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    Slight Misfire

    Hello Gents.
    I have had my 76 Stag for just over a Year now and have always suspected a slight misfire. I only have limited mechanical knowledge, but have tried to get to the bottom of it. Ive replaced the plugs/leads/cap/rotor arm and checked the timing (now 12 BTDC) and all seem aok. Today I did a compression test and have the readings as per the attached photo (engine warm, all plugs out - is this the correct method?).
    My question is I think these seem a little low, but are roughly within 10% of each other so I presume ok?
    The plugs all look the same (see next photo) - is this what they are meant to look like??
    The electronic ignition is Newtronic which I understand is out of the ark, could this be the problem and should I consider updating it?
    Sorry for all the questions but im not sure on most of this!

    Any advice greatly appreciated.

    Thanks.

    #2
    Your compressions are reasonable, mine are about 120-130 (40,000 mile engine) but do vary a bit. Your plugs look good too, I wish I could get mine to run that lean!

    I had a problem with a slight misfire earlier this year which turned out to be the bomb-proof Lumenition letting go. Since I put the PowerSpark in the engine pulls like a train. I have a spare PowerSpark in the boot just in case, and as soon as I've found which bit of the Lumenition is faulty I'll put it back - after all, it worked for 35 years, so must be pretty good! I fixed a Newtronic for somone last year, they seem alright but all these ignition systems are getting quite old now.

    Hope this helps,

    Richard
    Mabel is a white 1972 Mk1˝, TV8, ZF 4HP-22 auto,
    2016 RBRR finisher. 400 mile C2C 21/22 April 2018!

    Comment


      #3
      I checked the compressions on mine last week and all 8 were 150psi. Heads were skimmed and fitted with extra thick BGA gaskets (standard ones weren’t available) in Oct 2014, 12000 miles ago.
      Last edited by DJT; 15th April 2018, 08:18.
      Dave
      1974 Mk2, ZF Auto, 3.45 Diff, Datsun Driveshafts. Stag owner/maintainer since 1989.

      Comment


        #4
        Always difficult to compare readings between different compression tests, the key thing is that all cylinders are within about 10% of each other, which yours are Neil..

        As for the slight misfire, if you haven't changed HT leads/rotor arm/distributor cap in your ownership, they would be good places to start.
        Header tanks - you can't beat a bit of bling.

        Comment


          #5
          Originally posted by wilf View Post
          Always difficult to compare readings between different compression tests, the key thing is that all cylinders are within about 10% of each other, which yours are Neil..

          As for the slight misfire, if you haven't changed HT leads/rotor arm/distributor cap in your ownership, they would be good places to start.
          The PO stated that he’s changed leads/cap/rotorarm in post 1
          Mabel is a white 1972 Mk1˝, TV8, ZF 4HP-22 auto,
          2016 RBRR finisher. 400 mile C2C 21/22 April 2018!

          Comment


            #6
            I have found that the older ignition coils don't like electronic ignition. I had a misfire under load until i fitted a high quality coil (Flamethrower from Demon Tweaks). My misfire only started when warmed up, so the test was to carry a spare coil in the car, and swap when everything was hot and misfiring.
            Another thing often reported on here as being suspect for MkII Stags is the resistive wire in the loom which drops the coil voltage to 6V. Mk I Stags had a separate resistor on a ceramic block, and some Mk II owners have bypassed the resistive wire in the loom, and fitted a Mk I resistor.
            '72 Manual O/d Saffron Yellow

            Comment


              #7
              Having dragged my stag out of hibernation I found it had a big misfire at around 2500rpm.

              https://youtu.be/IfCmgLfnxmc

              misfire is more likely to be ignition related. in my case I think more likely was a dry connection to the Low tension feed to the coil

              Mine has a 12v coil and no ballast resistor btw and yes and in the vid^ I mistook the tacho feed as a starter boost feed etc etc

              As you can see I fixed that, BUT now I have a misfire at 1500rpm on light throttle but not heavy throttle.!! It is a big misfire so will likely be rotor or king lead into the distributor cap rather than individual ht leads.

              To fix I bought new 8mm silicon leads (LDpart) - there used to be a quality issue with non silicon green HT leads which is why I have the fatter 10mm silicon leads on the car at the moment. these are getting on a bit now

              there are also very current (has been for a decade now!!) quality issue with distributor cap and rotor. has your rotor got a river holding the metal blade down? is the metal blade around 1mm thick? send the bugger back!

              LDPart sell better quality cap and rotor, I bought from mine distributordoctor. same cap is used on Rover v8 hence my first hand knowledge of very poor quality caps and rotors. aghhh gah

              hope the above helps
              Last edited by richardthestag; 15th April 2018, 14:31. Reason: to confirm that I have NO ballast resistor with my 12v coil
              Stags and Range Rover Classics - I must be a loony

              Comment


                #8
                Thanks Richard.

                Photo below, looks like ive got the rubbish version from Rimmers then? Interesting about the Rover V8 similarities, Ive got a P5B also, so just out of interest I think I will have a look at the cap etc this afternoon.
                Going forward though, I think I will invest in the Pertronix kit and big coil, (lots of debates on here about different manufacturers, but there doesnt seem to be any coming out in front of any others) but I presume that I will need to change the base plate, or does it come with it?
                Thanks

                Neil.
                Attached Files

                Comment


                  #9
                  Originally posted by richardthestag View Post
                  ...Mine has a 12v coil and now ballast resistor btw...
                  I suspect that this was a mistype but just in case - if you have a ballast resistor you need a 6V coil and take the 12V feed for the electronic ignition from the fuel cut-off switch instead of the coil.
                  Simon

                  Comment


                    #10
                    Originally posted by Neil183 View Post
                    Hello Gents.
                    I have had my 76 Stag for just over a Year now and have always suspected a slight misfire. I only have limited mechanical knowledge, but have tried to get to the bottom of it. Ive replaced the plugs/leads/cap/rotor arm and checked the timing (now 12 BTDC) and all seem aok
                    When I drove the Stag at Christmas time, I was somewhat frustrated with it at first because it too had a misfire and only had that when it had come back from a very expensive service (while still under the ownership of my parents). I lifted the bonnet and noticed that one of the HT leads was arcing to the head, it was in very close proximity. All I did at the time was turn the spark plug cap (as they're kinked over) around so that the proximity to the head was different.

                    I may change the HT leads at some point but for now it has been fine since.

                    Comment


                      #11
                      Originally posted by mallardstag View Post

                      I suspect that this was a mistype but just in case - if you have a ballast resistor you need a 6V coil and take the 12V feed for the electronic ignition from the fuel cut-off switch instead of the coil.
                      Simon
                      typo. I do have a ballast resistor but it is in the shed
                      Stags and Range Rover Classics - I must be a loony

                      Comment


                        #12
                        Originally posted by Neil183 View Post
                        Thanks Richard.

                        Photo below, looks like ive got the rubbish version from Rimmers then? Interesting about the Rover V8 similarities, Ive got a P5B also, so just out of interest I think I will have a look at the cap etc this afternoon.
                        Going forward though, I think I will invest in the Pertronix kit and big coil, (lots of debates on here about different manufacturers, but there doesnt seem to be any coming out in front of any others) but I presume that I will need to change the base plate, or does it come with it?
                        Thanks

                        Neil.
                        when the rivet is that far out it tends to cause less problems. The problem ones had the rivet much closer to the centre of the rotor. the problem I think is that the rivet is too close to the distributor shaft and it arcs through that rather than the post. correct me if you know better.

                        If you look at the thickness of the metal strip is it 1mm thick? or a lot chunkier? chunkie is good

                        Also modern blue caps are not the best quality. that is why they are cheap.

                        I have a couple of the last of the mythical quality blue caps, bought over the parts counter at a land rover dealership over a decade ago though whether they were made by lucas or not is debatable, needless to say distributor doctor is cheaper and almost certainly better quality. One that I do not have any more was blue and had made in england cast into the top of the cap around the centre post, like lucas always did.

                        Alas everything I have seen in a green lucas box now.... aint

                        A landrover quality dealer tried to sell me a lucas black cap for a lot of money. fitting it to the rover v8 found that when I clipped it down I could rotate it 3-4degrees the fit was appalling, it also was NOT a snug fit on the top of the distributor meaning that it would rock back and forth I only noticed because when I started it up after fitting, I noticed that the cap was wobbling as it was running, I thought that I had left a clip off but hadnt. I went back and moaned like hell at them, they told me it didnt matter ! I dont go there any more but did get my money back
                        Stags and Range Rover Classics - I must be a loony

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