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Thread: 350x decompression adjustment

  1. #1
    DDQ's Avatar
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    350x decompression adjustment

    I know this has been covered before and I have searched and read every thread I can find and found no answer to my question.
    The manual indicates there should be about 1mm of 'free play' in the lever on the head at TDC. Even with the cable unhooked, I can not feel any free play anywhere in the entire range of motion of the arm. Is the free play supposed to be at the exact spot the arm stops moving down from the tension of the spring?
    I took a video and if someone can tell me what the time in the vid is the point there is supposed to be 'free play' I would be grateful


    https://youtu.be/4qGm-u2fmBE

  2. #2
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    It will help if you can pull the valve cover off and take a peak inside the head when you are moving the lever. This will let you see what it is doing.

    What you should feel is the lever should be pretty easy to rotate until it rotates counter-clockwise a short distance. This is the point where a little tab on the shaft you are rotating begins to contact the rocker arm. Just in case you didn't know, this is what the decompressor does: it releases some of the pressure in the cylinder by holding the exhaust valves slightly open.

    The fact that yours is moving so far might simply be due to where the engine stopped rotating. If the cam has the exhaust valves open (in your video) the decompression lever won't have anything to push against. I think in your video, this is when the lever gets to about 2 O'clock position.

    So, watch your exhaust rocker arm and rotate the engine until the exhaust valves are closed. Then, actuate the decompressor until you see the exhaust rocker arm move a small amount (I'm pretty sure that you can do this by hand). If not, just watch until the decompressor just touches the exhaust rocker arm. Back off from there the 1mm. This is where you want the adjustment to be.
    - Frank

    1984 200ES Big Red
    1985 350X (x2)
    1986 350X
    1986 250SX
    1984 Auto-X
    1984 ATC70
    1985 ATC70

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    DDQ's Avatar
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    Thanks! I pulled the head cover and can see what you are talking about. When the flywheel 'T' mark is lined up with the cover mark is the motor always at TDC on the compression stroke?

  4. #4
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    There’s tdc compression stroke and theres a tdc on exhaust stroke/ on the compression stroke both valves will be closed. On the exhaust stroke only exhaust valve will be open.
    Take out spark plug rotate motor/ put finger in/on hole you’ll know when its on compression stroke

    Shep

  5. #5
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    Thanks shep!

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    Shep for Forum President

    Glamy can be his campaign manager

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  7. #7
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    Hey guys, I've been wrestling with a decompression cable problem myself. My problem is that once I get the cable on, it feels like the cable is stretched to the max. There isn't any adjusting to do because it's so tight. Am I installing this thing correctly? The cable wasn't on the X when I got it so I'm trying to sort out someone else's problem.

    Hopefully this won't be a hijack.

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  8. #8
    DDQ's Avatar
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    Quote Originally Posted by wonderboy View Post
    It will help if you can pull the valve cover off and take a peak inside the head when you are moving the lever. This will let you see what it is doing.

    What you should feel is the lever should be pretty easy to rotate until it rotates counter-clockwise a short distance. This is the point where a little tab on the shaft you are rotating begins to contact the rocker arm. Just in case you didn't know, this is what the decompressor does: it releases some of the pressure in the cylinder by holding the exhaust valves slightly open.

    The fact that yours is moving so far might simply be due to where the engine stopped rotating. If the cam has the exhaust valves open (in your video) the decompression lever won't have anything to push against. I think in your video, this is when the lever gets to about 2 O'clock position.

    So, watch your exhaust rocker arm and rotate the engine until the exhaust valves are closed. Then, actuate the decompressor until you see the exhaust rocker arm move a small amount (I'm pretty sure that you can do this by hand). If not, just watch until the decompressor just touches the exhaust rocker arm. Back off from there the 1mm. This is where you want the adjustment to be.
    So the 1.5mm of free play has nothing to do with the return spring. It is when the arm is 1.5mm from contacting the rocker arm at TDC on the compression stroke?

  9. #9
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    You should be able to leave ONLY 1/14th of an inch of play left at TDC.

    In other words, that's ALL the free play you want at the lever, at that time.

    If the cam inside the case is in wrong, or the kickstarter shaft/cam, if the relationship is off at all, none of these adjustments will work...

    I've seen the wrong placement of a thrust washer stop the cam from working correctly.
    All our government does is distract us while they steal from us, misspend our tax $ and ruin our country

  10. #10
    DDQ's Avatar
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    I think I finally get it.
    With the motor at TDC on the compression stroke, when I lift up on the decomp lever and it contacts the rocker arm, that is where the play is supposed to be.
    Essentially, when adjusted, the decomp lever can only move 'up' 1.5mm before touching the rocker arm?

  11. #11
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    Quote Originally Posted by DDQ View Post
    Essentially, when adjusted, the decomp lever can only move 'up' 1.5mm before touching the rocker arm?
    ^ Yes, this.
    - Frank

    1984 200ES Big Red
    1985 350X (x2)
    1986 350X
    1986 250SX
    1984 Auto-X
    1984 ATC70
    1985 ATC70

  12. #12
    Ranvier is offline At The Back Of The Pack Arm chair racerAt the back of the pack
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    There was a post about adjusting the cable while the motor is running. This is what I do and it works well for me.

  13. #13
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    ^ DON'T tell him that!! Something jams up and snaps off it'll be on your call, LMAO
    All our government does is distract us while they steal from us, misspend our tax $ and ruin our country

  14. #14
    DDQ's Avatar
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    So what I did was...while the front valve cover was off the head cover I turned the motor over by hand until I could see the rocker arm was in it's highest position. Then, I adjusted the cable so it had about 1.5mm of free play before touching the rocker arm in that position.

  15. #15
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    No, that's completely wrong. Your gonna break something...

    This adjustment is for TDC on the compression stroke, we all know what that means, rocker arms are at REST!

    Put it at "T" on the compression stroke, adjust valves 1st and then the decompression free-play... And FOLLOW THE MANUAL!!

    IDGAF who says what, do it by the book and it will be right!!
    All our government does is distract us while they steal from us, misspend our tax $ and ruin our country

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