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Thread: 2-stroke compression too high?

  1. 11th January 2014, 09:59 AM #1
    geodon's Avatar
    geodon
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    2-stroke compression too high?

    In the absence (or can someone re-direct me?) of a small 2-stroke forum, can I ask why here why my El-Cheapo post hole auger has suddenly gone feral insofar as it's impossible to start due to being too hard to turn over?


    Yes, it has a decompression valve & it is working OK. I think. I removed it & can blow thru it when the mushroom valve is out.
    It's easier to turn over with the valve actually removed & very easy with the plug out. There's not much difference between decompression valve in or out though.


    I've pulled it to bits (I suspected a seizing piston) & everything seems perfect:


    no scoring in the bore
    rings are free to turn & undamaged
    very little carbon


    It was working fine then suddenly could not be started & the tension required to turn it seemed to have tripled.


    I suspect the decompression valve but don't know how to confirm it. I can blow thru it by mouth but only just. How much air should these things pass? Are they a generic thingy that can be bought? (I don't like my chances with the "service" back-up of the supplier)
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  2. 11th January 2014, 12:33 PM #2
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    Is the oil/fuel ratio correct, too much oil can lift compression, just a thought, Regards Frank.
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  3. 11th January 2014, 02:47 PM #3
    uninformed
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    I wonder if its sheared the key in flywheel and timing is out..... Long shot.
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  4. 11th January 2014, 02:55 PM #4
    pannawonica
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    carb diaphragm can and does give this symptom in chainsaws as they age and harden!
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  5. 11th January 2014, 07:05 PM #5
    Gumnut
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    Another long shot...

    Try pulling the muffler off and make sure it is not blocked.

    Mud wasps are fair dinkum swine for building nests inside, and that will prevent starting.

    Gumnut.
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  6. 12th January 2014, 08:06 AM #6
    llandro
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    Has the centifugal clutch stuck and you are trying to turn the whole auger assembly when starting?
    CR of a 2 stroke can never be much more than 8:1 due to the porting configuration.
    Mike
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  7. 12th January 2014, 11:21 AM #7
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    Tank wins the cigar!

    Well done, mate. Thanks!


    To quote Julius Sumner Miller: "Why is it so?"


    Simple really.


    The tolerance of the rings in the bore is set for a certain petrol/oil ratio. I was running 25/1 but the handbook says 50/1. Working on a hot day, evaporation probably made the relative amount of oil go up & that improved the sealing capacity of the rings = much higher compression than my poor arthritic shoulder can handle.


    Remember how to compression test an engine? If you find an abnormally low reading, you squirt oil don the plug hole and test again. If the compression does not go up = a burnt valve. If it does go up = poor seal in the bores, worn rings or a re-bore required.


    It still won't start but I can now easily crank it & investigate further.
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  8. 12th January 2014, 01:11 PM #8
    uninformed
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    IMO fuel oil ratio is not causing you compresiion to jump so much as you say it has.

    I run most my OPE at 40-1 and some at 32-1. With NO noticable difference in pull over compression.

    My neighbour ran his blower at 25-1 for 2 years before it finally had some problems (muffler gumming up was one of them) After I fixed at and set it up for 50-1, it ran fine. Again no noticable difference in pull over compression.

    Look at any Dolmar, Stihl or Husqvarna manual and it will tell you to run their product with THEIR oil at 50-1, if not run a good quality oil at 25-1! Note the mention nothing of retuning to mix.

    Running more oil in your fuel, i.e. 25-1, will result in similar actions as running lean (can lead to sieze)

    Your muffler screen, muffler could be clogged. You could have a partial lean sieze, or many other things.
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