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Help - Intermitten Rough Idle (Long)

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by FightinTXag, May 2, 2007.

  1. FightinTXag

    FightinTXag 1/2 ton status

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    I've got an issue with intermitten rough idle in my 85 K10's 305. Steps I've taken:
    -Rebuilt the q-jet (including new accel pump). The APT is set at 2.5 turns out as are the mix screws. The mix screws were set with a vacuum gauge.
    -Set initial timing to 4° BTDC (if the marks are accurate)
    -Disabled the ESC
    -Checked the vacuum advance can (which is hooked up to ported)
    -Checked the mechanical advance weights (they move smoothly and snap back)
    -New cap/rotor/wires/plugs
    -Checked the EGR (moves and returns freely, diaphragm seems good)
    -All new vacuum lines

    The idle quality seems to fluctuate randomly. If it's acting up, it will idle rough at every light or stop during that trip. On the return trip, it may idle ok at every stop.

    When it's acting up, it seems to load up, idling worse and worse the longer it idles until the whole truck is shaking noticeably. I always thought loading up indicated it's idling rich, but I get no black smoke or rich smelling exhaust. It passed emissions last weekend with flying colors when it was idling pretty rough. It never dies or seems to fluctuate wildly in rpms.

    Also when it's acting up, there will be a bog or flat spot when accelerating just off idle. This hesitation is sometimes present to a lesser degree when it's idling ok. Sometimes it'll ping for a fraction of a second when tipping in from idle. It never pings any other time - just when leaving from a stop.

    I have searched and searched for a vacuum leak. I replaced all vacuum lines. I've tried spraying carb cleaner in the area several times, and I haven't pinpointed anything.

    The carb itself hisses a whole lot. I suspected the throttle shafts. The throttle has a little bit of play in it, but spraying the driver's side linkage with carb cleaner doesn't seem to affect the idle. At one time the throttle was hanging up, and depending on how I released the accelerator the idle would vary by several hundred rpm. I put on a new throttle return spring and that seemed to cure it for the most part. If I gingerly release the accelerator it'll probably stay idling 50-100 rpms higher than if I let it snap back from WOT. If the throttle shaft was worn badly enough to cause this poor idle, I would think spraying carb cleaner on the driver's side linkage would affect the idle.

    I'm beginning to wonder if I have an internal leak (bad intake gasket)? Or the carb has an interal vacuum leak. I know that the air horn on my carb is pretty warped, but again spraying carb cleaner doesn't seem to affect it.

    #8 spark plug was kinda greasy. Could a burned valve only cause idle/low rpm issues, or does that usually cause a miss at all rpms?

    Any ideas?:(
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    Since your problem is intermittent, I'd be looking away from valve issues, and other "hard" parts that would affect how it runs in all conditions, at all times.

    I'd be concerned about the internals of the distributor (anything loose, cracked, or damaged that can warp or move enough to cause poor running?) and since you mention a warped air horn, PERHAPS a leak between the internal passages. Good visual inspection a good idea.

    Make sure you don't have any bad thermal vacuum switches or valves in the vacuum lines that are bad.

    Even the leaking q-jet internal passages would seem a stretch, since they would always leak, not just once in awhile.

    Another issue, if your throttle is/was sticky, and the throttle bore is shot, are you dealing with the idle dropping lower than it should only intermittently?

    I never noticed poor running from excessively worn throttle shaft bore, but I did notice that idle RPM varied depending on how I let off the throttle. If you set idle speed with the screw, and the throttle shaft isn't always returning to the same position, it may be set too low, and simply hangs up most of the time, artificially creating a higher idle.

    Did you adjust APT correctly? I've never paid a whole lot of attention to that aspect of the carbs, and I have no idea if it's not working right if that could affect idle.
     
  3. FightinTXag

    FightinTXag 1/2 ton status

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    Thanks for the reply, Dorian.
    Maybe I'll get a rebuilt distributor for an earlier year model, if that didn't solve the problem at least it would eliminate it as a possibility and get rid of the ESC completely.

    Seems like an internal leak at passages would be tough to see with the carb assembled. And of course if it's a problem at the air horn/carb body interface, pulling those components apart probably won't allow you to see the problem.

    Do Q-jets typically hiss rather loudly at idle with the air cleaner off? I don't remember my dualjet carbed Camaro hissing so much.

    Last time I pulled the carb, I accidentally broke the thermal vacuum switch. So it's new. I can't think of any other in-line valves, except check valves.

    Well, I should clarify. I'm a bit of a perfectionist. So when it's idling "ok" it's borderline unacceptable to me. Sometimes it is embarrassing the way the truck is shaking at a light. Other times, it just annoys me because I can still detect the slight roughness in idle.

    So I think the problem is always there, just varying monstrously in severity - from borderline unacceptable to flat out embarassing. When it's at an embarassing level, it will be that way every time I come to a stop. Driving to the next red light does not bring any change.

    Would that still point away from intake manifold gaskets and internal carb vac leaks?

    Well if a worn throttle shaft isn't likely to cause rough idling, mine is probably ok. I had wildly varying idle rpm, but a new throttle return spring seems to have cured most of that. What's left of the idle speed inconsistency is ok by me for the time being.

    I called a local carburetor shop and asked them how I should set the APT. They said they set them at 2.5 turns out when they do rebuilds, so that's what I went with. I'm sure that it would affect idle if it were out of adjustment since it dictates how low the power piston goes when seated (and thus how far the primary needs go into the jets).
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    I dunno about the hissing...it seems to be dependent on the engine, air cleaner, etc. If you had a leak or something causing the noise, you'd have found it I'm sure. I've noticed some engines just seem to make more of that hissing noise than others.

    Internal passage leak would certainly be hard to tell externally. Of course, with a tweaked air horn, it is entirely possible it is a problem. Those internal passages, when and where present, usually have very thin walls, and the gaskets are thin and relatively hard as well. Not a good combination for sealing things that aren't necessarily flat, when they should be.

    No change if the brake booster is disconnected and the line plugged, EVAP is no problem, AIR, etc? Those are the ones I can think of with check valves.

    If I were thinking a valve issue, I'd do a compression test. But an issue like yours that isn't *consistent*, can't be a valve, at least not by itself. If the valve isn't seating or lifting right, it's going to be a 100% repeatable problem. I can't imagine anything in the valvetrain being bad (bent pushrod, misadjusted rockers, bent stud, wiped cam lobe or lifter) that would be totally inconsistent like your problem is. Since you say it's not right all the time, there could be a "hard part" issue that is contributing, but unlikely IMO to be the cause of the major issue. You can run a compression or leakdown test, but that only tells you if the valves are opening or leaking, not if they are staying shut.

    I don't know if the Doug Roe book covers setting APT, but I doubt it is as simple as 2.5 turns for every quadrajet with APT out there. It wouldn't be adjustable if it were. :) When the carb is acting up, try tapping it with something. Sticky float, needles, etc. With a tweaked carb body, the float may be the culprit anyways...if it sticks (which I have dealt with) it can cause all sorts of issues.
     
  5. FightinTXag

    FightinTXag 1/2 ton status

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    Not sure what EVAP is.

    AIR doesn't leak vacuum. Isn't that pretty much all that can go wrong with it?
    When I read your comment, I remembered thinking the exact same thing when the local carb shop guy said it. Then I remembered I had found a technical explanation of exactly how to set the APT, and I just re-found it. Read Technical Procedure #2 on page 26 of this document:
    http://www.florida4x4.com/tech/quadrajet/QJet_Tuning_Paper.doc
    There's actually a ton of good info in that write-up, and it's basically the best explanation I've seen for setting up the APT. When I went though that procedure, low and behold it was damn near 2.5 turns out. I suppose that it's possible that the old coot at the carb shop has committed to memory that GM trucks with 305s get the APT set at 2.5 turns while other applications require other settings. That shop has been there for at least 20 years, and as plentiful as these trucks are/were down here, I wouldn't doubt it.
    I'll give that a try sometime. A new non-ESC distributor followed by a Jet Perf Q-jet is sounding tempting though. Or maybe even a TBI 350.
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    If I were contemplating EFI, I'd not spend money on a carb setup at all. :) Different distributor, no carb, etc.

    EVAP are all the lines from the fuel tank to the cannister in the engine bay, then to the carb. Purge valve on it is probably thermal vacuum controlled. Later on is electronic, but I suspect earlier there was a vacuum valve inline to purge at the right time.

    AIR I'd expect divert/manifold switching is handled by a thermal vacuum setup as well, again, later was electronic not much familiarity with earlier.

    Glad to hear the old guy is right. It's very likely that 2.5 was good for 75% (or more) of the carbs out there, since the rods and jets were still changeable at that time.

    A float problem might be something you can see when it's doing it, pull off the air cleaner (if you can in time) and check to see if it's spewing gas all over the place. Had that problem with mine one time, something was sticky, rebuild solved that issue.

    Good article BTW, hopefully more can get some use out of it.
     
  7. NewUser

    NewUser 1/2 ton status

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    maybe try swapping on a known good condition Q-jet to see if that fixes it or not? :dunno:
     
  8. FightinTXag

    FightinTXag 1/2 ton status

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    Newuser, I wish I knew someone that had one setup and running well on a 305.
     
  9. FightinTXag

    FightinTXag 1/2 ton status

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    [​IMG]
    This plastic thing that appears to just serve as a conduit for these wires and includes the little ground strap is cracked on my truck. But I'm pretty sure that those wires are ESC related. I've disabled the ESC so I don't think this is my problem.

    If the coil connections are at 3 o'clock, this thing is about 10 o'clock.
     

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