Throttle Problem 63 AFB Carburetor - NCRS Discussion Boards

Throttle Problem 63 AFB Carburetor

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  • David F.
    Frequent User
    • January 10, 2011
    • 48

    Throttle Problem 63 AFB Carburetor

    I am a "BACK YARD MECHANIC" finishing up restoration of a 63 Vette Stingray (DRIVER). It started in August, I had a problem with engine not idling down. I have to rev. the engine hard to get to return to idle RPMs. Each time I give it gas to drive the engine won't return to idle without kick (rev) multiple times.
    Where do I start looking for solution to this problem? I do have copy of 63 Corvette Shop Manual, but this is not covered in trouble shooting section.
    The carburetor is Carter AFB marked 3721SB. Need a little help.
  • Bob R.
    Extremely Frequent Poster
    • June 30, 2002
    • 1595

    #2
    Re: Throttle Problem 63 AFB Carburetor

    I have always had the same problem with my 63 with a correct 3461 carb so I'm interested in hearing the responses also. I have thought the throttle shaft may have wear and this may cause the throttle to stick but let's see what others think.

    Comment

    • Kent S.
      Very Frequent User
      • February 1, 1993
      • 203

      #3
      Re: Throttle Problem 63 AFB Carburetor

      Wow, I just solved this problem on my '63. I discovered that the throttle lever (the one your rod hooks into) was slightly, and I do mean slightly, loose on the shaft. Exact symptoms you describe. If you use a long screw driver to put slight pressure on the butterflies it would return to idle. No amount of pushing on the throttle lever would work. Try pulling back toward the firewall on the lower lever that has the fast idle adjustment screw. Several fixes: add a spring to the lower lever pulling it back (and shut) is easiest, but not correct; drill a hole through the fast idle lever and throttle lever and insert a roll pin or small screw; JB Weld between the two levers; remove carb and have a small TIG weld from the back securing the two levers together (of course carb must be drained and dry!). Hope this helps and solves your problem. I spent several days removing/reinstalling my carb before I found the problem - it's hard to see unless you are looking specifically at those two levers.

      Comment

      • David F.
        Frequent User
        • January 10, 2011
        • 48

        #4
        Re: Throttle Problem 63 AFB Carburetor

        Thank you. I will take a look at the throttle lever and linkage.

        Comment

        • Timothy B.
          Extremely Frequent Poster
          • April 30, 1983
          • 5177

          #5
          Re: Throttle Problem 63 AFB Carburetor

          David,

          I don't think anyone is going to be able to give you a answer to the problem without having the carburetor in hand.

          Could someone have removed and installed the throttle blades without centering them, is there a vacuum leak.. What you should know is that these AFB's are good carburetors and something is not right. Consider sending it to a good known rebuilder and tell him what the problem is, on my 63 the carburetor throttle shaft returns fine and it's easy to tell with carburetor in hand.

          Comment

          • Joe M.
            Very Frequent User
            • February 1, 2005
            • 589

            #6
            Re: Throttle Problem 63 AFB Carburetor

            check the linkage on the firewall. Had a similar problem with my 63.

            Comment

            • Stuart F.
              Expired
              • August 31, 1996
              • 4676

              #7
              Re: Throttle Problem 63 AFB Carburetor

              Over the years, since new, I have had 4 different AFB carburetors on my 63 icl. 2 different 3461S's, a 3720SA and a new replacement 3721SB. Every one of them presented "return to idle speed" issues. For the last 3 carbs I have used a small booster spring which is enough to keep the situation near normal. Other issues I have encountered are:

              1) 63's should never use a white painted return spring. Almost all vendors used to sell these as a 63 only spring. They finally learned it is wrong. A black spring, as used on 64 and later AFB's, is correct.

              2) My 3720SA did have the throttle plate screw heads broken off and then epoxied back in place. That did not hold.

              3) I have had the primary shaft re-bushed on my latest 3461S, but that only solved a low speed lean surge problem. It did not affect the idle speed situation.

              4) Most recently, with my current 3461S, I experienced a situation where my idle speed would increase to 1200 to 1500 rpm, but would come down as soon as I blipped the throttle. In addition, the engine was running very rich and would flood badly in a heat soak after shutdown. I had to give it full throttle each time I started it when hot (very uncharacteristic for this carb and my engine). I could not diagnose the problem with the carb on the engine, so I removed it and replaced it with my spare 3721SB.

              On the bench I noticed that the flooding seemed to be originating in the right rear (passenger side) secondary. I removed the top plate followed by the right side secondary cluster. To my shock I discovered that during my last re-assembly of this carb, I mistakenly used a primary cluster gasket which has a closer screw hole spacing. Over time the gasket, held by only one screw, turned allowing fuel to be drawn directly from beneath the cluster assembly instead of through it. This accounted for the irregular idle speed, the over rich condition, and the engine flooding badly during hot soak.

              When you take a carb apart and reassemble it as many times as I have sorting out it's original lean surge condition (over the past two years), s*** happens.

              Have fun.

              Stu Fox

              Comment

              • Frank D.
                Expired
                • December 27, 2007
                • 2703

                #8
                Re: Throttle Problem 63 AFB Carburetor

                The loose throttle arm plate at the linkage as noted above is often a culprit...the peened over shaft head doesn't always seem to secure the two plates together and the slop when the linkage returns to idle causes the inconsistent idle. Very easy to spot by just pushing forward on the plate to see if the idle drops to normal while watching those plates. An extra or stronger return spring can help but can cause its own wear issues..

                Also, putting the "S" spring on the accelerator pump incorrectly can cause excessive wear on the air horn's body. Namely the two "lugs" that guide the accelerator pump shaft into its bore....when the wear is excessive you'll have idle problems and even a sticky throttle.

                Note the wear on the rear lug in the first picture...making the pump shaft cocked in its operation.
                The second pic shows the peened shaft on the two plates that may work loose.
                Attached Files

                Comment

                • John P.
                  Very Frequent User
                  • January 1, 2006
                  • 162

                  #9
                  Re: Throttle Problem 63 AFB Carburetor

                  Hello David, Check and see if your secondary throttle plates are not sticking. When the condition exists, lightly tap the throttle plates closed with a screw driver. It may be something as simple as carbon buildup not allowing the throttle plates to seat. Any suitable carburetor cleaner sprayed around the throttle plates may cure the problem.

                  Comment

                  • Stuart F.
                    Expired
                    • August 31, 1996
                    • 4676

                    #10
                    Re: Throttle Problem 63 AFB Carburetor

                    David's;

                    Believe you said your AFB was a model 3721SB which means it was likely a replacement carburetor. The one I have I bought new in 1989 as a replacement. It could therefore be some 25 or more years newer than any 3461S originally installed on your engine. The likelihood that it endured a great deal of mileage wear is less likely than with a much older unit.

                    Stu Fox

                    Comment

                    • David F.
                      Frequent User
                      • January 10, 2011
                      • 48

                      #11
                      Re: Throttle Problem 63 AFB Carburetor

                      I am planning to look at the linkage and shafts this week end. Thank all of you for you input and experience with same problem.

                      Comment

                      • David F.
                        Frequent User
                        • January 10, 2011
                        • 48

                        #12
                        Re: Throttle Problem 63 AFB Carburetor

                        housing I think I found the problem. I spent the last few days checking out everyone's suggestions, multiple times (Thanks for all your suggestions). I found nothing wrong, very frustrating. I step away last night to review my investigation and realized I was missing the obvious. The run away engine occurred whe ever I mounted the carburetor air filter housing. There were little silver metal flex on the intake manifold under the throttle lever. Looking at the bottom of the air filter housing and I found scrap marks. It appears the throttle lever was binding on the bottom of the air filter housing. Using a mirror I could see it hitting and could feel on the throttle rod. Mounting the housing carefully I am able to test in my garage with no problem.
                        I hope to test drive on the road this weekend.
                        Thanks to all for your suggestions, I have learned a lot reading about your solutions and getting my hands dirty. I have found some other problems that I will request some help under title Carburetor AFB.

                        Comment

                        • Frank D.
                          Expired
                          • December 27, 2007
                          • 2703

                          #13
                          Re: Throttle Problem 63 AFB Carburetor

                          I hope that is indeed the problem but your symptoms as first described are not typical of that issue. Near idle the throttle linkage and carb plate are in the far forward position where air cleaner interference should not be a factor. That sort of interference (I would think) would occur at the top of the accelerator plate arc which would be 1/2 to near full throttle.... This would mean the engine would be revving at really high RPMs versus just an unstable idle.

                          Your latest post describes a 'run away engine'.....if that had been included in the initial symptoms you might have gotten different suggestions for a fix.

                          If that is what is happening then yes, you may have hit on the cause - let's hope so.

                          Comment

                          • Stuart F.
                            Expired
                            • August 31, 1996
                            • 4676

                            #14
                            Re: Throttle Problem 63 AFB Carburetor

                            David;

                            It may not seem like much, but are you using an air horn gasket? That would be the large diameter gasket that seals the air filter housing to the top of your carburetor. It's not real thick, but it may just be enough to eleviate your clearance problem. Also, are you using a stock air filter housing and not an after market open element jobber (one unit fits all type)?

                            I fought an unusual problem with my L-76 SHP (340 hp) chrome air filter housing: from day one in 1963, the chrome on the inside of the housing began to peel off. When I observed a loose piece I would grab ahold of it and peel it free down to the copper under plating. The obvious reason for this is I didn't want it to be ingested into the engine. I was stationed out in the middle of the New Mexico desert so wasn't able to rush to a Chevy dealer for service. Be glad you never had that problem.

                            Stu Fox

                            Comment

                            • David F.
                              Frequent User
                              • January 10, 2011
                              • 48

                              #15
                              Re: Throttle Problem 63 AFB Carburetor

                              Dear Stu & Frank, you are correct the throttle lever hitting the air cleaner is not the problem. I do have gasket but it still can happen if not carefull installing the air cleaner.
                              It is prime time for leaf disposal here in North Jersey so I can't work immediately on solutions. Now to my actual problem. I bought the 63 vette in March 2013 partially restored. I was told it had been sitting for about 10 years after loosing interest in finishing the restoration. I replace the tires, shocks, brakes (drum) and brake cables, steering column coupler, alignment shims, had radio repaired & installed new antenna cable and antenna and some other interior items. Under the hood were missing items: I installed radiator shroud seal, PVC valve-tube-adapter-elbow-grommet. Still missing is the lower clean air tube & hose (paragon #4000K-S), which I plugged until I can learn more about installing.
                              I have driven the vette for the past 3 summers (less than 50 mile trips) without a problem. Then in late August this year the carburetor problem started. The engine will idle at about 2500 to 3000rpm and not return to normal idle rpm without kicking the throttle hard. Even that sometimes does not work. Lightly touching accelerator or start to drive the engine goes back to the high 2500-3000rpms.
                              I have use carburetor cleaner and checked throttle valve plates and the choke opens fine when engine warms up. So now I am lost. Do I need to take it to a carburetor shop or is there something I can do?
                              Your advice is very much appreciated. David (52688)

                              Comment

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