'66 350hp idle speed - NCRS Discussion Boards

'66 350hp idle speed

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  • Rob A.
    Expired
    • December 1, 1991
    • 2126

    '66 350hp idle speed

    I've posted this for discussion before. When warm(and only when warm) the idle speed will sometimes increase by 200 rpm and remain there until the engine cools down. After it cools, and the engine is restarted, the idle rpm will return to where I originally set it. The distributor, weights, vacuum advance, etc. have been eliminated as possible causes, the choke is not engaged and the throttle lever is against the adjustment screw. It is DEFINATELY in the carb, an original #3367 with stock components. I asked Holley tech what they thought and he suggested I open up the secondaries about a quarter of a turn to try to balance out the increased vacuum that seems to occur when the engine is hot. He suggested that the increased vacuum was pulling fuel from the primaty side when hot, for some reason. Does anyone have any suggestions that wouldn't require removing the carb? It's more of a nuissance than a problem, but I wouldn't mind trying to eliminate it. Again, the idle speed only increases after operating the engine at normal temp, with the outside temp on the warm side. Could a vacuum leak somewhere in the carb at warm temp cause this?
  • Rob A.
    Expired
    • December 1, 1991
    • 2126

    #2
    Addendum...

    I've noticed recently, that in hot weather, I can hear a high pitch whistling noise at the carb at idle. I assume that is evidence of a vacuum leak, which might be causing the problem.

    Comment

    • Duke W.
      Beyond Control Poster
      • January 1, 1993
      • 15610

      #3
      Re: '66 350hp idle speed

      I don't know about Holleys, but AFBs have a "hot idle compensator" - a little valve/bimetallic strip between the secondary venturi clusters that can open when things get hot.

      Its purpose is to admit additional air when things get hot enough that fuel percolation might lead to overrich an mixture. Of course, if it opens it will increase idle speed.

      I don't know if Holleys have a similar device.

      As carbs get old, throttle shaft wear can cause varying alignment when they are closed, which will alter available flow area and sometimes lead to higher idle speed when the throttle is closed. This behavior can also be affected by tempearture.

      Something might also be causing a slight vacuum leak when thing get hot.

      There is probably a correlation between the "whistling" and increased idle speed, so If you can find the whistle you have probably found the culprit.

      I can't make any sense out of the "advice" you got from Holley.

      Dukr

      Comment

      • Rob A.
        Expired
        • December 1, 1991
        • 2126

        #4
        Re: '66 350hp idle speed

        Duke,

        There is a hot air vent on the primary bowl. It's operating correctly, open at idle. I lowered the primary float setting to see if that may help.

        Comment

        • Wayne K.
          Expired
          • December 1, 1999
          • 1030

          #5
          Re: '66 350hp idle speed

          Rob,

          If you haven't already tried it when motor is hot and idling with the whistle present manually open the secondary throttle just slightly to see if the whistle changes or put some pressure trying to close them and see if there is any change. It doesn't take much movement to make a difference in the air passage. I had a similar issue and after making a total assessment of my carb I decided to replace it with a new repro which to date has been flawless.

          Wayne

          Comment

          • Gary S.
            Extremely Frequent Poster
            • July 31, 1992
            • 1628

            #6
            Re: Addendum...

            My original 66 Holley had a slightly warped base. Discussions here indicated that this was not uncommon and the fix was some machine work. A thorough rebuild, including machining the base flat cleared up the problem. I would suggest that the warped base manifests itself only when the engine is hot and then you get the warpage and vacuum leak. Of course, you should also put a vacuum gauge on the engine to see what happens to the vacuum when the idle increases rpm and the whistling starts.

            Other areas that these Holleys tend to leak at are the holes that the shafts fit in. Over time they enlarge and require bushing them.

            Gary

            Comment

            • Rob A.
              Expired
              • December 1, 1991
              • 2126

              #7
              Re: Addendum...

              Gary,

              Would checking for warpage and any subsequent machining be something most carb rebuild shops are equipped to handle, or should I specifically ask whoever I decide to have rebuilt the carb if they are capable of detecting warpage and correcting it?

              Comment

              • Joe R.
                Extremely Frequent Poster
                • March 1, 2002
                • 1356

                #8
                Re: Addendum...

                Rob:

                Only the "best" carb rebuilders will pay close attention to things like warpage in the base plate and/or wear at the points where the shafts pass through the plate. These two problems are well known issues with the Holley carb, so you should make sure that any rebuilder you use will address them.

                If you check the archives you will find frequent mention of Jerry Luck and Bob Kuntz as rebuilders who do the job right, although I believe that Jerry may be having some issues with turnaround time these days.

                I think it is quite likely that the problem you have described is due to one or both of the above mentioned problems with the base plate assembly. You could try having the carb rebuilt by an expert, or you could try buying a new replacement base assembly and installing that yourself. There are also entire reproduction carbs available now from Holley, but be sure to keep you original dated carb if you decide to go that way on a temporary basis.

                Comment

                • Rob A.
                  Expired
                  • December 1, 1991
                  • 2126

                  #9
                  Re: Addendum...

                  Gary,

                  When you refer to the warped base, do you mean the surface that mates against the main carb body, the intake or both?

                  Comment

                  • Gary S.
                    Extremely Frequent Poster
                    • July 31, 1992
                    • 1628

                    #10
                    Re: Addendum...

                    I don't know the answer to that question but Joe seems to have covered it. Oth, Jerry Luck built my carb and it acted, looked, and performed like new. He did machine the base and it ran flawlessly.

                    I will tell you that if decide to go with someone like Jerry Luck, he is VERY difficult to get in contact with. He may have health issues if I remember that from the archives.

                    Gary

                    Comment

                    • Rob A.
                      Expired
                      • December 1, 1991
                      • 2126

                      #11
                      Re: Addendum...

                      Gary,

                      Jerry did my '67 tripower carbs.....8 months to get them back....need I say more. I'm not looking for cosmetic restoration this time. Chuck Smith, here in California said he would rebuild the carb for me. I'll have to check with him before I send it, to make sure he can re-surface the base as necessary.

                      Comment

                      • Joe R.
                        Extremely Frequent Poster
                        • March 1, 2002
                        • 1356

                        #12
                        Re: Addendum...

                        Rob:

                        Aside from a warped base, the other potential problem that must be evaluated and addressed is the amount of wear on the holes where the primary throttle shaft passes through the base plate (the primary shaft is the only one that tends to get worn). Wear in this area can cause air leakage around the shaft and/or misalignment of the throttle valves.

                        It takes considerable expertise to disassemble, machine, reassemble and align the throttle valve assembly, so be sure that whoever you choose has experience doing this procedure. Misalignment and poor seating of the primary throttle valves is one possible cause of the problem you have described, so you want to be sure that your rebuilder gets this right.

                        I believe that both Jerry Luck and Bob Kuntz are experienced with doing the necessary operations. If you are concerned about turnaround time with Jerry, you might want to talk with Bob Kuntz at 314-845-2566.

                        The other option, of course, is to buy a brand new throttle plate assembly from Holley.

                        Comment

                        • Rob A.
                          Expired
                          • December 1, 1991
                          • 2126

                          #13
                          Thanks for the additional info Joe.... *NM*

                          Comment

                          • Wayne K.
                            Expired
                            • December 1, 1999
                            • 1030

                            #14
                            Re: '66 350hp idle speed

                            Rob,

                            I might add that before installing the new Holley I used the carb to manifold gasket as a pattern to make a spacer out of 3/8 or 1/4 inch phenolic. I RTV'd the spacer to the intake and then put the gasket on top and then bolted on the carb. I figure that with all the problems with heat transfer that this would be good insurance. Everthing else hooked up just fine.

                            Wayne

                            Comment

                            • Wayne K.
                              Expired
                              • December 1, 1999
                              • 1030

                              #15
                              Correction, it was 3/16" phenolic. *NM*

                              Comment

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