Q Jet Tuning

Discussion in 'The Venerable Q-Jet' started by CJB72Skylark, Oct 6, 2008.

  1. CJB72Skylark

    CJB72Skylark Moderator

    Just rebuilt my Q Jet with one of those cheapie 35 dollar kits that came with some new bushings, and gaskets, etc. The carb was horribly dirty and starting it cold was a nightmare.

    Everything is A OK no except for a little tuning needed. It smells a little rich, and when I lightly press the gas it seems to stumble badly. If I go to WOT it never misses a beat....


    What do I need to adjust to help that little hiccup when I lightly accelerate?
     
  2. techg8

    techg8 The BS GS

    My first inclination when I hear light throttle hesitation is that you have a lean hesitation. Two things come to mind immediately, though it could be a few things.

    1) Accellerator pump. Make sure you see squirts of fuel in the barrels when you depress the pump. Also there is a choice of linkage holes in the pump lever...if youre on the inner one, mave the linkage to the outer hole for a slightly longer pump stroke ie more fuel per shot.

    2)Carb base gasket leak. listen thru some hose around the base of the carb while running...should hear a suction sound if a vac leak is there. Or you could try propane around the base gasket...If RPMs increase ('cause the propane is sucked into the intake thru the vac leak) then youve found a leak.

    Of course you said it smells rich...

    The rich smell could be from idle mix screws needing adjustment?

    Id also look at the choke, make sure its functioning right, ie closed when motor is cold, and opening when the engine warms up. This would make you real rich if it was staying closed.

    LoL I keep thinking of more things....

    good luck
     
  3. CJB72Skylark

    CJB72Skylark Moderator

    First of all the I dont have any choke at all right now- its blocked off. I don't know why it's like that but thats how I bought the car. I would need someone walking my though how to fix that problem.

    -I'll check the pump lever when I get home tonight

    -Listen through a hose around the base of the carb? You would me to put my ear at one end of the hose and the the other around the base of the carb and listen for a vacuum leak? Okay- Ill try that too.

    -The rich smell was primarily right after I got it running again, not so much anymore. I just remember driving it for the first time thinking whoa- that smells rich.
     
  4. techg8

    techg8 The BS GS

    Well,

    One end near your ear and the other end pointing to where you think a leak might be. move that end around the base of the carb little by little and listen. when you hear a hissing vacuum sound, you are pointing the end at the leak. Dont just wrap a hose around the base of the carb.

    Yep it works. Just dont put the vac hose like INTO your ear. If you do and you get too close to a good vacuum it will try to pop your eardrum. I know this from experience I hate to admit.
     
  5. CJB72Skylark

    CJB72Skylark Moderator

    Update:

    Bought a vacuum gauge- which I have no idea how to use. But I know Im going to need to use to diagnose and tune.

    I bought a Quadrajet book awhile back and read it cover to cover one night and remember it saying something about the two screws in the front and adjusting them into the carb all the way in then back them out like a turn and a half and to use that as a starting point.

    Anyway about the leaking air- I had the air filter and base off and pulled the throttle back and heard a WHOOOOSH sound. Between the loud headers and the loud WHOOOOSH sound I wasnt able to hear a damn thing with that hose up to my ear.

    Is that the sound I should('nt) be hearing?
     
  6. 71skylark3504v

    71skylark3504v Goin' Fast In Luxury!

    I'm sorry, but that way of diagnosing a vacuum leak sounds very substandard. Use a can of WD-40, carb cleaner, ect. and spray where you suspect a vacuum leak to be, if you find one, the engine will sound different for a second.:beer
     
  7. techg8

    techg8 The BS GS

    you wouldnt need to open the throttle to hear it. If you heard no leak at idle then there likely is no leak there.

    As for it being a substandard test, it has worked great for me.:Do No: Maybe it is a little oldschool if nothing else.

    sprayable things like WD40 or like I mentioned before, propane work well also.
     
  8. CJB72Skylark

    CJB72Skylark Moderator

    If that is not it than what AM I hearing. What is that whooshing sound right when I move the throttle linkage? I will try the other test tomorrow

    What are those screws/adjustments that I turned in the front of the carb?
     
  9. CJB72Skylark

    CJB72Skylark Moderator

    Just came across the screws in my book- they are idle mixture screws.
     
  10. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    You have a vacuum gauge. Hook it up to manifold vacuum. Get a base line reading with the engine fully warmed up, transmission in Park. Spray some carb cleaner around the carb base gasket. Watch the gauge. If you hit a vacuum leak, the gauge reading will go up suddenly, and the RPM's will increase.
     
  11. CJB72Skylark

    CJB72Skylark Moderator

    Ok- I need to pick some carb cleaner up tomorrow, I have zero tools/supplies at my apartment.

    You said to hook it up to manifold vacuum, I dont know where that is. I dont know what a low pressure reading is, I dont know what a high pressure reading is. I am a complete virgin when it comes to working on carburators.

    The only experience I have is changing diaphrams and jets on Briggs carbs when I used to race go-karts.
     
  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The Q-jet has 2 vacuum nipples on the front of the carburetor above the idle mixture screws. The larger one on the passenger side of the carburetor will be manifold vacuum. When you uncover the nipple with the engine running, it will hiss and there will be suction at the nipple. Hook the vacuum agauge to that nipple and read the gauge. Spray the carb cleaner around the base of the carb, and watch the gauge. It's as simple as that.
     
  13. CJB72Skylark

    CJB72Skylark Moderator

    Sounds easy enough. What is an ideal gauge reading with the car running and no leak? Will the gauge go up or down if it is leaking? The kit I had didn't come with new floats- should I check those out while Im at it?
     
  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member


    With a stock cam, look for a steady reading of about 20". A vacuum leak will produce a low(16-17"), but usually steady reading. With a longer duration cam, the reading will be lower (15"or so), and that would be normal. Those are general numbers and can vary. I'm not sure what you combination is. Is it a stock cam? If the float is bad and leaking, the carburetor would flood, so I wouldn't worry about it right now. Float level setting is critical though. A low setting will cause all kinds of problems.
     
  15. techg8

    techg8 The BS GS

    by the way, turning your idle screws all the way in and then out 1.5 turns as a start point is a very good place to start them.

    I set mine there on a 68 Qjet I rebuilt and havent moved them since.
     
  16. CJB72Skylark

    CJB72Skylark Moderator


    Check out my signature- thats the car Im working on. The cam has a .454 int/exh duration.
     
  17. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    No that's the valve lift. Intake and exhaust duration is 212* at .050. I would expect around 18" of vacuum at idle.
     
  18. CJB72Skylark

    CJB72Skylark Moderator

    Check out the picture that is attached so you know what part of the carb I'm taking about. The below steps are the checks I did roughly in the order i did them.

    1. Removed vacuum line from Point A and attached Vacuum Gauge.
    -With car warmed, in park, and idling I had 13 lbs pressure. I do not have a tach so I dont know what RPM I idle at, but I know it is kind of low, so that might be why the reading was so low. If I took the gauge off while the car was running and put my finger over the nipple there was suction there.

    2. With gauge attached I sprayed carb cleaner around base of carb where it attached to the intake as well was where the top of the carb bolts to the bottom of the carb.
    -Engine never gained any RPM or changed sound at all

    3. With motor off I disconnected vauum line from Point B.
    -I put my finger over the nipple and then pulled that linkage back, it stayed in place and never moved. So I think that part is ok.

    4. I replaced the vacuum line on Point A and Point B as a precautionary measure because it was old and hard and the ends of it were cracked.

    5. The vacuum line at Point C where it goes into the carb as well as where it goes into a gizmo on the distributor were so loose they slid off easily with the slightest tug.
    -I used a very small nylon tie strap to make it stay in place better.

    6. The line at Point D is hard and stuck on there and I dont even know what it does.

    7. Points F I dont know what these are really, but the one towards the front of the car will hold pressure and I could not test the one at the back of the car.
    -There is alot of play in the linkages attached to both pieces. I dont know what they do or how they work but they seemed to move very freely.

    8. Point E Im guessing this is my choke that is blocked off.
    -I tugged on the line itself as well as the bolt that was capping it off and they are both secure.

    9. Point G- I dont know what this is, but when I sprayed carb cleaner around it the line was secure and did not leak at all. It looks like a newer line.

    Forgive me for not knowing the correct term, but Im a work in progress....
     
  19. CJB72Skylark

    CJB72Skylark Moderator

    Maybe what I am hearing is normal. But the the airclean assy off, the car warmed up and idling- when I pull the throttle linkage back to gain some Revs, I am still hearing that loud whooooshing noise. What is that and is it normal? It almost sounds like a pop off valve.
     
  20. techg8

    techg8 The BS GS

    sounda like the normal aspiration of the motor to me. The engine gulping in air past the throttle plates.
     

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