No spark at plug after drop

[avemachina]

Longtime listener, first time poster. I've been stuck on a headscratcher for weeks with my '81 XS400 Special II.

I finally got it running, and riding (!!), and wanted to try out kickstarting it--I've never successfully done that, so why not, right? Well, why not is because my dumb self managed to drop it onto the ground (on the right-hand side). Scooped it up, tried again a day later--yes, dropped it again, same right-hand side, on my leg too. Some things aren't meant to be.

Long story short, I can't get the old boy to start again. The electric start will crank all day long, but no spark happens at the plugs.

The most obvious problem is that neither coil, despite being in range of the manual's specs, produces a spark. I've tested both the old-school method (plug against engine ground) and with an inline bulb-type tester.

Post-drop first aid:
- Changed the spark plugs.
- Disconnected the airbox hoses to let the (stock) filters breathe in case gas got on.
- Cleaned the carburetor (clean-clean, with spray carb cleaner, then carb dip, then ultrasonically)
- Left the plugs out of the cylinders to let anything that may have flooded it evaporate, then puffed a bit of compressed air in just in case.

More troubleshooting:
- Tested compression (120ish PSI on both cylinders).
- Changed wires and coils (4 ohms, 10.5k/10.2k ohms).
- Tested with another CDI.
- Replaced the starter/killswitch module in case it was damaged from the fall.
- Confirmed the magnet thing that spins above the stator is inline with the top metal bar at LF.
- Confirmed timing is OK.
- Confirmed valve clearances are good.
- Confirmed battery voltage and continuity is good (14.7V on a lithium battery)
- Confirmed continuity on fuses for Main, Ignition.
- Verified I've got fresh, clean gas flowing freely to the carb.
- Confirmed all wires from the CDI have continuity at their endpoints.

What could I be missing? I've searched and researched and tested and swapped, but I'm not even sure where else to check!

(edited; it's not a point system, so it's a CDI and not a TCI)

 

[CaptChrome]

Welcome to the forum! Maybe try the kickstart for the next few times while the bike is on the center stand!

You have done quite a lot of troubleshooting, and after reading the list, I get the impression you should focus on the ignition system. Just to avoid confusion, the bike does not use a CDI (capacitor discharge ignition), but has a TCI (transistor controlled ignition). Dropping the bike shouldn't necessarily cause a problem with the TCI, but based on everything you have checked, verifying that the TCI works is going to be essential at this point.

First, do you know if the spare TCI box you swapped in is a good one?

 

[avemachina]

Welcome to the forum! Maybe try the kickstart for the next few times while the bike is on the center stand!

You have done quite a lot of troubleshooting, and after reading the list, I get the impression you should focus on the ignition system. Just to avoid confusion, the bike does not use a CDI (capacitor discharge ignition), but has a TCI (transistor controlled ignition). Dropping the bike shouldn't necessarily cause a problem with the TCI, but based on everything you have checked, verifying that the TCI works is going to be essential at this point.

First, do you know if the spare TCI box you swapped in is a good one?

Thanks, I'm excited to be here! Hopefully I'll get to a place where I can give advice instead of needing it I'll give the kickstart a whirl when it's not so... able-to-fall-over-onto-me.

I'm actually not sure if it's a good TCI box--just a random used unit off eBay. Juuuust in case, I've gotten another spare that should arrive sometime next week. Third time's the charm, right?

I got an intermittent spark briefly on the LH plug from the TCI it was "born" with after swapping the electrolytic caps and reflowing the solder joints, but that's no longer the case. It's been a pretty unscientific process, changing handfuls of variables until the thing goes, and I'm going to challenge myself to just leave the poor thing alone until the new TCI gets here. I'm beginning to think the failure after the drop was coincidence and not necessarily causation.

The no-run issue is an old one I've been fighting a while. I got this bike as more or less a barn find six years ago, and spent the intervening time bringing it into the modern age and getting it roadworthy. It needed just about everything that wasn't inside crank case freshened up, replaced, adjusted, or cleaned. Thanks to the expertise of the good folks on this forum and the manual, I DIYed every fix except getting the rear tire on. After restoring it that long, I got it running about a month ago, and that little jaunt around the neighborhood felt amazing!

 

[avemachina]

Well! I've been diving into manuals, and I think I'm going to give the whole system a workover, because it's not adding up that every component is "fine" but isn't working. Back to basics!

A few things stood out to me--the Haynes manual says the XS400H that I have might be specced for 3 ohm coils, not 4. The plug gap is a minimum of 0.24in, and I think I've got mine much more open than that. Check the wiring connectors, etc. It's frustrating, but I'm not giving up.

And for posterity, I made this little signal flow diagram to track the spots I'll need to test.

 

[CaptChrome]

Back to basics!

Yup! Being detail oriented will be the key. Like the diagram also, just saying because it's my jam.

 

[avemachina]

Been working at getting basics squared away, and one of my big a-ha moments was that my spark plug wires might be just plain wrong. I was getting 14.4-20kOhm secondary readings depending on how I wiggled them, so I ordered up some copper ones that should be less inherently resistive. Meanwhile I'm going to play with some leads that are within spec, but a bit short and ugly.

Plugs are at 0.025in gap, and coils that rate around 2.8ohm are now in with 10-10.2kOhm secondary resistance.

Tested the rectifier as well just in case, had proper diode behavior across the board with only one-way continuity.

Since I have two TCIs now, I took the one of them and gave it a full refurbishment. I gave the circuit board a bath in 90% isopropyl alcohol for about, oh, 20 min total to help remove the rosin from the solder points so it wouldn't contaminate the new joints (there was also this gross white... skin? of wax I peeled away), desoldered and reconnected all components, replaced the electrolytic capacitors, and replaced the two transistors with Darlington ST901T transistors from DigiKey. Mouser had them too, but shipping was less from DigiKey. Everything should be in order there now.

Dying to try it all out over the weekend.

 

[gpounce32768]

Check resistance of crank position sensor coils- orange and grey leads on the TCI. They should both be in the 100 to 600 ohm range. If one or both are shorted or open then there will be no spark. My '82 had one that would fail open once the engine got warm enough, causing the engine to instantly go dead until things cooled off enough.

 

[avemachina]

Check resistance of crank position sensor coils- orange and grey leads on the TCI. They should both be in the 100 to 600 ohm range. If one or both are shorted or open then there will be no spark. My '82 had one that would fail open once the engine got warm enough, causing the engine to instantly go dead until things cooled off enough.

Thanks for the tip! I'll add it to my checklist.

 

[avemachina]

Y'all, still no go. New copper wires came in, got them in the right spec (2.7ohm primary, 8.5kOhm secondary). Continuity's all good throughout the ignition circuit I posted earlier and the crank position sensor coils.

Next thing on the checklist is the battery itself, the spark plugs (I wonder if yanking them out, putting them back in, doing all those little tests might have affected them), or something with the pick-up coil? To be continued.

Edit: it's called a pick-up coil, not thing. Still learning.

 

[avemachina]

Y'all, still no go. New copper wires came in, got them in the right spec (2.7ohm primary, 8.5kOhm secondary). Continuity's all good throughout the ignition circuit I posted earlier and the crank position sensor coils.

Next thing on the checklist is the battery itself, the spark plugs (I wonder if yanking them out, putting them back in, doing all those little tests might have affected them), or something with the pick-up coil? To be continued.

Edit: it's called a pick-up coil, not thing. Still learning.

Maybe I should clarify why I think the PUC is worth checking: mine has oil all over the cloth pushback wires, and I read a bit that sometimes the magnet gets "overwhelmed" by the reg/rect if it's bad (albeit from the XS650 forums, but it's the same idea). It's such a cheap thing to check and swap if that's the problem, so no harm done to try it. The test they say is to reg/rect and see if I get spark.

 

[gpounce32768]

here is the thread I wrote on the pickup coil drama I had with my '82 Seca

https://www.xs400.com/threads/air-lock-in-fuel-line.20919/page-2

 

[avemachina]

here is the thread I wrote on the pickup coil drama I had with my '82 Seca

https://www.xs400.com/threads/air-lock-in-fuel-line.20919/page-2

This is awesome!

 

[avemachina]

Reg/rect was fine. New pickup coil arrived, so that's going in this weekend, and one thing I need to check--that my coil packs are wired to the right connectors! I found a few threads where the problem was that someone had the coil wires swapped (left cyl coil fires right cyl plug). Considering how many times I've taken the coils off and put them back on, it's possible...

 

[avemachina]

SUCCESS! Y'all, we're sparking, starting, and running again. It was a wiring fault! I traced every wire on the TCI I rebuilt for continuity, and the black ground connector had gotten loose. I've never been so relieved to hear no beep. I should've trusted my first instinct--that the parts were all good--because they were, it was the wiring itself.

The moral of the story is to make sure your signals go to the right place, and your multimeter is your best friend.

For reference, the from-to spots are (dominant color listed first if there are two)
- TCI orange -> ignition coil orange
- TCI redwhite -> ignition coil redwhite
- TCI grey -> ignition coil grey
- TCI black -> ground wire to frame
- TCI blackwhite -> rectifier black
- TCI whitegreen -> pickup coil whitegreen
- TCI whitered -> pickup coil whitered

I guess now I just have a bunch of bonus parts, and a known-good TCI in case something like this pops up again. Huge thank you to everybody's suggestions!