less than $10. Summit Racing SUM-G1420 Summit Racing® Carburetor Adapter Plates | Summit Racing
     

Share This Page