Low engine vacuum?

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

  1. VortecZ

    VortecZ New Member

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    I have installed a GM SP357 crate engine in my 1980 Z. Part # 19367080 I have an Edelbrock Performer intake w/ spread bore opening. I have a Summit billet dist, remote coil and CDI ign box. Timing set at 12 BTDC. No vacuum adv. I cannot get this engine to make more than 14-15 in of vac. I tried a couple 'used carbs' I thought were good. ( Q jet, holley 4150, holley 4165, Edel Performer) All had idle and off idle issues. Works good at 2500 rpm & above. Always erractic idle w/ run on if idle screw is in too far. Mix screws always seem to change, but can't find smooth & crisp response. Performer seemed to idle best, but had not very good top ond. Just bolted on a new QF 600 Brawler mechanical sec. ( Also I have the adapter plate for under the Holleys. ) The new QF still has same idle conditions. 4 corner idle screws all respond. Plugs stay clean. At idle it is hard to tell if it is rich or lean. Introduce a vac leak, idle changes better a little. Choke the air horn, hardly any change. Installed a 4.5 Power valve to help a low speed hiccup. (better) Just jetted to 67 from 70. Drivability is worse. Just going to jet up to 72s. (not done yet) My question is has any body used this engine and had low vac issues? Am I missing some simple? I am wondering if valve timing is off. This engine has less than 2000 miles on it. I checked 1 valve adj. (seems OK) 3/4 of turn tight after 0 lash. I have never seen this engine run smooth since install. Pulls good at RPM. Sorry for long winded story. Any input would be appreciated.
     
  2. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    Sounding a lot like an internal vacuum leak, like around the intake ports. If you've done all the testing for leaks externally, pull the intake manifold and inspect the gasket. Good luck and welcome to NastyZ.
     
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  3. Dave Nelson

    Dave Nelson Veteran Member

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    Can you add vacuum advance?
     
  4. biker

    biker Veteran Member

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    X2 on vac leaks. If not...Cam specs? Initial advance may be a bit low. What do you get for total timing?
    I second the vote for vac advance. If you weren't responsible for adjusting the valves, I would go over all of them. Just me, but I'd go to 1/2 turn. And welcome!
    Edit: go richer than you might think with the idle mixtures.
     
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  5. sandlapper

    sandlapper Veteran Member

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  6. G72Zed

    G72Zed Veteran Member

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    Agree with the other members on the potential of a vacuum leak(s).

    If you are using good fuel and the correct plugs, and the lifters are properly adjusted, and no vacuum leaks are found, I would look into the timing.

    Those engines came with HEI's most likely, and they had, the vacuum advance canisters would be included, and for good reason.

    Your descriptions sound like a lean idle circuit combined with not enough initial timing, or even a rich idle will act like what you are describing with low initial timing, and no vacuum advance.

    A correctly tailored vacuum advanced setup is your friend, It cleans up the idle, make fine idle/transition carb adjustments easier, gives improved idle/off idle throttle response before the mains come in, makes the exhaust valves run cooler, and if the AFR's are in check, 2-3 more MPG's under lean cruise conditions.....WIN -WIN -WIN.

    I use a vacuum advance, would not even think of disconnecting it, for any reason, and I track and autocross my car, and beat it like a rented mule LOL, and I do not disconnect it for any events.
     
    Last edited: Oct 14, 2021
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  7. VortecZ

    VortecZ New Member

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    I used Edel gaskets on intake with sealer. Proper torque/loctite on bolts when installed. I used thermometer on header pipes when running. All run about same temp. +/- 50 degrees. I thought intake vac leaks would show here. The engine doesn't mind some timing. Specs for total are 32 @ 4000. Cam specs are 0.473 lift for both in& ex. Duration is 215 in & 223 ex. Centreline is 109 ATDC in & 107 BTDC ex. Cam is not that radical. I have had ign timing up to 16-17 degrees BTDC. Still idles the same. I have it set at 12 right now. There is 21 degrees in the dist. Checked w/ adj light. I am still wondering about valve timing? I hate to tear the front off the 'new' engine to check. I am hoping somebody else with this engine reads this. I am going to change pri jets to 72 real soon. I will update.
     
  8. VortecZ

    VortecZ New Member

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    I used GM HEI w/vac adv. at first. Lighter springs and adjustable adv. Could not find sweet spot with it. Always had older carbs on at this time. Ign. timing does not seem to make a big difference for the idle quality. Never ever smooth. Anywhere's from 4 to 20 initial I have tried. Going to do more this weekend. I will update.
     
  9. COPO

    COPO Veteran Member Lifetime Gold Member

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    14-15 is not low vacuum in a performance cam’d engine. 10 is low and is what I run at idle with VA.
     
  10. biker

    biker Veteran Member

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    I agree with Mark above, 14-15 is not terrible vac reading. But you sound like you have enough experience to know when something isn't running right, and that is a tame enough cam that it should idle fairly smoothly.
    I still say start with valve adjustment, maybe even a compression check just to make sure all valves are sealing up. Then check again after you re-set valves.
    Running on at shut down can be oil in the cylinders which won't help idle either. PCV can huck down an alarming amount of oil with the wrong valve and baffling. But, you said your plugs are clean.
    You say it runs on if idle screw is in too far... G72Zed can help you out with throttle valve settings to make sure your transfer slots have proper exposure to idle/off idle vacuum. Sounds lean at idle to me.
     

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