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Low vacume - Looking for mechanic advice and double check.

Discussion in 'The Garage' started by BadDog, Sep 15, 2003.

  1. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Ok, my son's K5 is driving me nuts. I don't think there was ANYTHING set right. It’s a 77 400 SB with lots of miles but seems generally sound. No smog anything, no EGR, no cats, no canister, nice and clean.

    Originally, it wouldn't even try to run. Got it running by advancing the timing about 15 degrees, and plugging several vac leaks and found idle air screws run out 6 FULL turns on one side, 5.5 on the other..

    Vacume was around 10 in hg but it was running as long as I kept the choke closed. That is with all vac ports plugged including PB, PCV, vac advance, and modulator.

    Did compression test. #1 @ 110. 2 cylinders @ 120, the rest at 130. This is a 77 400 with high miles so, this looks reasonable, not great, but not OH MY GOD I'VE GOT TO REBUILD! Plugs are black and mostly chalky, though the engine seems to be running lean. There appears to be no oil fouling.

    Took off carb. Bottom plate inverted screws (going up) were sloppy loose, 2 turns to snug. Several other needed snuging. Isolator was hard and severely deformed. Looks like over tightened. Checked base which appears to be within 0.05 of flat so it should seal, but I didn’t actually use a feeler gauge to check, just visual under a straight edge. Also noticed bad but not severe wear in the throttle plate bushings. Other than that, carb seems ok, looks like a reman with a few years on it. Replaced isolator with new FelPro and vac is up to oscillating between 13 and just under 15 in hg.

    Now it runs much better, but still not great. Idle air screws are run all the way in, opening up has little effect. It idles rough, and I know it will never idle right with the worn butterflies, but it should be better. After warm up, it runs best with about 1/4 - 3/8” air gap on the choke. I blocked the linkage to keep it from opening past that. Now vac comes up to around 20 and it runs great. Also picks up roughly 100 rpms over an open choke plate.

    I’ve been over everything again and again, I can find no more leaks. Looks like the factory manifold has never been removed, so I am hoping (praying) that it’s not leaking on the bottom of the gasket. I went around the thing all over with Propane and it never settled down, there was no effect at all.

    At this point I’m thinking I should just replace the carb with a reman. I frankly don’t have time to rebuild right now, and the bushings are a pain. With the worn bushings, something needs to be done about the carb anyway, that that *may* fix the problem. But I don’t like taking pot shots.

    Does anyone have any ideas what I may have missed or should check? I haven’t done any significant engine work or diagnostic since somewhere in the late 80s… For about 10 years I joined the “don’t even change my own oil” crowd so I have forgotten allot of what I once knew. I only mention that so that you won’t assume I’ve checked the obvious, since it may have slipped my mind. But just thinking through the whole “how an engine works” thing seems to indicate there aren’t many possibilities remaining. Any suggestions/testing tips will be appreciated.
     
  2. Zeus33rd

    Zeus33rd Smarter than you GMOTM Winner

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    Is it just an idle problem? How does it run past idle?
     
  3. 84_Chevy_K10

    84_Chevy_K10 Banned

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    What's the ignition system like?
     
  4. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Vacuum leak could be in the truck or a bad vac booster for the brakes. The HVAC controls are vaccum operated. SHould be a hose on the T behind the carb that runs through the fire wall. unplug that line and plug it and see if your vac comes up.

    Unless that thing has a lumpy cam it should pull 17-20hg no problem with the compression your seeing. 15degree of advance is a little high if it has vac advance. Most SBC's are timed at around 4-8. Also check the mechanical advance. Sometimes the advance hangs. What happens is oil vapors coke up in the shaft. Take the weightso off, drive the roll pin out of the gear and you can slide the inner shaft out and clean it.
     
  5. BorregoK5

    BorregoK5 1/2 ton status

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    Russ,
    The 400 has a much lower static timing than a 350, youre going to want to run it at about 2 degrees, could come up to 4 depending on your tuning. Check your valves and make sure you havent overtightened a lifter which will allow combustion pressure back in the intake, you'll see a regular flutter on the vac guage. If you have a steady leak somewhere your vac guage should still read steady but low. Did you change the plugs or wires recently? Bad plugs or wires could be contributing to the poor idle, slipping up the firing order could also cause all of this.
     
  6. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Hmm, and here I thought I was being thorough in my description.

    Joe: It runs pretty decent from about 1k on up, if you can get it there, but it stumbles pretty bad if I don't have the choke partially closed (even when fully warmed up). Once I get the revs up, the choke open is fine, but then at those rpms there is plenty of vacume to go around even with the secondaries open.

    Tim: The ignition system "seems" sound. Relatively new (appearing) generic wires cap and rotor. No visible signs of a problem. I have no way (that I'm aware of) to check the coil). On the boys "to do" list is to check the resistance on each wire just to be sure. We'll eventually replace with a set of Taylor’s...

    Grim: All external vacume was plugged at the engine for most of our tests. In any case, vac signal remains pretty much the same with all external vac accessories plugged in. The AC stuff ran off the intake tree behind the carb, but that has been completely removed (no doors or top anyway) and plugged. No cam as far as I know. Looks like it's never been open, though it’s impossible to tell. It also doesn’t sound like a cam, I’ve built a number of hot rods in my younger days so I know that sound. 15* is not total advance, that's with the "vac advance/retard" unhooked and relative to where the PO had it set, which was clearly BTD. When I work on this kind of problem, the vac retard stays unhooked/plugged till I get it settled down. I also ditched my timing light years ago (to many moves around the country) so I'll check it using my neighbors once the rest is settled down. I'm fairly confident/competent setting it by ear though. Mechanical advance is relatively clean and seems in working order.

    Mike: Like I told Grim, the timing was set by ear, and the degrees were relative to the initial location as set by the PO. I have always set by ear, then checked by light. Top does not appear to have been messed with, I know I have not. But the valves are something to check I suppose. I do have a variable rpm and vacume at idle. Not like a lope, I know that sound, more like a stead surge/ebb consistent with what you describe. Maybe the valve adjustment is off, heck, everything else that was adjustable was set WTFO. Firing order was already checked when I couldn't get it running to start with. The plugs appear relatively new, but you never know. He is supposed to check resistance on the wires this evening. However, it runs extremely well when the choke is partly closed (and the vac runs up near 20), which makes me think the wires/order would not be related.
     
  7. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Anyone else got any ideas?

    Do you think the carb could account for that much vacume loss? The only identifiable problem (that I could find) is the worn butter flies. But that should not have so drastic an effect.

    It will probably be a few days before I have time to check the rocker adjustments but that seems a likely suspect with the fluctuations I’m seeing. Hmm, it might also correct one of those low cylinder pressures if that is the problem… Those 3 cylinders are probably the first I should check.
     
  8. 84_Chevy_K10

    84_Chevy_K10 Banned

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    90% of "carb problems" turn out to be ignition. I'd set the timing correctly and change the wires and see what happens.
     
  9. SUBFAN

    SUBFAN 1/2 ton status

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    Spray around the base of the carb, if the idle increases, that is where your vacuum is going. If that does nothing, then spray around the inrake manifold....
     
  10. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Tim:
    I've heard that before, but the ignition seems fine (though he still has some testing to do) and I don't think ignition issues would cause what I'm seeing. Basically idles rough with 13-15 in of vacume fluctuating, then block off half the air flow, engine runs smoothly (I would even go so far as to say great) and picks up 100+ rpms and 20 in hg vacume. But, you may well be right. I've never seen the combo of symptoms before where I couldn’t find the problem. And this time the usual suspects seem to be innocent this time. I'll be looking more at the ignition when I get time to get back on it...

    Subfan:
    I've already done several different leak tests including WD40 and Propane with no change in behavior.

    Thanks for the continued suggestions...
     
  11. fatboy

    fatboy 1/2 ton status

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    if compression held on each cylinder I would think the valves would be fine. If you dont know I would do compression again and let it sit for a few minuts to see if you have any leakage and then you know the exact cylinder. Dont know if it helps but I find it interesting. /forums/images/graemlins/smirk.gif /forums/images/graemlins/burb.gif
     
  12. Rustheap

    Rustheap Registered Member

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    Sounds more like a fuel system problem to me. If your ignition checks out ok, I would check the fuel system start to finish. When you block half of your airflow with the choke, you are enriching the fuel mix. Sounds like it is running to lean for some reason, causing your stumble. That may also be why the idle screws were backed out six turns, some rookie trying to compensate for some other kind of fuel problem. /forums/images/graemlins/burb.gif
     
  13. 84_Chevy_K10

    84_Chevy_K10 Banned

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    If choking the engine makes it run right, you've got a vacuum leak somewhere. Keep searching, you'll find it.
     
  14. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    [ QUOTE ]
    Tim:
    I've heard that before, but the ignition seems fine (though he still has some testing to do) and I don't think ignition issues would cause what I'm seeing. Basically idles rough with 13-15 in of vacume fluctuating, then block off half the air flow, engine runs smoothly (I would even go so far as to say great) and picks up 100+ rpms and 20 in hg vacume. But, you may well be right. I've never seen the combo of symptoms before where I couldn’t find the problem. And this time the usual suspects seem to be innocent this time. I'll be looking more at the ignition when I get time to get back on it...

    Subfan:
    I've already done several different leak tests including WD40 and Propane with no change in behavior.

    Thanks for the continued suggestions...

    [/ QUOTE ]

    That's strange. Blocking off the airflow brings more signal to the carb and increases fuel flow at the carb. Quads need the vacuum to idle right and run right up till about 1k when they get some good air flow to get the main ventureis going good. With needing choke at low RPM's and the wide open idle mix screws all says big vacuum leak to me. That our a seriously screwed up carb.

    Get the base timing (no vac advance) down to 4-8 at 600-700RPM and see what the Vacuum reads at. It needs to be over 16hg or a quad is going to fall on it's face.
    Check the intake bolts as well and see if any are loose. A vacuum leak between the intake and heads from the bottom side is not unheard of. Loose bolts might indicate an issue.

    Double check the base gasket to the carb for leaks again with some carb cleaner. Also shoot the carb cleaner where the intakes and heads meet.

    If all the air isn't coming through the carb it will cause run problems that would require goofy mix settings. This really stinks of a vacuum leak somewhere.
     
  15. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Grim, your post clearly states exactly my thought process on this that led to my making the post. /forums/images/graemlins/laugh.gif I initially thought it would be ok if I could get around 14-15 in hg but clearly my memory was mistaken. Must have been confused from my days of camed Ponchos and SBCs…

    I did put a new carb base isolator/gasket in there and picked up an inch or so of vacume. Looks like the carb may have been over torqued and warped, plus the primary throttle plate bushings are worn so the carb is suspect. But I just can’t see it loosing 3-4 inches of vacume from the carb without visible issues. That’s got to be a pretty significant leak. I've checked and rechecked the intake and can find no problem on the top. All bolts are tight. I think I'm down to just going to get a carb and hope that fixes it. Either way, with the worn bushings and potential for warped body it could use a carb...
     
  16. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    [ QUOTE ]
    Grim, your post clearly states exactly my thought process on this that led to my making the post. /forums/images/graemlins/laugh.gif I initially thought it would be ok if I could get around 14-15 in hg but clearly my memory was mistaken. Must have been confused from my days of camed Ponchos and SBCs…

    I did put a new carb base isolator/gasket in there and picked up an inch or so of vacume. Looks like the carb may have been over torqued and warped, plus the primary throttle plate bushings are worn so the carb is suspect. But I just can’t see it loosing 3-4 inches of vacume from the carb without visible issues. That’s got to be a pretty significant leak. I've checked and rechecked the intake and can find no problem on the top. All bolts are tight. I think I'm down to just going to get a carb and hope that fixes it. Either way, with the worn bushings and potential for warped body it could use a carb...

    [/ QUOTE ]

    My Quad on the 75 has pretty worn bushings and it still pulls 19-19.5hg. Even with the mixture completly out of whack it pulled better then 17.

    Only ways I could see the carb causing that low of vac is a cracked body or one or both sets of butter flies in the base plate not fully closing. I could see if the secondaires were not fully closed it could pull enough air around the top doors to drop the vac to that range without actually opening the doors.

    Your sure that thing doesn't have an agreesive cam? That Vacuum is just through the floor.
     
  17. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    I'm 99% certain on the cam. I am very familiar with cam lope from mild RV style (barely noticeable) all the way to "won't idle below 1000 rpms". This thing has no lope. And it idles smooth as glass with the choke over half closed (and warmed up). But it "surges" with vacume fluctuations consistent with a single cylinder type problem. That's why the "rocker over tightened" mentioned above sounded interesting.

    Secondary butterflies stuck partly open was one of my first thoughts after checking the obvious external options (visual inspection and plugging all vacume lines). So, I’ve already checked both primary and secondary butterflies for seating. I can’t see light through either with the plates closed.

    I think it would need a hole the size of a pencil to drop that much vacume. I’m stumped. Current plan is to check the rocker idea, it needs valve cover gaskets anyway. If that yields nothing, I’ll borrow or buy a carb just to see. Then, I suppose I’ll have to pull the intake and put on new gaskets. But that will just have to wait till after BB03 because the boy is not up to that yet and I don’t have time…
     
  18. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    I'm just hoping its not a slight intake valve leak (bent, sticking, carboned) or worse on that 110 psi cylinder... That would be a rare bird, but just my type of luck...
     
  19. diesel4me

    diesel4me 1 ton status Premium Member

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    The valve guides(intake)could be loose enough to allow some vaccuum to be lost(engine sucks air through the guides instead of the carb)but I would try another carb first,especially if its a quadrajet--it may have been rebuilt with the wrong top cover gasket or have an internal vacuum leak.Lifters can stick in the overpumped position and hold open a valve slighty and cause this too. /forums/images/graemlins/crazy.gif
     
  20. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    [ QUOTE ]
    I'm just hoping its not a slight intake valve leak (bent, sticking, carboned) or worse on that 110 psi cylinder... That would be a rare bird, but just my type of luck...

    [/ QUOTE ]

    Intake valve not closing usualy will make the vac gage bounce. Seeing any of that?

    Yeah I would check the valve on that low cylinder. When I burned a valve I was only getting about 40 on that cylinder. That was exhaust side so never put a gage on the motor and it was a Toyota so not sure what it would have shown. So to me that sounds like it's not burnt. Hit the dealer and get some "Top end engine cleaner". Company work van here managed to stick a valve. It was bad stuck and would not idle. Dealer was saying they were going to have to pull the head. The mechanic decided to give the top end engine cleaner a shot.....it worked. Saved the company $600.
     

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