Inconsistent AFRs when returning to idle, driving us crazy

Discussion in 'The Venerable Q-Jet' started by jamyers, Jun 20, 2019.

  1. jamyers

    jamyers 2 gallons of fun

    A couple of years ago I bought a Q-jet from Mark at Qjet Power for our '69 Chevy C10 w/ stock 350 engine. It's been a great carb, but lately has us stumped. We've had a wideband O2 gauge installed for the last couple of years, which is very helpful in setting idle mixtures and seeing what's going on as various issues have cropped up (like a crazy vacuum leak, etc). (it's almost too much info some times)

    When it's running like it should, the jetting built into it is perfect - 14.7-15.0 at idle, 16 at cruise, and 12.5-13 under load, transitioning into the power circuit right when you'd want it to.

    Symptoms: The A/F mixture ratio when returning to idle is erratic - that is, while driving and you come to a stop sign, sometimes it will be right at 14.7-15, and the next time it may well go as lean as 19-20+, with rough running and engine dying. Pump the pedal and restart it, and the idle may be normal, lean, or slightly rich. Next stop sign, it may be normal or lean again. It rarely goes rich, and then it might be 12-13, most often it leans out.
    Above idle / transition (cruising down a road or highway), it's fine - generally right around 16, with great transition into power and secondaries.

    History: 1. A year ago we had problems with the old fuel tank sending dirt to the carb, so now the truck has a new, clean aluminum fuel tank, an electric pump regulated to 5psi, two fuel filters, and we've taken the carb apart a couple of times, spraying out all passages and double-checking float height, etc. Since the new fuel tank I haven't seen any dirt, dust, or anything inside the carb.
    2. We had a freaky issue with the intake gasket allowing a large vacuum leak, and replaced the gasket with a thick Cometic gasket - forgetting to open up the holes for the exhaust crossover passages, which meant we had to reset the choke.
    3.Ignition is a GM HEI unit with relatively new plugs and wires, maybe 15K miles on it.
    4.When it's idling correctly, it pulls 19 inches steady of vacuum and is rock steady. I haven't gotten a vacuum gauge on it when it's acting up yet - it behaves fine when at home (of course).
    5. Engine temps are always a solid 195, Voltage is steady 14.5V when running.
    6. The only vacuum ports are: Brakes, PCV, Distributor.
    7. Transmission is a 700R4.

    I'd think something is intermittently plugging something in the idle circuits, but for the life of me I can't find anything in there.

    Maybe Carb icing because of no exhaust crossover - in West Texas?

    I'm grasping at straws now - Any ideas?
     
  2. BUICKRAT

    BUICKRAT Torque Rules!

    I've seen worn throttle shaft bushings cause this.
     
  3. techg8

    techg8 The BS GS

    How about being in and out of the mechanical advance at idle? Make sure it's at 0 and stays there at idle.

    Weakish throttle return spring?
     
    BUICKRAT likes this.
  4. shiftbyear

    shiftbyear Well-Known Member

    Fuel pump or gas cap (vented/non-vented)?
     
  5. jamyers

    jamyers 2 gallons of fun

    I'm pretty sure Mark installed new bushings in it, but I'll double check that.
    Ignition timing is nice and steady at +10, and returns there every time I bump the throttle. It has 2 throttle return springs, both pulling from the front against the throttle linkage (instead of the usual stupid Chevy 'pull from the rear and eat the throttle shaft bores' setup)

    Vented gas cap. The new fuel tank is actually a fuel transfer tank / toolbox in the bed (70 gallon capacity, heh).
     
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    When you come off the gas and slow to a stop it should go rich then stabilize. How hot does the engine run? Do you have the exhaust crossover open? Doesn't take much to vapor lock the cat piss gas we have today.
     
  7. Gary Bohannon

    Gary Bohannon Well-Known Member

    I had a similar problem years ago. Finally decided to blow thru the 2 vents on my 67 GS gas tank, they were both plugged from age, rust probably.
    Installed new 2 vent stock tank and problem went away.
     
  8. jamyers

    jamyers 2 gallons of fun

    Exhaust crossover passages are blocked by the Cometic gasket. Engine runs at a rock-solid 190 degrees, even with a/c on a 100 degree day.

    Fuel tank has two of the huge vented transfer tank caps, both of which are relatively new.


    I'm in the middle of Summer Camp until July 5th, then we'll tear back into it - and either figure out what's going on or convert to a Holley Sniper throttle body injection system (which we've been thinking about for awhile now...)

    Thanks all for the inputs, I'm betting on either worn throttle shafts or some tiny crap inside the idle tubes
     
  9. Cliff R

    Cliff R Well-Known Member

    The first thing to do is to open up the exhaust crossover in the intake manifold. It's a wet-flow system and stock set-up, it requires the heat to get the intake up to a suitable temperature for best thermal efficiency and to make the choke work correctly.......Cliff
     
  10. TORQUED455

    TORQUED455 Well-Known Member

    What is the “freaky issue” you had with the intake gasket? Have you tried retorquing the intake bolts? Carb mount gasket look good? Carb bolts tight? Anything unusal about the carb mount pad and the carb mount gasket, as in are the carb and intake made for each other? Do the brakes feel OK when the problem is occuring? Have you done the “start test” for the booster? Does it hold vacuum?
     
  11. jamyers

    jamyers 2 gallons of fun

    Will do! Give me an opportunity to triple check the intake gasket
    Well, you won't believe me (and I'm not sure I would either), but the intake gasket 'walked' itself up from between the head and intake. When assembled, I distinctly remember making SURE that the gasket was in position, even used 'gask-a-cinch' to stick them to the heads. Over time (maybe 6 months), and after being retorqued at least twice, the back end of the drivers side (#5 and 7 cylinders) gasket shifted upwards to the point that there developed a whistling vacuum leak. I know gaskets 'don't move', but I'm telling you that this one did, and I have no idea why or how.
     
  12. telriv

    telriv Well-Known Member

    Were the intake sides of the heads or intake cut??? Look at the surfaces closely & look for irregularities.
     
  13. jamyers

    jamyers 2 gallons of fun

    New Summit heads (aka Dart iron heads with vortec chambers), so I'm assuming they're good and flat. The intake is from a wrecking yard, no idea if it's been resurfaced - but we did check it with a straight-edge, it's at least flat. (?)

    Strongly considering a new aluminum intake, possibly going to go EFI with a Holley Sniper. This truck is my son's, and he's wanting to take it to college with him in August - and I don't want him to have to mess with it.

    We'll be digging into it this Saturday, going to confirm ignition, vacuum, etc, then pull the intake, open up the exhaust passages, confirm gasket sealing once again, open up the carb once again, and see what we can see.
     
  14. TORQUED455

    TORQUED455 Well-Known Member

    Before you take things apart, pull the PCV valve and hose and introduce propane in a valve cover and see what happens to the idle.
     
  15. Cliff R

    Cliff R Well-Known Member

    "I know gaskets 'don't move', but I'm telling you that this one did, and I have no idea why or how."

    I've seen many of the aftermarket paper type move around especially if they are sandwiched between aluminum heads and intake.

    For this reason I stick with the stock type Felpro blue intake gaskets that are metal reinforced. They will stay in place where many others will not.....Cliff
     

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