rebuilt AFB but idle SUCKS ????

Discussion in 'Carter' started by bad news, Mar 18, 2006.

  1. bad news

    bad news old B gaser

    THIS time i deciede to save a couple bucks ...right iget a kit off a board member dis asemble and soak carb for a week////////////////read the manual three times and follow it to the a T .... ....................the result is carb runs great no lag no hesatation perfect EXCEPT it will not idle smooth........................ive no gas running in ive pluged or bypast ed all the vacum stuff ..the only way it will stay running is if i back out the mixture screws about 4 turns(factory says 1 turn) no smell of gas but the 4 turns tells me it is getting more that it shoud at idle////

    what hurts is it really runs great otherwise....SO any thoughts thanks pete
     
  2. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    Did you blow out all the passages with an air gun before reassembling? I had the same problem as you did a year or two ago, and it ended up being gunk in the idle passages. I still could'nt see anything when I pulled it back apart, but I cleaned it again, and blew out all the holes and nooks in the carb and reassembled it. It ran perfect after that.
     
  3. D BERRY

    D BERRY 72 Skylark 2 DR POST

    The mixture screws that your turning out actually are just that, a mixture of gas and air. I believe that backing out the mixture screws richens the idle. Are you sure the butterflies on the primary and secondary are closed. Are you sure the carb is not leaking around the basegasket, or the throttle shaft it sounds like the classic vaccum leak, and that's where my problem was. Is Tinley Park around Chicago, I'm from southern IL and it seems like everyone who posts here from IL is up there. Course most of the people from IL are from up there!

    Dave B
     
  4. bad news

    bad news old B gaser

    YES tinley is just south of chitown >>>AND yes i blew out the passages ...i cant say for sure that the butterflys are closing all the way but it is anther thing to check when i rip into it again thanks guys pete
     
  5. carbking

    carbking carburetion specialist

    Number one cause of a poor idle is a vacuum leak. Absolutely no offense meant my next question. Have you checked ALL vacuum lines (maybe a heater control, etc.) and the vacuum advance. I once had a passenger take a nap in the car, and in the process of getting comfortable, knocked off a heater control vacuum line up under the dash. Still ran fine until we got off the interstate. No idle.

    Assuming no vacuum leaks, the idle mixture control screws should be out from 3/4 turn to 1 1/2 turns. Some Buick carbs have the big air adjustment screw (between idle mixture control screws); most have the throttle positioner screw (drivers side). One should set the idle mixture control screws to the suggested beginning point (1 turn), then set the idle RPM with either the idle air screw or throttle positioner screw, depending on the carb.

    My guess is the carb is OK, and it is a vacuum leak somewhere.

    Jon.
     
  6. D BERRY

    D BERRY 72 Skylark 2 DR POST

    Before you even take it off the car grab the throttle shaft, primary, and try wiggling it up and down. You should just be able to feel movement, if it wiggles you'll have to get the throttle shaft bushed. Good luck!

    Dave B
     
  7. bad news

    bad news old B gaser

    ok i will go ck this does not have the center idle screw so my question is when you turn the mixture screws out do you ritchen the mixture?? jon what dues a choke housing cost the threads where the heat line on mine are bad pete
     
  8. carbking

    carbking carburetion specialist

    D Berry - with all respect, the throttle shafts are probably OK. This is a Carter AFB, not a Q-Jet.

    The factory design tolerances on the AFB were 0.016~0.022 inches. This is necessary due to the expansion of the large bearing surface area on the AFB. Too tight a clearance will cause binding when the engine is hot.

    Conversely, the factory tolerances on the Q-Jet are 0.004~0.006 and bushings should be installed at 0.010.

    Pete - you are wanting to spend your money too easily :error: Simply cut about 3/16 inch off the end of the thread, and rethread (it is a tubing thread, and the die should be readily available at your local parts house). Drill into the inside of the threaded area sufficiently to install a ferrel (same parts house) and use a new tubing nut (again, same parts house). You have now saved a piece, learned something, and spent a lot less money. :)

    Jon.
     
  9. carbking

    carbking carburetion specialist

    Oops, reread, and saw your other question.

    The idle mixture control screws do not richen the mixture. They provide more of the same mixture. This may sound like semantics, but it really isn't.

    As an analogy, think of a set of the older shower controls. There were three controls: (A) hot water; (B) cold water; and (C) volume of the mixture. You would adjust the temperature by turning both the hot and cold taps, and when you got the proper temperature, you would open the volume tap to control the pressure.

    In the carburetor, the idle mixture is pre-set at the factory! A pre-determined amount of fuel is drawn through the idle tubes (long slender tubes in the primary venturi cluster), mixed with an initial mixture of air through an "idle air bleed", squeezed through a "restrictor" to accelerate the velocity of the mixture, then mixed with additional metered air through the "bypass" and directed to the idle channel, where the mixture control screws can meter the necessary amount to adjust the idle. So the "hot" and "cold" mixture controls are pre-set; and the mixture control screws adjust the volume.

    Jon.
     
  10. awake13

    awake13 Well-Known Member

    Did you install the metal shield that goes between the carb and the manifold
    and make sure there were no holes in it?
     
  11. bad news

    bad news old B gaser

    YES i installed a new plate gasket on either side jon i will give that fix a try will try all the above double check for vacumn leaks and if that dont cure it illgo buy a holley LOL thanks guys pete
     
  12. D BERRY

    D BERRY 72 Skylark 2 DR POST

    Wow, .016 to .022 inch clearance, I had no idea that was the tolerence. :Dou: Does the same clearances hold for the Thermo Quad also??

    Dave B
     
  13. carbking

    carbking carburetion specialist

    No, the TQ is kind of a marriage of the AFB and the Q-Jet, all the good points of both, none of the bad points. Compare the throttle bearing surfaces of the TQ/AFB and you will find large differences. Also, the TQ does not have the huge one-piece aluminum combined bowl and throttle body. Use 0.006~0.010 inch clearance on the TQ.

    Jon.
     
  14. bad news

    bad news old B gaser

    AN update on my carb rebuilt attempt frist A BIG THANK YOU to all for the ideas ........ I checked for vac leaks found none but what i did note is that i had to crank it over a lot be for it fired(like 40 sec) so that tells me it is leaking down //any way i removed the carb (after frist running the eng) and found that the gasket right under the carb was stained on mostly the right side (with gas) idid not spill any on removal and gasket was new////

    I r emoved the top found that both bowls were about 2/3 full////idisassembled it found everthing tight all gaskets looked good///

    then i blew out every passage some i ran a pin through i then eyeballed each before installing i am sure all passages are clear//

    i did find gas in a little galley at the very back of the carb //it is very shallow and is 1/4 by 2 1/2 in and is on the back wall of the carb at the very top???? i have no clue what it is for ???

    i assembled and test ran SAME thing except it does run smoother but to get it to run the mixture screws have to be out 3 to 4 turns//////
    but it will idle at a lower rpm and is smoother (now it is like a 3/4 cam instead of a full race cam before)) lol so for those still interested im open for suggestions //////what i thought is ill let it sit see if it leaks down again and get some prices on a holley lol thanks for reading one more thought the frist gasket i installed was in the kit and it had a cutout in the front for exhaust gasses to heat carb?? that is the one that was loaded with gas i replaced it with a solid one (dont know if that could be what is helping it idle better////??? later pete
     
    Last edited: Mar 21, 2006
  15. carbking

    carbking carburetion specialist

    Pete - While there may be no gasoline in the bowl after the engine sits overnight, the carburetor is not "leaking down" (unless you store the car upside down on its top :pp ). When Holley brought out the type AA-1 carburetor (in 1939), the power valve was a diaphragm located in the bottom of the bowl, with a direct passage into the intake manifold. When (not "if") the diaphragm ruptured, the gasoline in the bowl would drain through the power valve into the intake. This issue was called "leaking down" and the term has been with us since. The AFB does NOT have such a passage; all fuel must go "over the top" through the main jets and out through the nozzles, or go out the fuel valves which are located in the top of the carburetor. Fuel does not normally run uphill.

    So where does the fuel go? It evaporates!!! Aluminum, remember AFB - (A)luminum (F)our (B)arrel, is a great conductor of heat. Once you shut the engine off, the heat from the engine comes up, and the fuel is percolated right out of the bowls. Tests on my shop truck (a brand with the same name as an ex-president who was a terrible golfer) with a Carter AFB would be bone-dry in roughly 30 minutes after shutting off the engine. Remember that the new fuel, especially ethanol based fuel, is MUCH more volatile than the fuel for which your engine was designed. The addition of an electric fuel pump cured the starting problems with my shop truck.

    As to your idle, and the 4 turns out on the idle mixture control screws: are you also rotating the throttle position screw to increase the idle RPM. You do NOT want the primary throttle plates completely closed.

    If the throttle plates are closed, and if you have the idle mixture control screws out 4 turns, then you WILL have an issue called "puddling" in the intake manifold. This issue WILL cause a rough idle, as the mixture will vary as droplets of fuel are swept into the cylinders.

    Jon.
     

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