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    Exhaust popping on overun

    If I was a few decades younger I'd love the noise but don't think it suits the Stag

    TV8, tubular manifolds, recently rebuilt Stroms and ZF4 spd. De-acellerating or If I manually select a lower gear going downhil get exhaust popping on overun, does anyone else get same - what causes it? Had it quite a while and was hoping Stroms would sort it but no - puzzled.....

    #2
    Worn valve guides is my first thought, although I am more familiar with older engines other than Stags. Regards, John.

    Comment


      #3
      Originally posted by SaphireSpud View Post
      Worn valve guides is my first thought, although I am more familiar with older engines other than Stags. Regards, John.
      Thats a good idea tho engine has probably only done 25k since rebuild, but easy to check by wiggling stems I guess?

      Comment


        #4
        I had a leak between downpipe and exhaust manifold on near side, after I fitted a new stainless exhaust, allowing air to be sucked in on overrun causing the popping you described. I had to grind a little off the upstand of the downpipe to allow the flanges to bolt up tight and stop the leak. I noticed a soot deposit in the area which drew my attention to the reason for the popping.

        Comment


          #5
          It can also be a lean mixture. The lean mixture causes mis-fires on over-run. So unburnt fuel gets into the exhaust and is then ignited by hot exhaust gases. Having said that, the Stag is good on lean mixtures so just a possibility.

          Comment


            #6
            I get that, and like you not overly fond of it.

            Its due to the engine being unable to fire the mixture in the combustion chamber, not enough air with the closed throttle and unburned fuel makes it to the hot manifold, add any extra air there it will explode. so yes air in the exhaust manifold from leaks is a usual suspect, Tubular manifolds don't help either..

            I'm not sure if you have by-pass valves on the side or pop valves in the throttle plates on the stromberg but if they are not working or adjusted correctly they are there to help that as well by introducing a bit of air on overrun. In my case with a manual I dislike the slow return to idle the bypass valves cause, but with an auto I suspect it would not be a problem. Mine are adjusted fully closed currently but I plan to start slowly tweaking them to try to reduce my popping without causing too much of a delay.

            May be time to check my flanges as macstag suggested, nothing seen last time but maybe there will be a telltale by now.
            Terry Hunt, Wilmington Delaware

            www.terryhunt.co.uk

            Comment


              #7
              Originally posted by MikeParker View Post
              It can also be a lean mixture. The lean mixture causes mis-fires on over-run. So unburnt fuel gets into the exhaust and is then ignited by hot exhaust gases. Having said that, the Stag is good on lean mixtures so just a possibility.
              yes the most common cause is a weak mixture. When tuning cars for the youngsters, to get the poops and bangs, you just lean off the the fuel on a closed throttle until 1500 rpm. Either that or remove the silencers.

              Comment


                #8
                Originally posted by MikeParker View Post
                It can also be a lean mixture. The lean mixture causes mis-fires on over-run. So unburnt fuel gets into the exhaust and is then ignited by hot exhaust gases. Having said that, the Stag is good on lean mixtures so just a possibility.
                I'd heard of that but last time it went for MOT, it was too rich and I had trouble getting it thru (reason I overhauled carbs)

                Comment


                  #9
                  Originally posted by trunt View Post
                  I get that, and like you not overly fond of it.

                  Its due to the engine being unable to fire the mixture in the combustion chamber, not enough air with the closed throttle and unburned fuel makes it to the hot manifold, add any extra air there it will explode. so yes air in the exhaust manifold from leaks is a usual suspect, Tubular manifolds don't help either..

                  I'm not sure if you have by-pass valves on the side or pop valves in the throttle plates on the stromberg but if they are not working or adjusted correctly they are there to help that as well by introducing a bit of air on overrun. In my case with a manual I dislike the slow return to idle the bypass valves cause, but with an auto I suspect it would not be a problem. Mine are adjusted fully closed currently but I plan to start slowly tweaking them to try to reduce my popping without causing too much of a delay.

                  May be time to check my flanges as macstag suggested, nothing seen last time but maybe there will be a telltale by now.
                  Thats a good point, I adjusted the bypass' according to details on this forum and they should open around 110F which I always thought a bit low - is it worth tightening to shut? [It had one carb poppet valve! but complete overhaul has none now ]. Exhaust leaks are possibility but if there are, further back after the silencers, the SS exhaust is about 2" too short and always having to juggle to seal and fit brackets!

                  Comment


                    #10
                    I think you are talking about the temperature compensation valve, (long, covered with a plastic cover) I would leave them alone. The bypass valves, if you have them, are just below them, there is a screw adjustment in the end. Info here in the tech section, page 49 of the pdf.

                    https://www.socforum.com/forum/forum...s-training-pdf

                    On some later carbs they had a pop valve on the throttle plate instead, it’s often removed. I suspect that removing that itself would not cause popping but it would contribute.
                    Last edited by trunt; 6 June 2021, 21:17.
                    Terry Hunt, Wilmington Delaware

                    www.terryhunt.co.uk

                    Comment


                      #11
                      Originally posted by trunt View Post
                      I think you are talking about the temperature compensation valve, (long, covered with a plastic cover) I would leave them alone. The bypass valves, if you have them, are just below them, there is a screw adjustment in the end. Info here in the tech section, page 49 of the pdf.

                      https://www.socforum.com/forum/forum...s-training-pdf

                      On some later carbs they had a pop valve on the throttle plate instead, it’s often removed. I suspect that removing that itself would not cause popping but it would contribute.
                      Yep thats the one, the plastic covered temperature valve , I've set as per forum views. The idle screw just screwed in.

                      Comment


                        #12
                        Originally posted by Sheepdip View Post

                        I'd heard of that but last time it went for MOT, it was too rich and I had trouble getting it thru (reason I overhauled carbs)
                        its not the mixture when the car is being tested, ie when the engine is being reved. It the mixture when the throttle is closed, the same as when it ticking over. If you alter the mixture for tickover to slightly richer then the poops and bangs will possibly go. If you fit an wide band O2 sensor to the exhaust and a gauge you’ll see what happens.

                        Even better get rid of the carbs and go fuel injection.

                        Comment


                          #13
                          damned poppet valve on the throttle butterfly is more about controlling emissions at idle. they rarely work properly and often do not seat properly. Didn't when first introduced and that situation has not improved. I always replace with a flat disc or solder the damned things shut.

                          the big screw near the temp compensator is for a very fine tune on mixture at idle and I doubt will make any difference to the high inlet vacuum when you let go of the throttle from higher engine revs. Default position is to wind them right in.

                          High "engine" speed deacceleration is what you are experiencing, my stag does it whether I am running 2.5%co or 4.5%co.It is usually when lifting the throttle at over 3k rpm, My stag has a recently rebuilt engine, great valve guides etc etc etc, standard exhaust manifolds but a quite noisy large bore stainless exhaust with all the silencers intact.

                          Quite late in the life of the Stromburg Zenith CD carb did the fickle bypass valve appear (mentioned by mr trunt above) to combat the emissions at high rev deacceleration. largely for the federal market. Not sure that I have ever seen one on a UK car. TR6 introduced them in 74ish and Stag 71. but only for west coast federal from what I have been able to find.

                          A set of carbs I recently overhauled were identified by me (no tags) as either TR6 175cd-2sevx (c3613l/r) or some bizarre Californian spec for the 72 stag which appeared to have been 175CD (not CD2) sev, (3428L/R)

                          You cannot just fit these devices to your carbs you would need to source a set of bodies as they are machined for the device. The device itself contains a small diaphragm which opens at high vacuums, my guess is that it was to combat the lean / high co mixture when letting go of the throttle completely from over 2k rpm.

                          Mr Buckeye describes the device I am talking about in "Part II overhaul" here https://www.buckeyetriumphs.org/carburetors

                          It is my theory that the car has always had the high vacuum issue at high rev deacceleration, but upgrades* such as stainless exhausts and other such shiny fings has amplified it somewhat.
                          Last edited by richardthestag; 7 June 2021, 09:40.
                          Stags and Range Rover Classics - I must be a loony

                          Comment


                            #14
                            Originally posted by richardthestag View Post
                            damned poppet valve on the throttle butterfly is more about controlling emissions at idle. they rarely work properly and often do not seat properly. Didn't when first introduced and that situation has not improved. I always replace with a flat disc or solder the damned things shut.

                            the big screw near the temp compensator is for a very fine tune on mixture at idle and I doubt will make any difference to the high inlet vacuum when you let go of the throttle from higher engine revs. Default position is to wind them right in.

                            High "engine" speed deacceleration is what you are experiencing, my stag does it whether I am running 2.5%co or 4.5%co.It is usually when lifting the throttle at over 3k rpm, My stag has a recently rebuilt engine, great valve guides etc etc etc, standard exhaust manifolds but a quite noisy large bore stainless exhaust with all the silencers intact.

                            Quite late in the life of the Stromburg Zenith CD carb did the fickle bypass valve appear (mentioned by mr trunt above) to combat the emissions at high rev deacceleration. largely for the federal market. Not sure that I have ever seen one on a UK car. TR6 introduced them in 74ish and Stag 71. but only for west coast federal from what I have been able to find.

                            A set of carbs I recently overhauled were identified by me (no tags) as either TR6 175cd-2sevx (c3613l/r) or some bizarre Californian spec for the 72 stag which appeared to have been 175CD (not CD2) sev, (3428L/R)

                            You cannot just fit these devices to your carbs you would need to source a set of bodies as they are machined for the device. The device itself contains a small diaphragm which opens at high vacuums, my guess is that it was to combat the lean / high co mixture when letting go of the throttle completely from over 2k rpm.

                            Mr Buckeye describes the device I am talking about in "Part II overhaul" here https://www.buckeyetriumphs.org/carburetors

                            It is my theory that the car has always had the high vacuum issue at high rev deacceleration, but upgrades* such as stainless exhausts and other such shiny fings has amplified it somewhat.
                            Thats very interesting - thanks (and also to other contributors). I've got 3 sets of carbs (inc one on teh Stag) and overhauled the best 2 bodies, no tags so unsure on spec but was told if they have temp compensators (all have) then they're fitted to the Mk2 Stags. Unfortunately we never know what previous owners have done and one carb had poppet valve! However the bodies were all teh same, don't have a bypass valve (but can see reason for left over gaskets now!), but Buckeye's description very enlightening - thanks. I'll try richening/weakening carbs and see what affect it has

                            Comment


                              #15
                              Originally posted by bullstarz View Post

                              its not the mixture when the car is being tested, ie when the engine is being reved. It the mixture when the throttle is closed, the same as when it ticking over. If you alter the mixture for tickover to slightly richer then the poops and bangs will possibly go. If you fit an wide band O2 sensor to the exhaust and a gauge you’ll see what happens.

                              Even better get rid of the carbs and go fuel injection.
                              I'll give that a try thanks. Yep injection would be nice.....

                              Comment

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