Using a timing light to check a CDI
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Thread: Using a timing light to check a CDI

  1. #1
    Senior Member lizrdbrth's Avatar
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    Just bought an '87, it's acting up and the P.O. asuured you that it just needed a carb cleaning? Or maybe your later model bike is just doing goofy stuff at inappropriate times, like stalling at high or low rpm's, or your bike runs like a scalded cat for miles, then quits. Maybe it won't restart. So you throw a few rocks at it, go in the house and numb your brain by watching Laverne and Shirley reruns er sumpin', come back later, hit the button and she fires right up. What gives?



    So you've come on this site, explained your problem, then followed the excellent advice offered by others and quite logically checked the tank vent, intake boots, float level, etc. Maybe you've even done a total carb cleaning or a total rebuild (or 3). After a month of shopping for answers maybe FINALLY invest in a multimeter. You check the coils and do everything the manual tells you to do and find nothing. You finally cross your fingers, swallow hard, and order up that high-rent CDI unit. You plug it in and life is good. Or not. $300 up in smoke for something you weren't sure you needed. I call this "The '87 Dance" because when it comes to the TW those are the bikes where this occurs most often, but it can happen to any year bike of any make or model and the drill is usually the same.



    What if there was a way to avoid the resulting court-appointed anger management. Diagnosing a failing, intermittent CDI has always been a pain in the butt. I have a low threshold for such agravation, so I did all of the above exactly once before I cooked up this method. It will tell you whether your issue is fuel related or spark related in advance, without going through all that.



    This is a 12 volt inductive timing light. Some of you young'ns may have never seen one. Basically it's a strobe light of high intensity. It's meant to be powered from a car's battery via those two alligator clips. There's a little gizmo that clamps around the plug wire which senses a spark and sends a signal telling the light to fire. It's very instantaneous and is capable of flashing several thousand times a minute, following the rpm. They usually have a trigger like a pistol which turns the light on and off as the trigger is depressed or relesed.







    They're reasonably inexpensive new and in the age of computer engine management and diagnostics they're fast becoming relics. Good used ones can often be had really cheaply at swap meets and yard sales. Since we're gunna modify it for use on a moving motorcycle that's where I'd get mine. For bike use get the smallest, most compact one that you can find.



    They have a lot of other uses, but for our purposes here we just want them to read the spark and flash accordingly.



    I'll take pics of my setup in use and add them to the explanation later so it will make more sense.



    First we need to lose the alligator clips. We need a way to power the light in such a way that the leads won't come loose while you're riding. If you already have battery tender leads on your bike the absolute best way is to cut off the alligator clips and wire the timing light leads to a tender accessory plug, then just plug it in to the bike.



    Next best is to bare the wire ends, loosen the battery terminals and hardwire them directly to the battery posts, then tighten. For one bike this method will get the job done as long as the connections are secure.



    Next we need a way to secure the trigger so that it's always "on". I've bypassed the trigger switch on mine, but in a pinch you can just hold it closed with some rubber bands, zipties or electrical tape around the trigger.



    Then we need a secure way to attach the light to the bike in such a manner that it can be seen while riding. The strobe will blind you if viewed directly, so I usually place it so the light bounces off the back of the fairing, then tape, bungee or otherwise secure it to the handlebars. Alternately you can place it off to the side and facing you in such a way that the edges of the light can be seen, but the direct beam won't fry yer eyeballs.



    Now we're gunna snap the inductive pickup onto the plug wire, make sure your light is flashing, and go for a ride. We need a stretch of road on which we can hold the bike at wide open throttle for an extended period, but we don't need to be in high gear. Any gear will do as long as you can wind it out and keep then throttle wide open when the engine starts acting up. I usually find a place where I can at least hold it in third without being run over. Higher gears are better because it takes longer for the bike to roll to a stop. More time to monitor the light.



    Regardless of whatever the bike may be doing, hold it at wide open throttle, in gear, don't touch the clutch and watch the light as it rolls to a stop.



    Light "on" and flashing consistently with rpm = fuel delivery issue.



    Light "off" at any point or flashing intermittently as the bike slows = CDI/spark/electrical problem.



    It's a bit trickier if it's acting up at low or mid-rpm's. The best way to monitor that is to find the sweet spot, then either twist on it or back off of it until it until it hits the point of failure. Watch the light.



    Your TW uses a wasted spark system. In other words it fires on every revolution of the crankshaft, including the exhaust stroke. It makes buckets of sparks. So basically if the wheels are turning and the clutch is engaged at no point should the light not be lit and flashing in cadence with the rpm, even if it isn't getting fuel.



    Foller dat?



    Same rules as above. No light = no spark, however briefly it may fail to flash.



    At any rate whenever a bike hands me "mystery meat" I may futz around looking for the obvious for a short while but I'll drag out the timing light long before I start tearing into a carb or attacking a wiring harness with a meter.



    If I had an '87 that was acting up I'd definitely take all the fun out of things by going to the light right from the git.



    Peterb, jbfla, youngdub and 3 others like this.
    I can explain it to you, but I can't understand it for you.

    Powdercoated '87 frame, extended swingarm, YZ fork legs, ATV tire, 14/55, XT350 tank, spliced quick-release seat, disc brake conversion, beeg headlight, beeger rack, Lizrdcooler, Lizrdventz and bunch of other stuff all covered in invisible ink.

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  2. #2
    Senior Member jontow's Avatar
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    Solid write-up Russ: I nominate this as a sticky! (we sure do get the 87 dance a lot..)
    --

    1997 TW, well loved, a bit modified.

  3. #3
    Super Moderator littletommy's Avatar
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    Very nice sir!! Yes! Make it a sticky!

    This will help a lot of folks!
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  5. #4
    Senior Member LittleCowTW's Avatar
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    Truly a terrific idea. HA! im picturing you blasting down the 14 with this rig.
    Persistence is the better part of genius.

  6. #5
    Senior Member lizrdbrth's Avatar
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    Quote Originally Posted by LittleCowTw View Post
    Truly a terrific idea. HA! im picturing you blasting down the 14 with this rig.


    Lol. Well, if you've ever done "The Dance" you'd be willing to ride down the 14 backwards wearing a showercap and a dress if someone told you that would help. It's truly frustrating.



    Using the rig isn't nearly the hassle I've made it sound, and if you score an old timing light you're looking at 5 bucks to make it into a usable tool. Even if you have to buy a new one it will cost you less than half an hour of dealer shop time.



    The title is a bit of a misnomer. It won't entirely establish that the CDI is in question. It will narrow it down to either a fuel problem or a spark problem so you'll know which one to chase. If you've already been in "The Dance" and have done all the proper electrical tests it will help confirm that it's time to make the $300 leap.



    No matter what the dealer or your cousin Ernie told you, there is no reliable way to test an INTERMITTENT CDI failure on a bench. If a dealer has told you that he has, he's guessing, same as you. The difference is that he's guessing at 90 bucks an hour on your dime. He's gunna order the CDI, install it and if he's right you'll both be happy. If he's wrong he's gunna dummy up, fix the real problem and sell you the CDI unit anyway and you'll be none the wiser. Of course an honest mechanic will be up front with you and tell you that it's a calculated risk, and leave the final decision up to you, but only after doing all the proper checks.



    There are some other, easier basic checks if you're suspicious of the CDI, as well. They don't always work, it just depends on the nature of your particular failure.



    With the bike parked and running (if it will run at all at idle. Some won't) try taking the CDI unit out of its holder. Manipulate its wires up, down and all around. Turn it upside down and sideways, etc. Listen to the engine. It will sometimes either increase rpm or die completely. Try squeezing the casing with pressure from your thumbs at various locations and see if it changes anything. You may find a "sweet spot" where it seems to run normally.



    Sometimes you getlucky with this, "luck" being relative. If any of the above give you positive results it means that some component inside the evil box has come loose inside the potting. Lots of T-dubbers with fairly sophisticated electronics backgrounds have attempted to dismantle and fix them but so far for we mortals we're still looking at a new CDI.



    I can explain it to you, but I can't understand it for you.

    Powdercoated '87 frame, extended swingarm, YZ fork legs, ATV tire, 14/55, XT350 tank, spliced quick-release seat, disc brake conversion, beeg headlight, beeger rack, Lizrdcooler, Lizrdventz and bunch of other stuff all covered in invisible ink.

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  7. #6
    Senior Member Rainman's Avatar
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    Excellent and solid advice. Thanks for taking the time to right this up. I agree with the others, lets sticky this sucker!
    If you can't find it, grind it

    1990 TW200

  8. #7
    Junior Member esw28's Avatar
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    This helped out a lot as mine was misfiring in neutral while revving as well, I hooked up my light and gassed it a bit and the light spoke the truth, then I noticed if I squeezed my CDI in my hand the bike would rev great, I guess it's new CDI for me

  9. #8
    Senior Member peruano's Avatar
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    Being old is an advantage sometimes. Good job Lizard. Tom
    Tom - TW200 2002, Kawasaki VN 500 2006

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