Loosening leans it out?

Discussion in 'The Mixing shop.' started by BuickV8Mike, Oct 30, 2018.

  1. BuickV8Mike

    BuickV8Mike SD Buick Fan

    I'm interested with respect the the air fuel screws on the front of a 750 cfm Q-jet. The backfires through the tailpipe was overwhelming before adjusting as well as the response on a "roll" of the throttle? Am I on the right path?
     
  2. johnriv67

    johnriv67 Well-Known Member

    Make sure timing is in check before making any mixture adjustments. If it is fine, then turn the mixture screws all the way in(GENTLY!!!!! DONT FORCE IT), and then 3 complete turns out and let it idle or drive it until warm. Yes, it will be rich most of the time, but once it warms up pull out your vacuum gauge.

    Plug into manifold vacuum with the gauge and tune* the engine by turning the screws in and out until maximum vacuum is achieved on the gauge. *Tune one screw at a time. If your idle jumps to over 900 if your engine has the stock cam and vacuum increases to over 25*, then decrease the idle speed courtesy of the screw that pushes on the throttle linkage on the driver side of the carb. Continue tuning to max vacuum and then once it is reached, turn each screw in 1/8 of a turn to achieve a lean idle.

    If that doesn't work, its something else! Please let us know how it goes

    And to your title, loosening makes it richer.
     
    Smokey15 likes this.
  3. BuickV8Mike

    BuickV8Mike SD Buick Fan

    Thanks John! The real problem is it feels like a dog and has a huge stumble out of the gate. It also constantly backfirers out the pipe coming off the throttle. I changed the rods and orifices recently from where it really ran strong..... which is why I'm going back too. I guess you will always smell gas when a '60's era car is running next to you.;)
     
  4. johnriv67

    johnriv67 Well-Known Member

    Does your accelerator pump shot work? Backfiring when coming off the throttle sounds dead lean. If it's a quadrajet and you've rebuilt one before, it would likely be in your best interest to jump up 4 primary jet sizes and see if your problems drastically change. If the problems are different the next time, it's directly related to fuel mixture and not much else.

    Edit: Go right back to the jetting you had before, if not 2 more primary jet sizes higher. If the car feels much better, then drop back to where you were before to compare for the final time.
     
    Last edited: Nov 1, 2018
  5. BuickV8Mike

    BuickV8Mike SD Buick Fan

    John,
    I am going back to 0.072 jets with 0.027" rods (how can you determine th code?) from the current 0.070 / 44B rods. Am I moving back in the right direction? Cheers, Mike
     
  6. johnriv67

    johnriv67 Well-Known Member

    Mike I have some questions for you about how the car ran before with the .072 and .027 sizes. (Also, they are mostly referred to as 72 jets/27 rods) If you stood behind the car's exhaust, did your eyes water? How rich did it smell?

    What engine is in the car? And any modifications?

    And what do you mean by "code" ?
     
  7. BuickV8Mike

    BuickV8Mike SD Buick Fan

    John, Thanks for all the help. The engine is a recent 455 stock rebuild, 0.010" off the heads by now. Pistons 0.05 in the hole.. 10 to 1 TA pistons. The code of the rods....CY or DA is what I'm talking about.. No eyes watering...but you can smell an engine running...
     
  8. buick64203

    buick64203 Right wing conservative Staff Member

    You should keep a 30 point spread between your primary jets and rods.
     
    johnriv67 likes this.
  9. johnriv67

    johnriv67 Well-Known Member

    Jason is right, you should look for the .030 spread between jets and rods, but in practice I've found it completely varies based on rod/jet size.

    A 68jet/38rod will run much much leaner than a 78jet/48rod at WOT due to the power tips being the same diameter.

    I would start at 72 jets and 37 rods with whatever secondary rod hanger you have. If you have CK of CH secondary rods, throw them in there. What do you have now?

    What jets and rods do you have so that we can make this combo work?
     
  10. buick64203

    buick64203 Right wing conservative Staff Member

    Swapping out jets and rods without a wideband in it is almost pointless. You have no idea what the air fuel ratio is at cruise, WOT or idle for that matter. I can tell you that minute adjustments to my mixture screws made big changes in my AFR.

    I have an Innovate Motorsports wideband with an extra long cable and the tailpipe adapter. I can put the adapter in my tailpipe, ran the cable into the car and drive it. Best thing I ever bought. Ive zeroed in nearly all my carbs
     
  11. johnriv67

    johnriv67 Well-Known Member

    Educated guesswork is better than having your car run like sh*t, but I completely respect the wideband decision, and totally wish I had the ability to buy one.
     
  12. BuickV8Mike

    BuickV8Mike SD Buick Fan

    Changed the timing to 8* BTDC from 10* and now it won't start. :(:(:(
     
  13. johnriv67

    johnriv67 Well-Known Member

    That seems unrelated to fuel unless it's continually cranking with no spark. Check your ignition connections, turn your mixture screws out a turn(because it's apparently quite lean anyway), and retest
     
  14. BuickV8Mike

    BuickV8Mike SD Buick Fan

    Will do John thanks. Backfiring through the exhaust is a LEAN condition? I'm sure if I put the timing back to 10* BTDC it will start. Thanks, Mike
     
  15. BuickV8Mike

    BuickV8Mike SD Buick Fan

    It wouldn't start because the distributor ground came off with the dwell meter........Replaced the 74/45 jet rod combo with the 72/42 combo. Starts great again! Still a little engine rattle when roll the accelerator off idle, first 400 rpm in the driveway, but much better. Does seem to smell more. Adjusting the idle screws is next. Can I do this at 1300 to better judge things....its hard to see the changes on the vacuum gage. A/F gauge for :) Christmas :) Thanks everyone! Mike
     
  16. johnriv67

    johnriv67 Well-Known Member

    Great! Now accept the smell, turn the mixture screws at idle out a little more to possibly eliminate that last rattle.

    The issue with adjusting idle at 1300 is that you may enter the primary circuit and leave the idle circuit while adjusting the carb at that high of an rpm. If you do this, when the idle returns to normal it will likely be dead lean since you had entered the richer circuit, often used under vehicle motion.

    Edit: Actually Mike, I would throw in the 74 jets with the 42 rods and compare how it felt before and now. I really recommend that
     
  17. BuickV8Mike

    BuickV8Mike SD Buick Fan

    John, Can I move the accelerator pump lever to the greater length?
     
  18. johnriv67

    johnriv67 Well-Known Member

    That would be an issue if it was stumbling on acceleration alone, but it sounds like its popping out the exhaust on deceleration as well. You could try it, but I don't expect anything.
     

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