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

  1. #11
    Join Date
    Dec 2018
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    on/off switch bad, or, handle brake??

  2. #12
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    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.

  3. #13
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    Quote 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 at 08:36 PM. Reason: Spelling and grammar...again. LOL
    Home, down in the valley behind the Red Angus
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  4. #14

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    Quote 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.

  5. #15
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    Oct 2013
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    Heat causing fuel vapor lock?

    Cylinder flooding?

    Gremlins?

  6. #16
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    I think it self floods somehow, the tank self pressurizes due to the heat and floods the engine after a bit of time, go to start immediately after and it will due to it not having enough time to pressurize the carb lines but let sit for a bit and bingo,

    let sit for way long and things vaporize and disperse...

  7. #17
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    When I bought my house in 1977 there was an old wooden shed, and when I was clearing it out, I found what appeared to have been a worm drive or maybe bevel gear drive chain saw. IIRC the axis of rotation of the engine was in line with the bar. I threw it out but now I wish I still had it as a curiosity. It may have been one of these:

    http://vintagechainsawcollection.blogspot.com/2012/05/ (Stihl BLK57 or BLK58)


  8. #18
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    May 2006
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    Although - maybe were going about this the wrong way, the OP is very well described -

    the engine ran ok before so it was something he did, the compression ratio change is the biggest change as everything else is pretty standard,,, we got to think outside the box now - like the piston is now coming up and cranking into the spark plug ground or some damn thing lol

    the change in relationship from piston to cylinder bore on a two stroke is huge,,, don't know just thinking out-loud here....

  9. #19
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    Start isolating.
    Try another fuel tank, you can change compression back with thicker gaskets things like that.
    Did you buy or make the gasket. .
    MAYBE IT ALREADY HAD this problem, so they got rid of it.

  10. #20
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    May 2006
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    We must always keep in mind that engines that "appear" to pass the spark test outside the combustion chamber do not pass the test under compression conditions

    is this theory linked to your mystery problem?


    Edit; also - you gave us a "squish area" AFTER the cylinder lowering - but not before so no way to judge approximately how much you upped the ante on the comprendo ratio,,,

    if it was a fair amount and your timing is fixed this is not a good combo, you created a faster burn rate with the higher comprendo therefor the ignition timing might have to be retarded some,,, yet it's fixed so the engine is running advanced

    even these old engines have very calculated critical systems that have to fall together... how much material did you take off the bottom of the jug?
    Last edited by A.K. Boomer; 03-14-2019 at 09:19 PM.

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