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Thread: *Clunk*, *knock*, then no compression. And then back to normal!

  1. #1
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    *Clunk*, *knock*, then no compression. And then back to normal!

    Strange thing happened today going to work. I have a new rear tyre on, which is much taller in profile (it's a BT45 4.00-18, closer to the recommended figure which is 4.10-18) than the old one, which was much lower in profile (100/90-18 -- I see these all the time on CB250RSs, can't work it out, as 4.10-18 is a lot bigger, isn't it? The 100/90 worked very well though). Coupled with me fiddling with sprockets recently, the result is 1st gear is a bit taller than I've got used to lately -- so I was very silly and stalled the bike hopping up onto the footpath where I park my bike.

    It didn't just stop like it used to; with the fatter spark, it kept wanting to turn over long past when it would normally cut out. So it made a horrible bottom-end knocking noise, *klonk klonk klonk* and then it stopped. Fine. I'll push the bloody thing to my parking spot.

    On a whim, I then tried to kick start it, in case I had somehow fooked the engine (it was a really horrible noise, I promise). Nothing! Turned over fine...

    So, bump start. Go to pull it back to TDC against compression, but it just kept going. Next to no compression. Getting scared now :P I jump on it down the hill anyway, eventually fires, runs like a dog for a few seconds, then magically it goes back to normal. Ride back up the hill, park it again, starts first kick. Running sweet since then.

    Why?!?! I can only assume losing compression and then getting it back like that is a result of valves sticking open or something? It did something like this once before -- the engine got super hot, then I left it out overnight and it got super (frost) cold. Valves seemed to be sticking open, but cleared eventually by themselves. Why would a horrible knock (detonation? I've switched to 91 and it pinks sometimes when hot at too low RPM when I open throttle too far) cause valves to stick?

    All very strange. Did an oil change tonight for interest's sake, didn't see anything interesting floating in the old oil.

  2. #2
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    26th February 2005 - 15:10
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    I have (very rarely) experienced similar on big singles. Happened last a year or so ago on the Yamaha (interestingly, also just after I raised the gearing).

    I stalled it in second gear, it made nasty clogga clogga noises and died. Thumbed the electric starter, motor turned over excessively fast with more noises. Oh oh , shit. Then fired , and all was well

    Without any evidence or logic whatsoever, I suspect the automatic decompressor system.

    I wouldn't worry about it. Just one of those big single things, all part of lifes rich tapestry. 's called character.
    Quote Originally Posted by skidmark
    This world has lost it's drive, everybody just wants to fit in the be the norm as it were.
    Quote Originally Posted by Phil Vincent
    The manufacturers go to a lot of trouble to find out what the average rider prefers, because the maker who guesses closest to the average preference gets the largest sales. But the average rider is mainly interested in silly (as opposed to useful) “goodies” to try to kid the public that he is riding a racer

  3. #3
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    I thought you might say something like that With the design of the automatic decompression system this motor has, I can't see it sticking open like that; that is, unless the knocking and stalling was unrelated, and it just stuck open when I went to start it.

    Still, like you say, put it down to character. My bike's got character -- the character it plays is a high-strung, expensive, hypochrondriac, needy girlfriend. At least I have fun riding her I suppose.

  4. #4
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    I'll put my money on it just running on a little because of the way it was stalled.
    If it wasn't for a concise set of rules, we might have to resort to common sense!

  5. #5
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    First things first , a 4.10 is a low profile 4.00 , I used to run a 110/90 18 BT45r on mine when I was richer . As to the noise no idea mines never done it but decomp might have stuck down but if its running fine ignore it .

  6. #6
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    Quote Originally Posted by xerxesdaphat View Post
    I thought you might say something like that With the design of the automatic decompression system this motor has, I can't see it sticking open like that; that is, unless the knocking and stalling was unrelated, and it just stuck open when I went to start it.

    Still, like you say, put it down to character. My bike's got character -- the character it plays is a high-strung, expensive, hypochrondriac, needy girlfriend. At least I have fun riding her I suppose.
    Anything automatic or electronic is always automatically suspect.
    Quote Originally Posted by skidmark
    This world has lost it's drive, everybody just wants to fit in the be the norm as it were.
    Quote Originally Posted by Phil Vincent
    The manufacturers go to a lot of trouble to find out what the average rider prefers, because the maker who guesses closest to the average preference gets the largest sales. But the average rider is mainly interested in silly (as opposed to useful) “goodies” to try to kid the public that he is riding a racer

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