Help me find spark on my SR500

scott s

XS650 Guru
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Rock Hill, SC
A few months ago I picked up a crusty, survivor 1980 SR500. Got it for a great price. The chrome was shot, so I sourced new shiny bits, rebuilt a set of XS650 forks with the adjustable pre-load, got some nice shocks, upgraded the brakes, etc. Picked up a pumper carb and cool vintage exhaust.
I'm well on my way to having a cool Thumper.


I'll post up an "after" when I'm done. But I'm banging my head against the wall trying to get spark on this thing.

First thing I did was trim back the wire on the coil and install a new cap and plug. When that didn't work, I ordered a new coil from Rex's Speedshop.
This one:

When that didn't work, I tried a new CDI unit.
This one:

When that didn't work, I did these tests here:

I can find no fault with the wiring harness or controls. I checked all connections. I checked the harness from end to end and with each component/section. Checked grounds.

Gave up and sent it to my buddy's shop. With his better meter, he diagnosed a bad stator. Sent the stator to Rick's for a rewind and it still has no spark.
Confirmed stator, rotor and flywheel match. Confirmed flywheel/rotor hasn't slipped on crank.

We confirmed that I ordered the correct CDI from Rex's (I did). We asked Rex's to test both the old and new CDI units but they declined, saying that taxes, import duties and fees make it not feasible.

-New correct coil from Rex's (includes cap)
-New plug
-New CDI
-Stator rewound
-Harness and controls checked
-Confirmed stator/rotor/flywheel match
-Confirmed rotor has't spun on the crankshaft
-Checked, tested and cleaned multiple grounds

Now what?
Also, I have wondered about the magnetism of the rotor. When I touch a screwdriver or allen wrench to the rotor, I can feel a slight tug, but not strong. I've worked on a few CB/CL350's and the rotors on those bikes are strong enough to grab a wrench or screwdriver. This one is not.

If the magnetism is weak, what can you do about it?
Also, I have wondered about the magnetism of the rotor. When I touch a screwdriver or allen wrench to the rotor, I can feel a slight tug, but not strong. I've worked on a few CB/CL350's and the rotors on those bikes are strong enough to grab a wrench or screwdriver. This one is not.

If the magnetism is weak, what can you do about it?
Yes, that can indeed be your problem. The CDI box and the coil need to receive energy, in order to produce a spark. I had this issue with a Villiers 2 stroke engine from the 50s. Weak to no spark. I sent the flywheel to a guy that "re-magnetized" it, and the problem was solved. Flywheel magneto with points, are equally dependent on the magnets as are CDI magneto setups.
We're going to do a test to confirm that the stator wasn't rewound backwards.
My mechanic said the flywheel pulls hard when installing it on the crank. I have since been informed that the SR500 flywheel has the magnetism on the inside, and that it's not unusual for it to feel weak on the outside.
We did the test today and the stator is wound correctly. Both pick ups attracted the South end of the compass.
We also checked over a few things that had been checked in the past and simply CAN. NOT. GET. SPARK.

Tried spinning the rear wheel in high gear. Tried giving the coil a second ground to frame. Tried eliminating the ground to the CDI. Tried a different plug cap on the new coil. Tried a different gap on the plug. Tried running power straight to the coil. Checked gap on the low speed pick up. And on and on and on.
EVERY single suggestion that I've been given has been tried. Key switch works. RH control works. Main wiring harness tests good. Flywheel and stator are matched. Flywheel has magnetism.

I, and my mechanic, are truly at a loss. I'm open to any suggestions.
Also, contacted a couple of places about remagnetizing the flywheel. Neither of them can do it (scooter shop can't handle the size, tractor shop can't do flywheels for lighting systems), but I was given some tests to try on the magnets. At this point, I kinda HOPE the flywheel is weak, as that would explain everything. Both shops confirmed that sitting for extended periods can cause them to lose magnetism, and if even ONE of the magnets is bad it can cause issues.
My mechanic is going to check and get back to me.

I'd hate to do it, but the Powerdynamo system may be my only option at this point.
Rex's HTC10 coil tests: Primary 1 ohm, secondary is 8.8 kOhm without the cap, 13.7 kOhm with the cap.

All of the magnets on the flywheel are strong enough to hold a 19mm wrench against gravity.

Low speed pickup appears to be grounded properly to the unit frame.

The tests continue.
I'd say that's good. All of the magnets are that strong.


Someone gave me some instructions for a "pulse test" on the CDI and we'll try that next.
I don't know much about the SR's but does it require the clutch to be pulled in, or does it have any safety components that restrict spark in some way, like the kick-stand being down or something like that?
I was just briefly looking over a few wiring diagrams on the web (there's so much garbage out there, so I'm not sure if the ones I saw are legit...but one them said it was a wiring diagram for a '79 SR500. IF it's correct, it did show a kick-stand safety switch as well as a clutch safety switch. You may want to check and see if your bike has wiring going to the clutch as well as wiring going to the kick stand. If they are there, either bypass them or make sure they're working....may be why you're not getting spark.

Again, if you have a manual, that would be a better source.
I do have a manual but at the moment it's up at my mechanics.

Rex's keeps going back to the stator, specifically the low speed winding. That gives me some pause, because that's the one that Rick's rewound a little fat on the first go round. It was touching the flywheel. We sent it back and he repaired it, but....

Per Marcus at Rex's: "There’s a ring terminal thats integral to the winding that the mounting bolt goes through, check here.

Another thing I’ve seen - the ‘ear’ that the low speed pick up mounts to breaks off. We’ve seen several bonded and even one welded repair that was so poor there was no earth to the rest of the stator! Check the stator hasn’t been repaired here."

He also said the coil tests are good.
"Tried running power straight to the coil."


I can't see any evidence at all here that you have tested and reported back that you have eliminated any part of the system. So the fault could still lie with any part of it and/or have multiple faults. So on a quick read of this I'm mystified. (Its a quick read so sorry if I'm talking out of turn).

Have you bench tested the coil / leads / plug as a system?

If you don't know how to do that, come back.
I do have the owners manual at home and it has a wiring diagram in it. It doesn't show any safety switches and I don't remember seeing any when I was doing all the testing at home. And trust me, I did a LOT of testing before taking it to my buddy's shop.

Damn. Coil tests are good. Cross posted

Yes, according to Rex's and any information I can find, the coil tests good.

Coil tests: Primary 1 ohm, secondary is 8.8 kOhm without the cap, 13.7 kOhm with the cap.

We also made sure the ground to the frame is good and even ran an external ground.
I probably misstated earlier about power the coil. I believe what he did was bypass the harness and went straight to ground and coil.
My 79 SR did not have any clutch or side stand safety switches. Have you checked out the SR500\400 form?
I'm sure you know this but for the sake of those coming after.

There are two windings in the coil. Supplying 12V to the primary coil creates a magnetic field in both coils. Collapsing the magnetic field in the primary coil creates a large voltage in the secondary coil making a spark. So, set up your coil on the bench or bike and ensure your spark plug has an earth by lying it on the cylnder head - if on the bike - or running a cable and croc clips back to the battery if on the bench. If you alternately supply power to the first coil across both terminals and disconnect, as you disconnect the power the plug will spark.

If you get a spark then this coil, ht lead and spark plug set are good. If not then one or more of thse things are bad.

For all bikes of this period this part of the ignition system is always the same. The second part of the ignition system is the bit that supplys and breaks the power to the coil. This can be a cam with a set of points. The opening of the points breaks the supply of power to the coil. Or it can be an electronic device that does the same.

If you checked out the coil with gauge and the readings are in spec the coil should be good. But I like to see things working as I'm a bit of a Doubting Thomas.