1975 cb500t intermittent ignition/firing on left cyl. - Page 2
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1975 cb500t intermittent ignition/firing on left cyl.

This is a discussion on 1975 cb500t intermittent ignition/firing on left cyl. within the Technical forums, part of the Caferacer.net Forums category; So the valves are set vm32 carbs have been set up and bench sync has been done. Both spark plugs have spark. Cold compression is ...

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  1. #11
    Member photoputz83's Avatar
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    So the valves are set vm32 carbs have been set up and bench sync has been done. Both spark plugs have spark. Cold compression is 15 pounds off of what the specifications say it should be warm. Still doesn't run on left cyl. Would the caps do this? Starting to get pretty frustrated. Haha. Any tips or ideas at this point?

  2. #12
    Senior Member Cyorg's Avatar
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    Do both plugs have a nice fat spark? There are cases where there is a spark, but not enough to light things under compression. Caps are worth a shot. At this point, if it were mine, I would introduce some propane into the left side at idle and see if it starts firing on that cylinder. If it starts firing, then you have a fuel problem. Something like maybe a bit of old o-ring jammed up between the idle mixture screw and the low speed circuit. I can't remember if that bike has a wasted spark and I'm too worn out from working on my own shit to look it up. If it does have a wasted spark, you could switch the plug leads and see if the problem goes to the right side.
    Last edited by Cyorg; 08-29-2015 at 09:53 PM.
    "Non urinat in ventum"

  3. #13
    Senior Member parkwood60's Avatar
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    Quote Originally Posted by Cyorg View Post
    I can't remember if that bike has a wasted spark and I'm too worn out from working on my own shit to look it up. If it does have a wasted spark, you could switch the plug leads and see if the problem goes to the right side.
    No, almost all Honda twins are 180 degree with a separate coil and points for each cylinder
    Whitworth - The whole Western World economy runs on total lies and BS: I'm a part of the action.

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  5. #14
    Senior Member parkwood60's Avatar
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    Have you checked the timing for each set of points? Just because they have the same gap, and are on the same plate does not mean they are both timed correctly.

    Also, when you swapped the carbs did you use new fuel lines? If I recall correctly this bike has 2 outlets on the petcock, and a line to each carb. It is possible the line is kinked, or one outlet is clogged or partially clogged. Make sure you have the same fuel level in each carb
    Whitworth - The whole Western World economy runs on total lies and BS: I'm a part of the action.

  6. #15
    Senior Member drgonzo's Avatar
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    Quote Originally Posted by parkwood60 View Post
    Have you checked the timing for each set of points? Just because they have the same gap, and are on the same plate does not mean they are both timed correctly.

    Also, when you swapped the carbs did you use new fuel lines? If I recall correctly this bike has 2 outlets on the petcock, and a line to each carb. It is possible the line is kinked, or one outlet is clogged or partially clogged. Make sure you have the same fuel level in each carb
    switch the fuel lines at the
    petcock and see if the problem moved to the other side.

  7. #16
    Senior Member hillsy's Avatar
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    Quote Originally Posted by drgonzo View Post
    switch the fuel lines at the
    petcock and see if the problem moved to the other side.
    You can do the same with the coils / leads and see if the problem moves.

    BTW - do you have new spark plugs in there?

  8. #17
    Member photoputz83's Avatar
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    New lines and petcock was rebuilt when bike was redone. I will however try the swapping of the coils. Thanks a bunch.

  9. #18
    Senior Member bmartin's Avatar
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    A couple of clues you stated - above 2500rpm's the ignition is fully advanced. Check for spark with another plug in the side that's not running and turn the engine over. If your getting a spark - then problem is coming from the low speed side of the carb where there is no gas flowing through. Gas start flowing through the mains after 1/4 throttle.
    Bob - Palmyra NY
    2 - 69 CB750, 1 Turbo
    1 - 71 CB750

  10. #19
    Member photoputz83's Avatar
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    Brand new carbs, both flowing fuel through idle circuit and at full throttle. Thinking it's an electrical issue. Coils, points, or plug caps. Valve clearance is at .0035. A hair over spec. But just as a saftey precaution to take the chance of premature wear out of the picture.

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