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

  1. #41
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    Speaker, It's probably the most suspect with what we know - I was pretty sure we had it tracked down to "variable spark" or "variable mixture" and the reasons for either abound,,,
    Im sure he's tried other plugs and also had the coil to flywheel gap spot on as he seems very thorough in his approach...

    compression on a two stroke is fairly stable as long as the crank seals are good and Andy covered that right off, compression on a 4 stroke can vary with heat like this with certain engine layouts and expansion rates for things that have to do with valve clearance - but most of the times "gain" clearance "with" heat...

  2. #42
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    Quote Originally Posted by markx View Post
    I guess as long as the system is able to operate with the reduced spark gap, there is no need for immediate action

    I agree and good job with the persistence - just run that sow lol

  3. #43
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    The hot coil issue is not uncommon with an older coil (the OP replaced his, presumably with a good one).

    Older coils may develop a problem of low resistance insulation when hot, so they either have leaky insulation when hot, or develop an internal spark-over when hot. A new coil presumably fixed any such.

    Gaps in the magnetic circuit, or weak magnets are the other half of a possible problem. One-piece coils leave the flywheel gap or weak magnets as causes, multi-piece coils can have crap between pieces that adds a gap and effectively weakens the magnets even if the actually are good.
    1601

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  4. #44
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    Quote Originally Posted by vpt View Post
    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
    Exactly, this was my first thought as well as I too have seen a lot of both ignition and pick-up coils do the same when hot and then return to normal when cool. This was also why I mentioned the high tension lead from coil to plug in my first reply as I have seen resistance type plug wires increase in their resistance when hot. At that point the OP had not yet disclosed that the wire had been replaced.
    The symptoms described fall exactly inline with a weak or non existent spark when ignition components are hot however it was mentioned that all of these elements were present during the no-start condition, this was a bit confusing to me. If all those elements are present it has to start, and yes the more air pressure in the combustion chamber the more voltage required to cross the plug's gap. I learned that graphically in 1968 when I worked in a shop that had a Champion spark plug cleaner and tester. It would apply an arc across the plug and while doing so you would increase the air pressure in the chamber while watching the arc. This allowed one the insight as to where on the pressure scale the plug would fail. Very graphic point that has been burned into my cranium ever since.

    I think this is a lesson for all of us in regards to trying to diagnose various issues long distance, too many subtle elements get lost due to just not being there yourself to see evidence first hand and following one's routines diagnostic process.
    Also an interesting lesson to be learned here in regard to how fine that line is between fully operational to dead in the water. Thanks
    Home, down in the valley behind the Red Angus
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  5. #45
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    It's why I had to eventually have to ask this question; "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?"


    We all covered so much ground it was time to start actually pinning him down with definitives/specifics due to the fact that he could not initially just divulge this info or the puzzle would have been very easy to isolate in the first place...

  6. #46

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    Quote Originally Posted by Willy View Post

    I think this is a lesson for all of us in regards to trying to diagnose various issues long distance, too many subtle elements get lost due to just not being there yourself to see evidence first hand and following one's routines diagnostic process.
    Also an interesting lesson to be learned here in regard to how fine that line is between fully operational to dead in the water. Thanks
    Too often when trying to describe a situation to others I tend to omit some details which subconciously seem irrelevant or selfexplanatory to me, because I can see mnemonic picture of the situation in my head. But I have to remember that the audience can not and may thus be misled by the lack of information. Do believe me, I was not trying to cheat and mislead anybody on purpose

    E.g. the ignition coil HV wire.....I know for a fact that it is for all intentive purposes attached to this particular unit permanently and can not be removed without violence, so I skipped to mention that the high voltage conduit was changed with the igniton coil unit as one piece. Seemed irrelevant, but viewed from the side of audience it is very much a justified concern if the old conduit was left on the machine or not.

  7. #47
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    You were very detailed in your initial description - after that it was all up to us to start asking the appropriate questions,,, I knew I had to start pinning you down with them, this could just as easily turned out to be carb/mixture related... just like you stated "it has spark" you also stated "it has fuel" well - could have been too much fuel due to a pressurized tank or whatever...

    but the latter would not have matched "why" the change after the rebuild - the former does - but there are other factors carb related that might have changed, after so many questions and answers the possibilities should get increasingly narrowed...
    Last edited by A.K. Boomer; 03-15-2019 at 01:53 PM.

  8. #48
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    I whole hardheartedly agree AK, I'm as guilty of this flaw as anyone. Sometimes hard to list all of the thought processes and procedures that one subconsciously goes through when assessing a situation.

    The Husqvarana saws that I have been working on (365s and 372s) however do have a separate lead independent of the coil, I assumed the 254 was the same. Perhaps another flaw due to not being there. I'll have to look it up.
    Oh and while on that note, the Chinese make some excellent hard parts as well as some that are not that great but still serviceable if you don't mind changing them more frequently. In my experience the coils are however a crap shoot, actually more crap than shoot.
    Home, down in the valley behind the Red Angus
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  9. #49
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    We all did good - not much wasted questions lots of narrowing down --- but in the back of my mind was the fact that this was "one sneaky customer" that had to be asked point blank about something he was actually trying to conceal from us to some degree, that's not "his bad" its what he had to do to create a good puzzle.... and he did just that without lying about anything...

  10. #50
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    Quote Originally Posted by markx View Post
    Too often when trying to describe a situation to others I tend to omit some details which subconciously seem irrelevant or selfexplanatory to me, because I can see mnemonic picture of the situation in my head. But I have to remember that the audience can not and may thus be misled by the lack of information. Do believe me, I was not trying to cheat and mislead anybody on purpose

    E.g. the ignition coil HV wire.....I know for a fact that it is for all intentive purposes attached to this particular unit permanently and can not be removed without violence, so I skipped to mention that the high voltage conduit was changed with the igniton coil unit as one piece. Seemed irrelevant, but viewed from the side of audience it is very much a justified concern if the old conduit was left on the machine or not.
    Not to worry Mark, like I said I'm as guilty as the next guy, I realize that I often take things for granted when trying to include all pertinent info into a description of events. I knew you were not being maliciously deceptive.
    Hey we are not all English language majors nor technical writers. We only have what is written on these pages as evidence to judge and base our response.

    No biggie, like I said we all learned something here and ultimately that's what counts.
    Home, down in the valley behind the Red Angus
    Bad Decisions Make Good Stories

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