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testing for spark plug misfire

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  • William F.
    Extremely Frequent Poster
    • June 9, 2009
    • 1363

    testing for spark plug misfire

    How to test for a sparkplug misfire without removing plugs in my 340 hp 62 without burning hands by removing plug wire at plug? Will removing one wire at a time from distributor cap cause current tracing on cap? Another way?
    Thanks
  • Duke W.
    Beyond Control Poster
    • January 1, 1993
    • 15672

    #2
    How do you know it's a "spark plug"? There are a number of issues that can cause misfires. Some fault in the ignition system is the most common cause, but it can also be a fuel system problem or something more serious like a burned valve.

    If you can provide a well thought out description of the symptoms you will receive better advice.

    Under what operating conditions does it occur? Continuous? idle? Steady speed? Acceleration? Speed and RPM range?

    Questions like this come up on the Corvette Forum all the time, and withing 24 hours there are 24 responses, almost all different, and NOBODY bothers to ask for any details, so its pretty much useless to the OP or anyone else.

    Let's not turn the TDB into another useless CF.

    BTW, what you described, pulling one plug wire ome at a time is called a "cylinder balance test". If an engine feels like it's only running on, say 7 ro 8 cylinders all the time, which may be a burned valve, this test is called for. On a healthy engine the idle speed drop for each disconnected plug wire will be the same. If there is a burned valve or dead plug there will be no drop in idle speed.

    Duke

    Comment

    • Owen L.
      Very Frequent User
      • September 30, 1991
      • 868

      #3
      Perhaps this is a CF sort of answer as it is just replying to the question asked...

      Using a timing light, attach the pick-up to each wire and watch for mis-flashes.

      Comment

      • William F.
        Extremely Frequent Poster
        • June 9, 2009
        • 1363

        #4
        Won't know if plug misfire but want to test. Was running fine when I filled tank with nonethanol 93 grade in Oct 24 and put it away in temp and humidity controlled garage for winter. Drove for first time in 25 yesterday. Idles fine but has a slight to moderate "buck" at low speeds. Don't notice, or not as much at higher speeds, but as you know hard to detect if a slight miss in a high power car. Has the one wire M and H electronic trigger in distributor. All plug wires are connected to cap and plugs . No detected vacuum leaks. Advance weights not binding (no vacuum on '62 340 hp). Advance weight springs just seem subectively weak but haven't been messed with since I bought car 10 years ago or who knows if ever changed.- do they "age" . Few years ago got a miss but at high speed and turned out to be water in gas. Haven't checked intial timing but hasnt been changed from when running fine. So; would think next thing would be to check if plugs are firing.

        Comment

        • Keith W.
          Very Frequent User
          • May 25, 2018
          • 200

          #5
          Originally posted by Owen Lowe (20119)
          Perhaps this is a CF sort of answer as it is just replying to the question asked...

          Using a timing light, attach the pick-up to each wire and watch for mis-flashes.
          Owen, could you elaborate on using a timing light, attach the pick up to each wire and watch for misflashes? Interesting thought and would like to understand this more and what to look for when conducting this test. Looking to learn more about this. Thanks.

          Comment

          • Mark E.
            Extremely Frequent Poster
            • April 1, 1993
            • 4542

            #6
            Agree with Duke's comments.

            Given it ran fine when stored and your symptom, I'd start with fuel then plugs.

            Drive through at least half a tank of fuel then top off and see if drivability improves. If not, check for fouled plugs.
            Mark Edmondson
            Dallas, Texas
            Texas Chapter

            1970 Coupe, Donnybrooke Green, Light Saddle LS5 M20 A31 C60 G81 N37 N40 UA6 U79
            1993 Coupe, 40th Anniversary, 6-speed, PEG 1, FX3, CD, Bronze Top

            Comment

            • Owen L.
              Very Frequent User
              • September 30, 1991
              • 868

              #7
              Originally posted by Keith Wiethe (64808)

              Owen, could you elaborate on using a timing light, attach the pick up to each wire and watch for misflashes? Interesting thought and would like to understand this more and what to look for when conducting this test. Looking to learn more about this. Thanks.
              Yep, that's basically it. Hook up the light as you would if you were checking the engine timing. Instead, watch the flashing of #1 for regularity. Move the pick up to #3, repeat, then #5, #7, #2, #4, and so on. If a plug misfires, the light won't flash.

              Comment

              • Keith W.
                Very Frequent User
                • May 25, 2018
                • 200

                #8
                Originally posted by Owen Lowe (20119)

                Yep, that's basically it. Hook up the light as you would if you were checking the engine timing. Instead, watch the flashing of #1 for regularity. Move the pick up to #3, repeat, then #5, #7, #2, #4, and so on. If a plug misfires, the light won't flash.
                Thank you Sir. Much appreciated! Will tuck this away for future use(hopefully not needed)👍

                Comment

                • William F.
                  Extremely Frequent Poster
                  • June 9, 2009
                  • 1363

                  #9
                  I pulled one of the AC 45S plugs and it was badly carbon fouled. If rest are same will replace with Denso W14-U's. After I then set idle making sure it's not rich are are there other things I should check on the AFB carb to see if its running rich when at cruise and full throttle?
                  Also, been a while since I changed a small block's plugs. Any tips other than doing from below to minimize skinned knuckles?
                  Thanks for all suggestions and tips.
                  William Ford 50517
                  '62 240 hp
                  '67 L79 C60

                  Comment

                  • Mark E.
                    Extremely Frequent Poster
                    • April 1, 1993
                    • 4542

                    #10
                    I'm glad you found the likely cause and it's a minor issue. Plug fouling is common on carburetored engines that are driven infrequently and/or much of the run time is during warm up and the choke is engaged. So there's a good chance your carburetor is working as designed but make sure the choke fully opens during warm up.

                    Running for at least 30 minutes after the choke and thermostat opens will minimize fouling.

                    Changing plugs on a small block with shielding is no fun. Even worse if AC equipped. Just take it slow and watch out for the sharp edges on the shielding.

                    ...yet another reason why big blocks are easier to service. 🙂
                    Mark Edmondson
                    Dallas, Texas
                    Texas Chapter

                    1970 Coupe, Donnybrooke Green, Light Saddle LS5 M20 A31 C60 G81 N37 N40 UA6 U79
                    1993 Coupe, 40th Anniversary, 6-speed, PEG 1, FX3, CD, Bronze Top

                    Comment

                    • Owen L.
                      Very Frequent User
                      • September 30, 1991
                      • 868

                      #11
                      Originally posted by William Ford (50517)
                      I pulled one of the AC 45S plugs and it was badly carbon fouled. If rest are same will replace with Denso W14-U's. After I then set idle making sure it's not rich are are there other things I should check on the AFB carb to see if its running rich when at cruise and full throttle?
                      I've not used one, but there are air-fuel ratio (AFR) meters that are inserted into the exhaust pipe. Maybe that will provide hard numbers to start.

                      I just run the car for a tankful and pull the plugs to examine the porcelain insulators. Be aware that types of driving affect plug appearance: lots of idling and sub-50mph will show darker insulators than sustained highway driving.

                      Comment

                      • Thomas H.
                        Extremely Frequent Poster
                        • August 31, 2005
                        • 1058

                        #12
                        Originally posted by William Ford (50517)
                        Also, been a while since I changed a small block's plugs. Any tips other than doing from below to minimize skinned knuckles?
                        Thanks for all suggestions and tips.
                        William Ford 50517
                        '62 240 hp
                        '67 L79 C60
                        Wearing mechanics gloves helps. I put off using them for years and now I'm a convert - especially when working around sharp edges like shielding.

                        Tom

                        1958, 283/245, White/red - Top Flight, October 2016
                        1960, Black/black, 283/230 4sp
                        1966, Black/Red, 327/350 4sp w/AC
                        1967, 427/390, 4sp, Goodwood Green, Coupe
                        1971 LS5, 4sp, coupe, Bridgehampton Blue
                        2007 Z06, Lemans Blue

                        Newsletter Editor, Delaware Valley Chapter

                        Comment

                        • David H.
                          Extremely Frequent Poster
                          • June 30, 2001
                          • 1528

                          #13
                          William

                          You might try tack welding an alligator clip to a spark plug. You can attach to any engine ground and test spark. I remove part of spark ground - makes center electrode to ground easier to see.

                          Dave
                          Attached Files
                          Judging Chairman Mid-Way USA (Kansas) Chapter

                          Comment

                          • William F.
                            Extremely Frequent Poster
                            • June 9, 2009
                            • 1363

                            #14
                            Using every combination of rachet, rachet extensions and bruised hands, I pulled old plugs and carefully replaced with new Denso 14's. All the old plugs were severely carbon fouled. I'll next make sure idle isn't rich. What about cruise mixture on the AFB should I check? Pull the jet mixture needles and make sure they aren't sticking up out of jets?
                            Thanks

                            Comment

                            • Timothy B.
                              Extremely Frequent Poster
                              • April 30, 1983
                              • 5186

                              #15
                              William, What you describe sound like excessive idle time on the engine. The air/fuel idle mixture is set by the idle air bleeds and the idle jet which is fixed unless you change the primary venturi clusters. You can lean the emulsion screws some and see if that helps and get it out on the road a little more.

                              Comment

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