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

  1. #31
    Join Date
    Oct 2002
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    Conyers Georgia
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    324

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    what about that wrong bolt

    air leak when hot
    George from Conyers Ga.
    Remember
    The early bird gets the worm, BUT it's the second mouse that gets the cheese.

  2. #32
    Join Date
    May 2006
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    I thought he correct that ----- ?

  3. #33
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    Feb 2016
    Location
    Edmonton, Alberta, Canada
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    990

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    checked the fuel lines for cracks? same with the line to the primer bulb if present and the primer bulb itself.

  4. #34

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    Quote Originally Posted by A.K. Boomer View Post
    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?
    Aye.....I think the first suggestion pretty much leads to the final answer to this "mystery" at least according to my own train of thought and actions

    So to not stretch the attention gap too far with this topic I shall disclose the solution....albeit I'm not 100% percent sure about the cause of the actual problem, but here's what I did to cure it and get back on track.
    The question "does it have spark under compression when it appears to have a healthy spark in the "relaxed" state and all else that should be working is working?" being the main topic here. As it turned out it does not have spark under compression! I should have known.....in fact I did, but I just deemed it too unrealistic according to calculations to try out the simplest thing in the world. Reduce the damn spark gap at the plug below specs and give it a pull!!

    But nooooo, of course not.....instead I dismantled the whole system countless times, exchanged all the bells and whistles that I could and was driven to the edge of putting the poor saw out of it's misery
    And then, in the very end, after pulling my arm sore in another episode of "no go when warm" I decided to pull out the spark plug the last time and just arbitrarily reduce the gap below the stated spec to see if it really was an obstruction in the spark path that causes the problem. The damn thing sprung to life at the very first pull after that! And keeps on doing so, cold or hot no matter.

    I think the severly raised compression caused the dielectric properties of the combustion mix to change so far in the hot state that a spark could not pass through it under specified gap dimension any more. I think it was borderline, because when it was running it was perfect: the fast spinning flywheel generated enough magnetic flux to drive the spark through with confidence. But try to achieve the same pulling by hand....not a chance. It was so borderline that in cool condition the spark passed and the saw would start right up, but not when hot. The momentary shut down effect though: saw would start when tried immediately after hot shut down, but not after more than five seconds had passed?
    I think was caused by fuel evaporating from the carb and making the combustion mix a bit richer, which in turn led to the spark not being able to pass the gap any more. Then further pulling would suck in more fuel and off we go. Although I'm not completely sure about that bit....
    The problem appearing after few hours of running time I also think can be attributed to the condition being borderline, just at the very edge of being able to pass the spark through. The engine broke in a little bit during these first hours, compression got a wee bit higher and the problem appeared.
    I must say this is the first time I've encountered such a situation and it confused the hell out of me....sort of works, sort of not. Usually if there is a healthy spark then it's a go and if not then all is clear, no inbetween limbo type of condition.
    I was actually thinking of such an effect regarding the ignition system might arise when I assembled the saw after rebuild, so I adjusted the spark gap strictly to specs as it was a bit larger before. But I did not have the common sense to reduce it further when I was facing the actual problem. What a bonehead.....the simplest move in the world, taking about a minute out of my life could have corrected everything without any mechanical interventions. Well at least it now has a new ignition coil, leads, new gaskets all around and basically only the carburetor housing remained from the old setup. And it runs strong as a beast!!

    P.S. regarding the lowering of the jug...it was done by adjusting gasket thickness. The original gasket was monstrously thick, so no material had to be taken off the cylinder mating surface. And yes the stripped bolt was replaced by a custom made one. I also rethreaded the messed up bolt hole properly to next upper size. It should be the strongest one on the saw now.
    Ah yes, just remembered that the "master" who fixed the bolt before my time was on another level really. He did not remove the cylinder from the saw to replace the bolt, but instead tapped through the cylinder bolt hole into the crankcase below. So the threads were also cut into the bolthole on the cylinder and then a random screw was inserted to fill the gap, but provide no actual tightening force at all. The screw would pull itself against the cylinder, but not the cylinder against the crankcase as it should have. Amazing engineering....but I corrected all that.

  5. #35
    Join Date
    Dec 2009
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    Stevens Point, WI
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    Honestly coil failure was the first thing to cross my mind. But then you mentioned a new coil and "it has spark".

    Reason it was the first thing to cross my mind is because I ran into that issue before.

    In fact I have a video, when the motor/coil would heat up the spark would go out. When it cooled you could start it again. Over and over again all day long. I suppose with your saw it was fine while running because of airflow. Shut it down for a few seconds and it would heat soak the coil. Is it an oem coil or aftermarket?


    My waterwitch that would die when warm.

    https://www.youtube.com/watch?v=YTn8wveCv4U
    Andy

  6. #36
    Join Date
    Feb 2012
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    Metcalfe, Ontario, Canada
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    New coil and no spark when hot, with slightly raised compression?

    Might indicate weak magnets as mentioned by Speaker in post #21.

  7. #37
    Join Date
    May 2015
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    Somerset UK
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    The ignition timing depends on the points gap, wider advances the ignition. You might try closing the spark plug gap slightly, with higher compression, partly from lowering the barrel/head and partly from the better sealing from the new ring with the correct gap could require higher voltage at startup cranking speed.

  8. #38
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    May 2006
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    Quote Originally Posted by cameron View Post
    New coil and no spark when hot, with slightly raised compression?

    Might indicate weak magnets as mentioned by Speaker in post #21.
    Yup - I would have to concur and that's why I wrote what i wrote on post #23 about magnet and or coil, things were borderline after all so the spark showing on the outside was not conclusive like he just stated,,,

    somethings weak and he's replaced most everything else so the magnet is highly suspect or the plug has internal carbon tracking on the porcelain or something,

    There's no way the little bit extra compression should shut this thing down like this - they should be able to jump a standard gap regardless as there's overkill built into the system - he's most likely a little above sea level right?
    this means that the unit would gain more effective compression if it was taken down to sea level and if stock would not run right from the factory - or days with higher barometric pressure - or running it in death valley or on and on,

    somethings still "jacked" but he found a way to compensate so it's fine if the gap is not shut down too far...

    way cool - good puzzle - thanks for the brain teaser.

  9. #39
    Join Date
    Jul 2012
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    11

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    The reason that I thought "magnet" was that I remembered this thread on another forum where they chased a mystery chainsaw problem. http://opeforum.com/threads/troublesome-ms660.3086/

  10. #40

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    Quote Originally Posted by A.K. Boomer View Post
    Yup - I would have to concur and that's why I wrote what i wrote on post #23 about magnet and or coil, things were borderline after all so the spark showing on the outside was not conclusive like he just stated,,,

    somethings weak and he's replaced most everything else so the magnet is highly suspect or the plug has internal carbon tracking on the porcelain or something,

    There's no way the little bit extra compression should shut this thing down like this - they should be able to jump a standard gap regardless as there's overkill built into the system - he's most likely a little above sea level right?
    this means that the unit would gain more effective compression if it was taken down to sea level and if stock would not run right from the factory - or days with higher barometric pressure - or running it in death valley or on and on,

    somethings still "jacked" but he found a way to compensate so it's fine if the gap is not shut down too far...

    way cool - good puzzle - thanks for the brain teaser.
    Ya....that's what I also thought "no way is the gap an issue, it should just spit over it like there was nothing"! That's what led me to believe something else was faulty and hence long tinkering around with all the other parts of the mechanism. But I also have to concur about something in the system being a bit weak if that raise in CR was able to shut things down in the electrical department. Magnets are most probable, as the thing is ancient and in these times the magnetic materials were not so durable. It may very well be that they are tired and thus can't generate enough voltage for the system to operate reliably. Also the fact that changing ignition coils did not make any difference seems to point at something outside the coil assembly and only the flywheel is there. But as far as I know the magnets can not be removed and this can only mean the changing of the whole flywheel. Them things are hard to find these days, OEM that is and not the cheapest. I could try a used one, but there is no guarantee it is not even more tired. Chinesium aftermarket crap is also an option, but this be a real gamble. Could work out fine, but could also mean disaster.
    I guess as long as the system is able to operate with the reduced spark gap, there is no need for immediate action
    Last edited by markx; 03-15-2019 at 10:50 AM.

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