TS250/1 Crank seals????

Forums Technical Two Stroke: Help needed TS250/1 Crank seals????

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    • #7646
      ‘AAA Dummy


      I have a TS250/1 which has been stood garaged for 3 years and displays the following symptoms:
      1.Starts reasonably easily but only if some throttle is applied.
      2.Immediately revs up to around 3000 rpm and stays there regardless of the throttle-stop/cable/airscrew/choke lever settings.
      3.After a minute or so the revs drop slowly and engine eventually dies.
      4.If I blip the throttle to stop it dying it returns to 3000 rpm and stays there again.
      5.It does smoke a bit but I wouldn’t say unduly for a ‘just started’ 2-stroke.
      6.The plug looks quite wet afterwards (not over hot).

      I have tried all the usual checks I can think of for both the fuel/carb and electric/ignition systems including air leaks around the carb/manifold, carb&jets cleaned, points, timing etc. and I am reluctantly arriving at the conclusion that one or both of the crankshaft oil seals are knackered. I did wonder if it could be a cold start/choke problem but I’m probably clutching at straws.
      I welcome any suggestions of alternative causes but assuming it is the crank seals can anybody help with any of the following:
      1.Point me at a good text or link or video describing the job of replacing the seals. Preferably as a standalone job rather than a complete engine rebuild. I have a Haynes manual but it’s not easy to extract just this job from the full rebuild of a 250 4speed with additional text for 5 speed models at the end.
      2.Do the symptoms suggest one seal (L or R) rather than both or do you always do both?
      3.Am I correct in thinking I can change the seals with engine in bike?
      4.Which (if any) special tools will I need to complete the job.
      5.Any advice on make of seals, or are they all the same?

      Any suggestions or advice very gratefully received.

    • #7655
      Membership Man

      Hi Pete,
      Your symptoms do sound like an air leak. provided the manifold screws are tight then the crank seals are the most likely culprit. I would guess it was the generator side and its easiest to start with that as you only need the minimum of tools.

      Job can be done in frame as follows.
      1.Remove dynamo stator – 2 screws and note where wires go. Extract rotor, you need a long threaded 8mm bolt to pull it off the crank.
      2. Inspect seal for obvious signs of leakage or damage but since its sucking inwards there may not be much.
      3. Remove the circlip which retains the seal. There are two types, early ones are proper flat circlips, later are just wire.
      4. remove seal by carefully drilling two small holes near the edge of the seal 180degrees apart. Do this carefully as you don’t want to damage the plate behind the seal.
      5. Screw in two self tappers – gently again and they may well force the seal out. If not, once they are firm use pliers to pull the seal out.
      6 Fit new seal – inner side to engine.
      7 fit circlip and dynamo bits – job done.

      Probably best to get the seals from TheMZshop but if you are confident in ordering such parts they can be bought for less from a bearing stockist such as Simply bearings. Spares book will give you the sizes 72 x 30 x 7 from memory.

      There is a better seal for the drive side with double lips but these should not be used on the generator side.

      If that does not do the trick then you may have to tackle the drive side which does need special tools. However its less likely to go as it runs in oil. Come back for more help if necessary.


    • #7656
      Membership Man

      Sorry, it should be a 10mm bolt to pull the rotor – senior moment. 8mm is for BMWs.

    • #7657
      ‘AAA Dummy

      That’s brilliant. Thanks very much Peter, that is exactly what I needed. I have ordered the seal and will see how it goes… If there’s no improvement I will tackle the drive side which I guess needs a clutch puller.
      Thanks again,

    • #7670

      “Push” or “Pull” but what do I know.
      As I recall I took my stator off by placing a short
      piece of rod in there, then used the bolt to bear on it and jack the stator off.
      So I would say push it off, or does that confuse things more so.

    • #7691
      ‘AAA Dummy

      Thanks Rob, Yes, I believe the spacer method would work equally well. I just got a 100mm M10 bolt and screwed it in, popped straight off.

      Well, I changed the generator side seal….. No improvement, so I got myself a clutch puller and an engine locker and changed the clutch side seal. Guess what…. no improvement.

      I will revisit the carb and manifold, although I thought I had been pretty thorough the first time. Perhaps I will try a different carburettor if I can get my hands on something that fits. The only thing I can add is that by trial and error I discovered that if I drop the throttle needle right down to it’s lowest position (top notch – weakest?) then it will run at a lower tickover (2000 rpm) but still if I blip it revs up and takes ages to settle back down to 2000.

      At least I have eliminated the seals as the cause, I know I have a good set of seals in and I won’t feel as daunted at tackling the job on other bikes in the future.

    • #7853
      ‘AAA Dummy

      Replacing the carburettor sorted this out. I bought a BVF copy/clone from Poland (£45 including post). Started straight up and ran perfectly. The quality is pretty dire though (as you might expect for the money). Longer term I will be looking to put a Mikuni on it.

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