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Qjet Help!

Discussion in 'The Garage' started by shimniok, Mar 21, 2005.

  1. shimniok

    shimniok Registered Member

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    Been working on getting a Qjet running right. Except for WOT, everything is good: idle quality, off-idle, cruise, throttle response, part throttle, etc.

    At WOT around 3500rpm and up in 1st it starts surging; The surging (surge-stall-surge-stall) gets worse if I lock out the secondaries which suggests to me it's not a secondary issue, confirmed by the fact that adjustments on the air valves and secondary rods had no effect whatsoever. What do you think is the most likely cause:

    • incorrect jetting (lean or rich? The jets and rods appear to be factory and the carb is pulled from a junkyard and is for a '78 472 Caddy but am running on smallblock with mild cam, large exhaust, 4v performer intake)
    • incorrect float shape (the float gets thicker vertically on the end opposite the needle, rather than right next to the needle)
    • fuel pump (running generic 4psi electric with return line)
    • I've set and triple-checked the float level to factory specs for that carb.
    • The previous carb (AFB) never exhibited this problem.

    Carb (and especially Qjet) gurus -- help! :D

    Michael
     
  2. Stoopalini

    Stoopalini 1/2 ton status

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    Maybe it could be a timing issue? Have you put a timing light on it yet? And how is your vacuum advance setup?

    When it starts bogging out, do you hear any pinging?

    Thomas.
     
  3. shimniok

    shimniok Registered Member

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    Yup, timed it as part of the swap, was pretty careful about setting it. I will doublecheck vacuum hoses. The distributor / mech advance is common across the working carb (AFB) and the Qjet.

    I don't get pinging, just get a regular oscillating bucking/surging on-off-on-off kind of thing, like it's accelerating and kind of dies out, then starts accelerating again.

    Then it shifts and bogs and stumbles for awhile then power comes back on and I run out of road.

    Not sure if that makes sense.

    Michael
     
  4. fatboy

    fatboy 1/2 ton status

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    sounds like a lean condition. If you lock out the secondaries you get less gas and you said it gets worse. Makes sure your float is set corect. and if it is an old one get a new brass float. Then make small adjustments and test it. Maybe a different primary rod that is thinner at the tip when the rod is out of the jet that is your WOT point. anyway here are some good links that may help


    http://http://www.buickpartsdirectory.com/carbs.htm
     
  5. sandy78

    sandy78 Registered Member

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    Q-jet problem

    Sounds like it's running out of fuel. Check the fuel pump output, filter, and needle and seat in the carb. If you rebuilt it, the parts supplied may actually restrict fuel flow. Doug Roe has a great book on Rochester carbureators complete w/ P/N's on high-flow needle/seat combo's.

    In the jetting dept., you could run leaner than what the carb was jetted for simply by the lack of a restrictive air cleaner. In which case, bigger jets or smaller rods are in order.

    Good luck
     
  6. shimniok

    shimniok Registered Member

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    thanx so far

    Thanks for the input so far, everyone. I have some things to try out tonight. I have posted to a few other lists and asked a couple friends. I think we're all on the same track (hopefully the right track) as far as possible causes.

    One interesting piece of trivia on the Qjet -- the tip size is the same on all Qjet rods 0.026" thus jet size is the way to tune WOT, while rod size tunes light throttle, cruise, and idle. Interesting, eh?

    My current favorite theories are insufficient fuel flow or pressure ... and... incorrectly shaped float (causing fuel shutoff on acceleration as fuel sloshes towards the back of the carb)

    Michael
     
  7. callmewaylon

    callmewaylon Registered Member

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    How are you 'locking out' your secondarys? I had the same thing happen and it turned out that my seconary air valve adjustment was so loose that the secondarys would just flop open instead of opening gradually.
     
  8. shimniok

    shimniok Registered Member

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    lock out

    There's supposed to be a small lockout lever with a notch that fits over the lever attached to the secondary throttle plate shaft. It's actuated by the choke linkage junk behind the choke. The linkage junk behind the choke and that little lever go together (ie, they aren't necessarily interchangeable across carb models). I just use bailing wire to wire that lever into the lock position. Hope this helps.

    Michael
     
  9. fatboy

    fatboy 1/2 ton status

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    hey thanks for the info on the jets. I do remember reading about that but I havent had a qjet in years
     
  10. JimCooper

    JimCooper Registered Member

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    One of the drawbacks of the Q-Jet is the small fuel supply in the float bowl. Sometimes one can use it up. Some folks recommend trimming the float and the spacer, while I think the best thing would be to improve fuel delivery to the carb, ie fuel pump.

    Jim
     

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