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

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  1. #1
    Join Date
    Nov 2018
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    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.

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