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O.T. old chainsaw restoration and a mystery problem

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  • O.T. old chainsaw restoration and a mystery problem

    So this a fun case and quite a puzzle regarding what followed afterwards....

    I have an ancient Husky 254 manufactured in year 1986, that I bought years ago in a "slightly, but passionately used" condition. It was running quite good and reliable, but out of curiosity I removed the muffler to look at the condition of the piston and ring. What I saw in the saw was not on the good side of things: a massive scoring mark that ran over the piston and ring. Clearly at some point a foreign object had passed through the pipes and left a mark. So apart it comes...

    One can clearly see the stripe running on the side of the piston:


    The ring was also scored and just plain worn to oblivion and beyond. The gap measured while placed into cylinder was around 0,8-0,9mm....ridiculous. The cylinder itself was luckily unharmed. There was a visible darker stripe inside the cylinder matching the scoring on piston, but it could not be detected physically. Also all of the bearings on crank, piston and rod were intact and in working order. Apart from one cylinder fastening bolt that had been stripped and loosely replaced by a next size garden variety screw....in an untightened state. Basically the thing had been running on three bolts holding the cylinder down.

    So it was time to make some upgrades. I opted to replace the ring (obviously) and to bring the cylinder down lower than it was originally, until 0,6mm squich remained between piston and cylinder in TDC.....for increased compression ratio. I decided to not touch the ports, just some minor cleanup on port edges to smooth things out and reduce swirling. Also renewed the clutch as it was in a sorry state with the housing and pads worn out. They were replaced and the housing machined to a clean straight inner surface. The new aftermarket clutch housing was a total disaster in terms of tolarances and crooked all over, so I machined the old one and used it instead. Also everything was cleaned and inspected.








    So after putting the contraption together again, boy does it have compression now. Fired right up, runs strong as ever and it will get even tighter with time. Everything seems golden. And then after a few hours of running time the mystery problem begins.
    Once the saw gets nice and warm it will not start again. If shut down momentarily it will fire right up again, but leave it for longer than five seconds and it's dead in the water. Let it cool down or mess around with removing and reinstalling the spark plug for long enough so it cools down and one can get it started again. If you let it run, no problem, just keep on cutting all day. Idles perfect, runs perfect, massive power, full range of revs....but shut it down for a refueling or any other reason and you are done for the day.

    So I began to tackle the problem to find out what was wrong with it:

    Spark plug replaced and gap adjusted to spec: no go
    Carb adjustment series: no go
    Higher octane fuel: no go
    New induction coil: no go
    Full carb dismantle, clean in ultrasonic and rebuild kit installed: no go
    All filters changed and cleaned repeatedly: no go
    All gaskets replaced around carb: no go

    Fuel gets to the combustion chamber.....so does air....so does spark.....compression is good.....but no chance of a go when warm. I got so frustrated that I was about to throw the thing into a lake. In the end I solved the mystery, but can any of you guess what the problem was?
    Last edited by markx; 03-14-2019, 08:11 PM.

  • #2
    Crank seal?
    Andy

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    • #3
      Nope

      Comment


      • #4
        High tension lead from coil to plug.
        Home, down in the valley behind the Red Angus
        Bad Decisions Make Good Stories​

        Location: British Columbia

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        • #5
          Gas cap vent allows pre-load draw but total shutdown if draw is below certain threshold?


          Both Andy's and Willy's are good guesses also


          Carb missing heat insulator block or heat shielding plate?


          give me a few minutes i'll have more maybe
          Last edited by A.K. Boomer; 03-14-2019, 07:52 PM.

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          • #6
            Raising the CR on a two-stroke generally requires a richer main jet...

            -js
            There are no stupid questions. But there are lots of stupid answers. This is the internet.

            Location: SF Bay Area

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            • #7
              Nope I exchanged the whole lot.....new induction coil, leads, gaskets, tubes. Basically everything I could think of. The carb could be tuned clearly into a too rich state, so it was not shortage of fuel. Besides it ran perfectly hot or cold through all the range of the revs. Just could not turn it off for longer than 5 secs once it was going

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              • #8
                I will say this much - your method of increasing comprendo ratio on a two stroke would not be the approach I would take - i would replace the piston with a slightly higher dome

                to move the entire jug down (ports and all) is not good and retards both your intake and exhaust ports which I would think would reduce efficiency and increase heat...

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                • #9
                  Originally posted by A.K. Boomer View Post
                  I will say this much - your method of increasing comprendo ratio on a two stroke would not be the approach I would take - i would replace the piston with a slightly higher dome

                  to move the entire jug down (ports and all) is not good and retards both your intake and exhaust ports which I would think would reduce efficiency and increase heat...
                  It's not perfect, for sure and the altered timing issue was and still is circling my mind. But I made do with what I had at hand. I must say it was a giant leap forward as far as power and performance goes.

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                  • #10
                    This things pretty old - what type of ignition system does it have? You have the full recipe for what it takes for an engine like this to run, the only variable I can think of when you state "it has fuel and air getting to the combustion chamber, it has comprendo, it has spark...

                    so "when" does it have spark? how is ignition timing controlled? is the spark occurring at the wrong time through a certain "heat cycle" for some reason or another? all i got for right now, don't tell us till we get it but do answer questions we ask... we will get this but sounds like a toughie lol

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                    • #11
                      on/off switch bad, or, handle brake??

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                      • #12
                        I think i can answer that for him - it does not have a handle brake and the on/off switch must be functional if it still has spark but will not run.

                        Comment


                        • #13
                          Originally posted by A.K. Boomer View Post
                          This things pretty old - what type of ignition system does it have? You have the full recipe for what it takes for an engine like this to run, the only variable I can think of when you state "it has fuel and air getting to the combustion chamber, it has comprendo, it has spark...

                          so "when" does it have spark? how is ignition timing controlled? is the spark occurring at the wrong time through a certain "heat cycle" for some reason or another? all i got for right now, don't tell us till we get it but do answer questions we ask... we will get this but sounds like a toughie lol
                          Yup all good points. This is obviously a heat soak or at least heat related issue. If all of the ingredients are there as you say it should run, hot or cold, but obviously one thing is missing when a restart is attempted (over 5 sec), unless sufficient cooling has been reached.

                          Fuel gets to the combustion chamber.....so does air....so does spark.....compression is good.....but no chance of a go when warm.
                          So not everything is in place for this to happen, however you say it is. This is the part that has me confused.

                          Also the .8-.9 mm end gap is not out of line with factory specs.
                          I'm rebuilding a Husqvarna 372XP at the moment and the factory service manual says that the gap is not to exceed 1.0 mm. Remember that theses things can get very warm in the summer and gaps will close.

                          Also looking at your piston I see a heavy score mark on one side of the piston and what appears to be a smaller one one the other side yet none in the center where the exhaust port would be located, these marks are indicative of a cold seizure. This is damage that occurs when the saw is put to hard use before cylinder and piston have had ample time to warm up and equalize in temperature. Cant see all of it very well but that would be my guess from where I'm sitting.
                          Last edited by Willy; 03-14-2019, 09:36 PM. Reason: Spelling and grammar...again. LOL
                          Home, down in the valley behind the Red Angus
                          Bad Decisions Make Good Stories​

                          Location: British Columbia

                          Comment


                          • #14
                            Originally posted by A.K. Boomer View Post
                            This things pretty old - what type of ignition system does it have? You have the full recipe for what it takes for an engine like this to run, the only variable I can think of when you state "it has fuel and air getting to the combustion chamber, it has comprendo, it has spark...

                            so "when" does it have spark? how is ignition timing controlled? is the spark occurring at the wrong time through a certain "heat cycle" for some reason or another? all i got for right now, don't tell us till we get it but do answer questions we ask... we will get this but sounds like a toughie lol
                            Yea... it is ancient, but a real good one The ignition system is dead simple, magneto type with fixed timing, there is really nothing to adjust, except gaps at spark plug 0,02" and the flywheel/coil gap 0,012". The only way to tamper with timing is to shear the key at the flywheel crank mating point. And I did not tamper with it, didn't even try to remove the wheel.

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                            • #15
                              Heat causing fuel vapor lock?

                              Cylinder flooding?

                              Gremlins?
                              http://pauleschoen.com/pix/PM08_P76_P54.png
                              Paul , P S Technology, Inc. and MrTibbs
                              USA Maryland 21030

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