~Spoke too soon, spark box dying at higher revs on one side AGAIN!!!


XS400 owner, bike lover
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Port Hadlock, WA
This time it can't be the fuse box either. Evidently the spark box only works when it's warm, so I guess it's bad. I really can't figure what else it can be, that is the only variable, that made it work was when I put it in the windshield of my car to try and get any moisture out. I am just about at wits end with this thing.
Checked everything again. pickup coils fine (it's cold but they show up 130), the coils are fine, spark plugs new, one new cap and one still serviceable one. The only thing I can think of is that TCI box. Yesterday when it worked it was only after getting the thing very warm and then it was warm out. Soon as it cooled down outside same thing.
There could still be moisture in the TCI module. Open it up and heat it with a hair dryer for an hour or 2.

It is also possible that there is a bad solder joint in the TCI module. Find someone who can solder and have them reflow all the connections that are visible when the cover is removed. Should take 20 minutes.

And enough with the hissy fits. You are frustrated, fine. But we don't need to hear it every time you don't get your way.
Well I did make the mistake of jumping in on this damn thing without eating and being obsessed all morning. Had a Hungry Man and things are much better. There is way too much damn truth to those silly Snickers commercials! Same thing I did yesterday tooo, and I've got to stop doing that because my attitude goes in the toilet when I do.

In any case, I did away with the fuse box entirely. The metal was so corroded it was beyond getting truly clean. I just used some wire from an old computer power supply, the fuses, and electrical tape to make a makeshift fuse box, until I can by proper holders and convert it to blade type fuses.

I found one point where the guy had wired some extra stuff to some kind of relay or junction box and it had stripped, giving the appearance of being tight when it wasn't, and I put a computer screw and a motherboad mounting foot on the other side and it's tight now. Then I went over it with electrical tape to make sure itr won't accidentally ground.

So then when I fire it up it's back to fine again. I can't really say whether it was that light lead, or if it was the hot leads from the battery (sprayed them with power lube and worked them in and out a bit to get a better connection), but the good news is that this proves the fault is not in the TCI unit and I can finally stop worrying on that one.

It's frustrating that I still don't know what it was that made it lose so much voltage that it hurt the spark, but it's working now, so I will see how that goes. Got to say that ruling out the pickup coils, ignition coils and TCI unit are HUGE. That's stuff I would have a very hard time affording. As it is, it's probably more frustrating, but at least I know that if it's just something in the harness, with time and patience, I should be able to solve it for very modest expenses.
Yep that was it. I don't know what this unit is, but it has screws on it and the guy wired it in to the system. He ran the main hot from the starter solenoid switch and where that one screws in it will start to feel tight, but it's stripped; any small movement will jar the connection and make it loose. Putting the computer screw and motherboard foot (had no other clean nut and bolt the right size handy) solved the issue. Lots more to do, but it's a runner. Just had a beautiful scenic drive down Chicken Coupe road and it never cut out once!
Yeah, or replace that unit, or even another nut and bolt and locktite later. More than anything though, I need to remember to eat before I go to work on this stuff. I have so much an easier and pleasant time of it if I do!
The most excellent news is, this morning on my little ride around it's fine. That bike and I have a love/hate relationship, but she is really fun to ride. Not quite the power my old Nighthawk 450 had, and no sixth gear, but very nice to ride all the same.
Well, silly as it sounds, there is a lot to that not eating thing. People really do have a better attitude when they take care of that stuff and when I get on a project I want as bad as something like this, it's easy to get a little off track.

Just took it for it's first real ride. about 35 mile round trip down part of US 101, then Washington 20, it ran great the whole trip, no real backfiring to speak of even decelerating down that twisty hill. I'd love it if I had a gauge, but one thing I can say, I am DAMN good at bench syncing carbs!
I tear 'em down, do the jets with carb cleaner (if they don't look too bad), but remember to take your O-rings off (they do shrink back down if you forget), on your needle seats, then I boiled the stripped bodies in vinegar water solution for like an hour turning occasionally.

My trick for bench syncing (I think a lot of people use it), is that there are tiny pinholes where the butterfly closes (idle jet I guess?), and I reference those. Just dial it in until the butterfly valve is as exactly equally distant relative the pinholes on both sides. I would love to be able to do them right, but don't have a gauge right now so this is fine and I got them really good I think this time because it really performed nicely, and idles SO smoothly.
Well here we are AGAIN, same problem. Keep cleaning up wiring and the problem keeps coming back. The flaw almost has to be in the spark box. Gonna try drying it one more time, but if that doesn't work this thing may have to go back up for sale. Though the spark boxes someone linked me to have gone down...
Found a suspect place on the harness! Wow, now I need a wiring diagram that can be read by a human who isn't some kind of Yamaha motorcycle wiring expert!
I have the exact same manual in print, the 82. Almost 50% of the colors on the key are represented by a black dot. How am I supposed to translate it in places where it goes into the harness and I can't find where it comes out?

In any case I think the point is moot. That was the problem. I still don't know what it was, but it was shorting to ground and now it runs all lights work, no loss of spark. It may have been wired in for the auxiliary lights I removed, but I can't tell for sure.