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Need some help with 2 stroke motor please

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  • Need some help with 2 stroke motor please

    Guys I could do with a little help with a problem motor. I've got to the point I'm going round in circles and I know I'm overlooking something simple but I just can't see it.

    Story. Motor is a 125cc 2 stroke in a late 70's Trials bike. Bike been left in a farm shed and not looked at for last 5years plus. I had need for some speedy off road transport and asked to borrow bike from owner. When we tried to start the bike we flooded the carb, fixed this I thought, then subsequently blew out the head gasket (hydraulic lock up?) Anyway I got bike back home, put on a new head gasket, stripped the carb and cleaned it up all looked OK, all jets clear all airways clear.

    Tried to start bike, starts fine with choke out but can't get it to run without choke. If its running with choke out even the slightest opening of the throttle kills the motor. The carb is a Dellorto PHBL 24BD. The choke is not a choke as such but a fuel enrichment circuit that bypasses the main venturi but it would seem as if the motor will only run on this. The idle and main circuits just don't want to work. If I choke the carb intake by hand I can get a throttle response but again only if the "choke" enrichment device is on.

    In an attempt to find an answer I've stripped the motor back and checked
    crankshaft seals which are perfect and replaced all relevent gaskets. I'm convinced this is a carb problem but I just can't see it.

    Anyone with words of wisdom to offer? I'd be grateful for any help anyone can provide.


    West Sussex UK

  • #2
    Is it a reed valve engine?
    Free software for calculating bolt circles and similar: Click Here


    • #3
      Hard to diagnose a running problem via long-distance. You don't indicate how the motor is running with the choke on.............smooth or rough. Carbs are always a source of trouble in older motors of all types.

      The most common 2-stroke problem I've seen is carbon build-up over the cylinder exhaust port. A sheet of carbon can cover this port completely, and is sometimes hard to see due to the darkness of either the carbon or the internal cylinder. When this carbon is present, the motor cannot 'exhale' and therefore, cannot 'inhale' either. Possibly, the choke is forcing enough fuel into cylinder to make it run somewhat.

      This condition will cause it to act like it isn't getting enough fuel to spool-up properly...............and that would be the case, exactly.


      • #4
        I would look at the carb...

        I would take a good look at the carb and let it soak in some carb cleaner for a few days. If it won't run with the enrichener off it would sound like fuel can't get through the carb. High speed and low speed jets need to be pulled and everything soaked and blown out several times.


        • #5
          you might take the exhaust pipe and blow from the cylinder side with some air and see if you think the same amount is coming out of the other end.might have a ratsnest in there.

          leon holmes


          • #6
            Hmmm.... What about the fuel supply? Clogged screen/filter in the tank? Plugged fuel line or air vent to the tank?


            • #7
              Guys, thanks for the input. Sometimes putting your problems down as text helps you see them in a different light. As I said in my post above I knew I was missing something major, as I finished writing my post I realised that whilst I had given the carb a good scrub out I did not recall seeing a jet for the main circuit. Mad panic and dash around the workshop looking for a jet, couldn't find a thing so pull the carb to bits again, then the penny dropped as I unscrewed the bolt for the fuel bowl I realised that the bolt also was the main jet, and of course being at the bottom of the bowl was full of crap.
              Duely cleaned and re-installed motor now runs, hurrah.

              So thanks all for the input, I'm putting this one down to a differet way of analysing problems by writing them out.
              West Sussex UK


              • #8
                I would have definitely said crank seals on a bike that’s been sitting that long, are you positive they’re OK?
                Another question to add to Evans query about reed valves – is it piston-ported or disc valve? Answering these 2 questions can also help diagnosis.

                First check the spark plug. If it’s wet there’s too much fuel getting though, if it’s dry there’s too little, although if it runs on ‘choke the latter is more likely.

                Air leaks – get it running and spray some WD40 around the joints from carb/manifold and manifold/cylinder. If the engine revs dip then you have an air leak in that area.

                Carbs in General

                – Idle and off-idle fuelling is controlled by the pilot jet. These have tiny little orifices which block at the drop of a hat. With ancient varnished old fuel it is likely the pilot circuit is blocked and must be thoroughly cleaned through and checked. The choke bypasses the idle circuit.
                – From about 1/8” throttle until about ¾” throttle fuelling is controlled by the needle jet/emulsion tube, and the emulsion tubes can also block. Make sure that the needle is being lifted by the slide – it should be held in place by a circlip, usually set at the middle clip position. However, the pilot circuit is the more likely culprit from your descriptions
                – Full throttle fuelling is controlled by the main jet.

                If is too much fuel then you float valve could be the problem, and needs to be cleaned and the float heights checked.
                The fuel tap may also be clogged with varnished old fuel, so the filter screen in this needs checking.

                And finally, take the baffle out of the exhaust to make sure it is not clogged up.

                Edit: oops too slow! Sounds like it's sorted.


                • #9
                  Clue is it runs better with choke.

                  Your main jet is plugged.


                  • #10
                    Peter Neil beat me to it. I think the crankcase seals could be bad from sitting so long. They are often overlooked yet they are critically important to good two-stroke operation. Other than that, there's not much to look at on a two stroke other than compression, ignition and fuel.

                    Good luck.


                    • #11
                      pilot jet is plugged
                      "the ocean is the ultimate solution"


                      • #12
                        I rode a 2 stroke 200 twin for a year with the choke on until an old machinist told me that a weak spark will fire a rich mixture, it takes a GOOD spark to ignite a correct mixture. Replacing the points that looked OK to me cured the carburetor problem. Good luck.


                        • #13
                          Originally posted by Alan Smith
                          ....I realised that the bolt also was the main jet, and of course being at the bottom of the bowl was full of crap.
                          Duely cleaned and re-installed motor now runs, hurrah.
                          Uhhh.... Hello???


                          • #14
                            I once had a 2-stroke commuter. It was a single cylinder 400cc Yamaha, bright yellow, and it was a hell of a fun bike to ride. I used to ride it from Newport Beach, CA to Monrovia to work. Somewhere around 25,000 miles in it the ignition started to go and it began running rough. I was a pretty good wrench being a desert racer at the time, so figured it was correctable, but I never did find the problem. It finally quit on me in El Monte. I nursed it off the freeway and got to a bar where I could make a phone call for help. Went outside to get my helmet and goggles and someone had already stolen it. Walked off with it, obviously. They didn't get much

                            That was my last two-stroke until I bought a 125cc Hodaka Wombat street model basket case a few years ago - still doesn't run, but it's fun to work on.