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Thread: Cylinder stops firing when bike heats up

  1. #11
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    Typical symptom of a coil failing is it stops firing when it gets hot.

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  2. #12
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    Typical symptom of a coil failing is it stops firing when it gets hot.
    Thanks for the idea to try - Just checked the coils and they read fine on the ohmmeter both before and after heating up though.

    Appreciate the help on this one guys but I'm at a loss to what is wrong.
    I spoke to a mechanic about it and they believe the cylinder is trashed due to the 30 psi difference in compression. Basically recommended I just sell the bike considering I only spent $1500 on it...

    But I have a hard time believing it since it was running fine before parking only a few months back. And the mechanic didn't actually look at or test the bike himself due to them being booked up with spring appointments.

    I guess the issue was around before and the winter tipped it over the edge..? Blows my mind.

  3. #13
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    The ohmmeter doesn't tell the whole picture. One failure mode of a damaged coil is that the HV side will spark internally when hot (as everything expands, insulation is no longer sufficient). A multimeter however can only tell you about the most serious of damage, as has to use very low current and voltage for measurement.

    As suggested by others, you might try swapping the coils as a start.

    However, since it does fire again at 2k-3k regardless of heat, a damaged coil might not be the culprit, but rather fuel delivery. Low vacuum paired with a so-so petcock and incorrect fuel level in the bowl...dunno. Because if a cylinder stops firing and resumes later on, that usually results in a mighty bang because of the unburned fuel in the exhaust. You haven't described that, so...sounds like not enough fuel.

    Also, provide some pictures of what you're describing as 'clogged', in case you go in there again. It sounds a bit like you're confused by the fuel's surface tension.

    I wouldn't mind the 30psi difference too much, at least for now. 120psi is more than enough for those engines to work.
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  4. #14
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    Quote Originally Posted by roeme View Post
    As suggested by others, you might try swapping the coils as a start.

    However, since it does fire again at 2k-3k regardless of heat, a damaged coil might not be the culprit, but rather fuel delivery.
    Did try swapping and same result. Spending more time with the issue I've noticed that the cylinder gradually fails before any amount of significant heat reaches the coils. Or even the carb for that matter. The cylinder will only work for about 2 minutes until it starts sputtering out at idle. You can raise the throttle and get it going, but as time goes on, it continues to sputter at higher and higher RPM's. Basically it doesn't fire at 2-3k after awhile either.

    Quote Originally Posted by roeme View Post
    Low vacuum paired with a so-so petcock and incorrect fuel level in the bowl...dunno. Because if a cylinder stops firing and resumes later on, that usually results in a mighty bang because of the unburned fuel in the exhaust. You haven't described that, so...sounds like not enough fuel.
    What you said here about the fuel level in the bowl set me off to spend a lot of time fiddling with different float levels. I did a float adjustment last year because there was too much fuel in the bowl and it was leaking out. I appeared to fix that issue for a few months before winter but it's possible that I had done the incorrect level all along.

    But last week I did spend time raising the fuel level, testing, raising or lowering again, testing, etc. When it started to leak from too much fuel I lowered it down again.
    Nothing I did changed anything with how the bike fired - it always did the exact same behavior of slowly losing the one cylinder over time.

    And yeah it doesn't have the bang. There isn't any obvious vacuum seal problem from the hoses, petcock, or the rubber boots from what I can see. I also took them off and cleaned them.

    Quote Originally Posted by roeme View Post
    Also, provide some pictures of what you're describing as 'clogged', in case you go in there again. It sounds a bit like you're confused by the fuel's surface tension.
    Here is a picture of what looking through the (pilot jet? - the small one with multiple holes on the sides of it) is like before/after running it for a few minutes. I don't notice any obstruction on other jets.

    https://imgur.com/a/Nvs3MWC - link for both pics because it's only letting me insert one in this post

    carbbefore.JPG

    I'm not certain if that's a foreign substance or the fuel itself doing that. What do you think?

    I could definitely see how it would impair the firing of that cylinder, though. Unless it just gets pulled through on every fire.

    If that IS the issue - I guess I've just not cleaned it well enough. It just doesn't make sense why that would only make the cylinder not fire after a few minutes, and then work again after the bike sits for a few hours. It'd be there regardless.

    Quote Originally Posted by roeme View Post
    I wouldn't mind the 30psi difference too much, at least for now. 120psi is more than enough for those engines to work.
    Good to know
    Last edited by epytus; 05-08-2021 at 12:14 PM.

  5. #15
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    Carbs seem OK, according to previous posts.
    I did not see this issue though:

    Each coil fires two cylinders and you have a problem with only one misfiring.

    Small break in wires could be culprit as the break would close when cold, and open more when hot and even a weak spark combined with compression could still fire at higher RPMs

    Change the plug wires and connector.
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  6. #16
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    Set up a test spark plug with an extremely large gap. See what the max gap each coil will jump with the engine cold. A failing coil normally cant pass this test. The bad coil should have weaker spark, and the color will be different too. Also check the voltage to each coil when it dies, you could be loosing power. If its new enough to have electronic ignition, one of the drivers in the ignition box could be failing when hot.
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  7. #17
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    Quote Originally Posted by metalfab View Post
    Set up a test spark plug with an extremely large gap.
    I know, i am a bit of a nerd :-)
    Love the old Suzuki Service bulletins.
    Like this one : they warn to not let the gap exceed 8 mm or you may blow the coil. (page 5)
    Guess they mean extended testing as they describe removing the side electrode altogether on page 3.
    Last edited by Rijko; 05-12-2021 at 02:42 PM.

  8. #18
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    Quote Originally Posted by argonsagas View Post
    Carbs seem OK, according to previous posts.
    I did not see this issue though:

    Each coil fires two cylinders and you have a problem with only one misfiring.

    Small break in wires could be culprit as the break would close when cold, and open more when hot and even a weak spark combined with compression could still fire at higher RPMs

    Change the plug wires and connector.

    It's a 450. Coils fire one cylinder.
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  9. #19
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    All the electrics talk got me on the right track - it's fixed! But for a pretty shameful reason...

    The battery was bad. Yeah that's it.

    I originally thought this wasn't going to be the case because I tried with the battery on the bike and separately with a car battery jumper hooked up. The jumper is basically just a motorcycle battery in a case, actually the exact same internal battery I have in a scooter I own. So I figured it would have ruled out the battery issue. But nope.

    I bought a new battery because I noticed this one wasn't taking a full charge anymore. Still could crank the bike up but apparently not good enough.

    I'm guessing the battery was losing charge after a few minutes of running. Either that or the cables were coming loose on one of the nodes when it was heating up. Then the cold might've tightened it up again as it contracted. Therefore causing the 2-3 min working time and the working again after sitting for awhile.

    Thanks to everyone who chipped in I appreciate the thought you guys put into it and I probably would still be stuck thinking it's my carb without this forum!

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