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    Triumph 650 twin problems

    Yesterday I drove by a motorcycle in a parking lot driveway, and saw the man was beside it with his tools already out.

    I went back to see if any help was needed and he gave me a problem I could not solve.
    This was a 1970 Triumph 650, twin, with kick-start only.

    The problem was that he had gone for an hour ride for enjoyment and on his way home the bike began losing power and cutting out.
    The power loss grew in frequency and duration until he had to stop and park the bike where I found him.

    Once stopped, the bike would show signs of starting, but would do so for only seconds before cutting out.

    A change happened since the bike was parked: after tickling the carb he tried to start it and it failed, but there was backfiring.
    The backfiring happened also when start effort was stopped while ignition was ON

    There seemed to be some movement in the positive terminal on the battery, but it showed 13.2 volts, so it was still working , and connections were tight and clean, however the power monitor light in the gauge cluster was dim/weak, suggesting low voltage at that point and likely poor connection in the harness .

    The bike was a barn find for him and he had rebuilt the engine entirely by himself.

    He was certain that the coils were good....they were both new and he had the bike running on one coil two days before.....but the symptom of slightly erratic running under no load was close to what had just happened.
    Plugs were clean and dry, no fuel fouling, and spark plug/coil wire connection was good.

    All gaskets were new, and compression seemed good to him as the bike has only kick-start and he could detect no change in effort.

    Carb had fuel and fuel filter and lines to/from carb were checked. All was clean. Carbs seemed tight to the engine, but we had no means of checking air leak so that cannot be ruled out.

    Bike has electronic ignition, and the timing had already been checked while he was there. (remove the cover, remove the timing check bolt at the back of the engine and rotate the engine until it reaches the detent. )

    It seemed there was some spark as the bike would still start/run for three to four seconds before halting. (possible intermittent connection or ignitor problem?)

    He had already called CAA (same as AAA) and was waiting for their truck to collect the bike and transport it home. (I could not help with that as it will NOT fit into the trunk of a Mustang.)


    Ideas?
    "If you scare people enough, they will demand removal of freedom. This is the path to tyranny."
    Elon Musk Jan, 2022

    #2
    I wonder if the bike had resistor plug caps?
    This thread from a triumph site has a lot of similarities to the fellow on the side of the road.
    Can't imagine it being a gas cap vent, after sitting for a while the pressure would drop enough to feed the carbs again.
    Good on ya for offering some assistance.
    2@ \'78 GS1000

    Comment


      #3
      Thanks for that. I agree about the gas cap as the fuel flowed freely when he tickled the carbs, and again when he removed the line.

      Interesting exchanges there, but apart from comments on the engine displacement the posters are largely describing what he did with the bike while parked, excepting carb adjustment which he was confident was spot on.
      The bike was loaded onto the truck and carted off to his home. I don't know what type of plugs he has but will pass on that info if I hear from him. He has my phone number but he did not give me his, so I cannot follow up.
      "If you scare people enough, they will demand removal of freedom. This is the path to tyranny."
      Elon Musk Jan, 2022

      Comment


        #4
        A Bonnie or a Trophy? If she's twin carbs possibly a split / rot in the carb balance pipe or the carbs are over tightened - both are not uncommon, the latter in particular. I forget what the torque setting us but it's very low - not much more than finger tight as the flange seats can distort. Could also be a jammed choke mechanism though she'd probably run in that case, just poorly.

        Which type of electronic ignition is fitted? If it's the old optical version (mental block - I've forgotten the brand name at the moment) it could be a bit of muck messing with things. If it's Boyer then check for bad earths. Are the coils getting really hot?
        79 GS1000S
        79 GS1000S (another one)
        80 GSX750
        80 GS550
        80 CB650 cafe racer
        75 PC50 - the one with OHV and pedals...
        75 TS100 - being ridden (suicidally) by my father

        Comment


          #5
          Originally posted by hampshirehog View Post
          A Bonnie or a Trophy? If she's twin carbs possibly a split / rot in the carb balance pipe or the carbs are over tightened - both are not uncommon, the latter in particular. I forget what the torque setting us but it's very low - not much more than finger tight as the flange seats can distort. Could also be a jammed choke mechanism though she'd probably run in that case, just poorly.

          Which type of electronic ignition is fitted? If it's the old optical version (mental block - I've forgotten the brand name at the moment) it could be a bit of muck messing with things. If it's Boyer then check for bad earths. Are the coils getting really hot?
          Must admit I have forgotten what they difference was, as I have had nothing to do with Triumphs for many years, so I dont know if it was a Bonneville or trophy.
          He told me it had chrome fenders, but they were badly rusted so he sanded them out and painted them black.

          He told me the ignition was Boyer. The ground/earth at the battery was sound, but there was something else at fault in his system as there is a power monitor lamp on the dash which he said should be bright when ignition was ON, but it was dim even though battery voltage was at 13.2
          "If you scare people enough, they will demand removal of freedom. This is the path to tyranny."
          Elon Musk Jan, 2022

          Comment


            #6
            Sounds like it's the battery down on power. The test is to switch on the headlight, foot on the rear brake to light up the brake light, hold both on for a minute and then hit the horn. Anything less and the Boyer boxes don't seem to like it.
            79 GS1000S
            79 GS1000S (another one)
            80 GSX750
            80 GS550
            80 CB650 cafe racer
            75 PC50 - the one with OHV and pedals...
            75 TS100 - being ridden (suicidally) by my father

            Comment


              #7
              Originally posted by hampshirehog View Post
              Sounds like it's the battery down on power. The test is to switch on the headlight, foot on the rear brake to light up the brake light, hold both on for a minute and then hit the horn. Anything less and the Boyer boxes don't seem to like it.
              Agree re Boyers. If the battery measured up at 13V plus, it's probably the harness. Replacement aftermarket ones solve a lot of OE quality control problems....

              Comment


                #8
                Originally posted by GregT View Post
                Agree re Boyers. If the battery measured up at 13V plus, it's probably the harness. Replacement aftermarket ones solve a lot of OE quality control problems....
                Thanks for that corroboration and replacement suggestion. I am hoping he will call, so I can pass on the info from this thread.

                I have torn apart and rebuilt harnesses on GS bikes and know it can consume a lot of time, so a replacement may be a good option for him..
                "If you scare people enough, they will demand removal of freedom. This is the path to tyranny."
                Elon Musk Jan, 2022

                Comment


                  #9
                  Tho old Boyer on my MK III Commando, as I recall, wants 10v or spark gets erratic. Newer stuff may need better voltage than that. The Boyer in my MK III has a continuous advance, which I think is crap, but it was all I could find in 1983. I'm going to switch it out for something else with a more defined curve, someday.

                  Either way, it sounds like an electronic ignition problem, which could be caused by bad grounds or low voltage.

                  I don't think that it is a coil that goes bad as it warms up, because of the way it backfires when the ignition is turned on.
                  sigpic Too old, too many bikes, too many cars, too many things

                  Comment

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