High rpms at idle

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  • Sola gratia
    Frequent Contributor
    • Feb 2024
    • 5

    High rpms at idle

    Hello everyone, I'm new to this forum and Atomic 4s in general. My Atomic 4 will only start with the choke on, and it takes a bit of carb cleaner to start too. When it gets going, it idles at 2000+, and without touching the throttle, it will surge to 3000+.


    -took apart the carb and cleaned everything jets and all
    -new spark plugs (I get spark)
    -new exhaust manifold (engine was overheating fixed that)
    -The fuel pump works fine
    -Watched and did everything in the "carburetor troubleshooting" video on YouTube

    kinda of at a loss any tips or tricks would be great, thank you
  • hanleyclifford
    Afourian MVP
    • Mar 2010
    • 7020

    #2
    It sounds like your idle system is not functioning at all. You are "idling" on your main jet. The fact that you need to choke the carb to get started suggests a lean condition. Your idle adjustment screw should be set initially at around 1 1/4 turns out from seat. Remember, turning the screw in towards seat is enriching your mixture which may seem counter intuitive.

    Comment

    • hanleyclifford
      Afourian MVP
      • Mar 2010
      • 7020

      #3
      You may also have timing advanced too far. There is also the possibility of a vacuum leak.

      Comment

      • Dave Neptune
        Afourian MVP, Professor Emeritus
        • Jan 2007
        • 5099

        #4
        Sounds also like you may have a vacuum leak. Check that the base gasket and spacer gasket if so equipped are both in properly and sealed. Be sure that the scavenge tube assembly, the small copper tube that connects to the manifold is also sealed and tightened properly.

        It is normal for the A-4 to need the choke when cold or even after shutting down for a while after running. This is normal for an updraft carburetor.

        Be sure the throttle linkage is working properly too. The A-4 likes to idle nicely at 700 or even less RPM's when all is well. The idle screw on top of the carb Hanley mentioned actually adjusts the air for the idle and not the fuel and in most all cases set at 1 1/2 turns off the seat works well.

        If you have a PCV valve plug it off until you have the idle sorted out.

        Dave Neptune

        Comment

        • Sola gratia
          Frequent Contributor
          • Feb 2024
          • 5

          #5
          Whoa, you all are so fast!

          I've adjusted the idle, but I'll do it again to ensure it's around 1 1/4 turns out from the seat. I'll also check for vacuum leaks (do you have any tips or tricks for checking them?) and report back!

          Comment

          • ndutton
            Afourian MVP
            • May 2009
            • 9777

            #6
            Originally posted by Sola gratia View Post
            Hello everyone, I'm new to this forum and Atomic 4s in general. My Atomic 4 will only start with the choke on, and it takes a bit of carb cleaner to start too. When it gets going, it idles at 2000+, and without touching the throttle, it will surge to 3000+.


            -took apart the carb and cleaned everything jets and all
            -new spark plugs (I get spark)
            -new exhaust manifold (engine was overheating fixed that)
            -The fuel pump works fine
            -Watched and did everything in the "carburetor troubleshooting" video on YouTube

            kinda of at a loss any tips or tricks would be great, thank you
            Since the intake and exhaust manifold are the same casting, maybe the smoking gun here.
            Neil
            1977 Catalina 30
            San Pedro, California
            prior boats 1987 Westsail 32, 1970 Catalina 22
            Had my hands in a few others

            Comment

            • hanleyclifford
              Afourian MVP
              • Mar 2010
              • 7020

              #7
              Originally posted by ndutton View Post
              Since the intake and exhaust manifold are the same casting, maybe the smoking gun here.
              I think Neil is right. First thing is to make sure no leaks in the manifold. Check torque on 3 studs. Make sure all plugs got back in. Make sure scavenge tube is secure at both ends. Run a vacuum check if you can. Next make sure carb gasket(s) are sound and properly torqued. Make sure throttle turns and closes in throttle body. If all of that is secure you may have to revisit the carb. When you cleaned it did you have access to compressed air to clean those pesky passages? Cleaning the jets sometimes is not enough.

              Comment

              • Dave Neptune
                Afourian MVP, Professor Emeritus
                • Jan 2007
                • 5099

                #8
                Oops, just realized this was your first post so welcome to the MMI Forum.

                Checking for vacuum leaks can be problematic until the engine is actually running. What you do is spray around the engine intake manifold and carb connections with a flammable and WD-40 works and is pretty safe as compared with starting fluid. When you spray at an area and the RPM's increase the leak is there. But until the engine is running the "CVI" (close visual inspection) method is about all you can do. Excess air is getting in somewhere causing the high RPM's if the throttle is closed.

                Dave Neptune

                Comment

                • Dave Neptune
                  Afourian MVP, Professor Emeritus
                  • Jan 2007
                  • 5099

                  #9
                  One more thought it could be an "intermittent fuel supply". If the RPM's increase slowly like the engine is surging, fuel supply is a distinct possibility. This could be caused by a sticky float. If it is a rapid increase the fuel is doubtful.

                  Dave Neptune

                  Comment

                  • Sam
                    Afourian MVP
                    • Apr 2010
                    • 326

                    #10
                    Along the same thoughts, the last time I had "engine surging & choke to start warm engine after dying" was due to some water at bottom of tank and a few waves to mix it up a bit. Pumped out the tank and fresh fuel solved problem.

                    Comment

                    • Sola gratia
                      Frequent Contributor
                      • Feb 2024
                      • 5

                      #11
                      Something is definitely wrong with the timing. My static timing light DC circuit tester is not even lighting up. I followed all the steps on the YouTube video for ignition timing. Not sure what to do next

                      Comment

                      • Dave Neptune
                        Afourian MVP, Professor Emeritus
                        • Jan 2007
                        • 5099

                        #12
                        First, what is the "static DC circuit timing light tester"? And what are you doing with it?

                        Do you still have spark?

                        Is there a chance that you did not get the plug wires back on extatically the same cylinder's plug? Did you move the distributor? If the answer is no then the timing should not of changed and be the same as before.

                        Does the engine still start and run to fast?

                        How did the engine run other than hot before you started working on it?

                        Dave Neptune

                        Comment

                        • Sola gratia
                          Frequent Contributor
                          • Feb 2024
                          • 5

                          #13
                          Originally posted by Dave Neptune View Post
                          First, what is the "static DC circuit timing light tester"? And what are you doing with it?


                          Dave Neptune
                          This
                          Static Timing light (DC circuit tester)

                          SKU: TOOL_584

                          Comment

                          • Dave Neptune
                            Afourian MVP, Professor Emeritus
                            • Jan 2007
                            • 5099

                            #14
                            OK, now check the checker by checking it on the battery. If the light comes on it still works. It is nothing more than a light bulb hooked to a probe.

                            You may of lost voltage to the ignition. It is better and almost the same process to use a volt meter of which you can check for far more things like voltage drops and resistance.

                            Turn the ign on and see if you have voltage at the + positive terminal of the coil.

                            Answers to the other questions will assist in diagnosing for us.

                            Dave Neptune

                            Comment

                            • Dave Neptune
                              Afourian MVP, Professor Emeritus
                              • Jan 2007
                              • 5099

                              #15
                              Also unless you are using "points" that tester will not indicate the switching of the circuit in the EI.

                              Dave Neptune

                              Comment

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