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Thread: O.T. old chainsaw restoration and a mystery problem

  1. #1
    Join Date
    Nov 2018
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    100

    Default 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 at 07:11 PM.

  2. #2
    Join Date
    Dec 2009
    Location
    Stevens Point, WI
    Posts
    7,854

    Default

    Crank seal?
    Andy

  3. #3
    Join Date
    Nov 2018
    Posts
    100

    Default

    Nope

  4. #4
    Join Date
    Oct 2005
    Location
    British Columbia
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    6,571

    Default

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

  5. #5
    Join Date
    May 2006
    Posts
    14,537

    Default

    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 at 06:52 PM.

  6. #6
    Join Date
    Nov 2013
    Location
    SF Bay Area
    Posts
    388

    Default

    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.

  7. #7
    Join Date
    Nov 2018
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    Default

    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

  8. #8
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    May 2006
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    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...

  9. #9
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    Nov 2018
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    Default

    Quote 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.

  10. #10
    Join Date
    May 2006
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    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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