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Yamaha Sc500 No Spark

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Can anyone help I have a 1973 SC500 but cant get a spark, I have power up to the coil but when the coil wire is connected it looses it. I tried another coil and same problem. If I un plug wire power back again?????? :cry:

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Just tried a different cdi modual and same thing, does anyone know the specs on the output from the generator and pulser coil.

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On my 1975 YZ 125 I had a problem with my coil, my boss said he had a similar problem, he took an old piece of metal, shaped it to fit over the coil using the same two screws holding the coil and it fired right up weird, but it worked.

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Does anyone have a wiring diagram for this model SC500

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Just tried a different cdi modual and same thing, does anyone know the specs on the output from the generator and pulser coil.

 

 

You test both the signal and exciter coils for a certain amount of resistance.  If I can dig up my factory manuals I can give you the test procedure.  Chanced are the insulation has broken down just enough to make them nearly out of spec and producing a weak spark.  I had this problem on my Big Horn (as seen in my video series) which works similar to the SC500 (I also have one)

 

EDIT:  I have the wiring diagram and the magneto coil resistance values for your SC500 as well as the MX360/MX250 up on my website (click the link)  The testing procedure should be self explanatory. 

 

Service Manuals

Edited by thejunkman

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Great Info  Just checked mine here are the results     F145-01   brown to black =174 and red/white to green/white =80  Is this the problem?  Also is there a check for the cdi box. How Much out can you go to the original spec.

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I too own a sc500.The BEST thing you can do to these bikes is an ignition upgrade.Even if you get your old stuff to work the advance curve is wrong and the old cdi boxes are terribly unreliable.Late 70's yz400 stuff works good or PVL.If you plan to use the bike much you wont regret it.

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Great Info  Just checked mine here are the results     F145-01   brown to black =174 and red/white to green/white =80  Is this the problem?  Also is there a check for the cdi box. How Much out can you go to the original spec.

 

Yes this is your problem.  The exciter coil (brown/black(is not providing enough resistance to harness the magnetic field created by the magnets on the flywheel as they pass by.  Likely causes are just age of the paper and varnish insulation between the wires and the anode has broken down enough to allow current to "leak" to a ground.  It appeasers that your signal coil (red/white green white) is "ok" FYI the signal coil is like the breaker points providing a signal to the CDI box to tell the ignition coil to "fire".  Bottom line is you will need to find  replacement coil(s) they can sometimes be hard to find but they do come up (evilbay).  Remove your flywheel and observe the wiring so you know which coil is which.  Replacement will require a little bit of soldering.

 

 

I too own a sc500.The BEST thing you can do to these bikes is an ignition upgrade.Even if you get your old stuff to work the advance curve is wrong and the old cdi boxes are terribly unreliable.Late 70's yz400 stuff works good or PVL.If you plan to use the bike much you wont regret it.

 

This is not a bad idea either (can be more expensive).  Also if you find a first edition Clymer manual it has a "performance" section that has other compatible ignition upgrades to achieve the same thing.  Only the first edition has the performance section, subsequent publishing remove it for some reason.  I would also pickup the specific factory manual as well (covers the MX250/360 too) as some things the Clymer manual omits or is just confusing because they condense some of the "Enduro" models in there as well.

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I will see whats for sale ive seen a mx360 setup but minus the cdi so if i can get cdi to match i will try that or mybe look into other setups.

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Use a peak voltage meter and check the peak voltage at the wire meeting the coil.Should have somewhere in the park of 35-40 volts.If its nowhere near there you have either a exciter coil thats just not charging the cdi good enough.

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Use a peak voltage meter and check the peak voltage at the wire meeting the coil.Should have somewhere in the park of 35-40 volts.If its nowhere near there you have either a exciter coil thats just not charging the cdi good enough.

 

As stated in my post above doing a resistance check on the coils (per the manual specs found on my website) does the inverse of this only it is a more accurate test as this is how the factory manual explains how to determine if the exciter and signal coils (sometimes labeled as pulsar and charging coils) are in good shape.  Basically improper resistance too low or too high will not induct electricity for the system.  For other SC500/MX360/MX250 owners with spark problems that information is found here http://www.thejunkmanadv.com/repair-and-service-manuals.html

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 Also if you find a first edition Clymer manual it has a "performance" section that has other compatible ignition upgrades to achieve the same thing.  Only the first edition has the performance section, subsequent publishing remove it for some reason.

any chance you could post the performance section for mx/sc models?  i have the later edition and it's lame....

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Keep checking the website every once in a while, I will try and get more information up.  It takes time to scan, crop, and upload.  My site is a hobby site.

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ok an update bought a full m100 cdi setup off a 1975 yz 360 and it sparks ok just need the rotor nut and I can try to fire it up. Does the front of the exhaust just sit against the flange (nut ring) on the cylinder or does it push into it ? mine just mates up with nut ring and spring holds it against it.

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