XJ750mk just won't stop dying on me.

Fuel levels:
It's hard for me to remember exactly on the nose where the fuel levels were because it's been a year since I had to do it. Now that I'm up and thinking about it I do think that most of them were right around 4mm from the line you measure from. I remember thinking that the numbers I was getting were on the low side but that they were still in spec. All that I have to do is grab some of the clear line I'm sure I still have kicking around, get the bike on the center stand and jack up the front to level out the carbs again, and I can verify the levels, make sure they haven't dropped for some reason.
Church:
What I meant when I said that the shaft seals were the final thing replaced is that it was the last thing I hadn't done. I've gone through the process of cleaning things, buffing and inspecting the slides, replacing fuel rail o-rings, unbuggering floats and needles and passages, with new gaskets and shaft seals all over the place. I simply didn't do all of it in one sitting, rather it was spread out over I think about a week.
Syncing:
I did a bench sync by lining up the plates with the ports in the throat of the carburetor, I think that Polack was the one that suggested the visual approach, which worked better for me than the card one. I'll be the first to admit I don't have a gunson colortune. They're costly and when I was looking around for them I couldn't find them from a reputable source that wasn't further jacking up the price. Instead I relied on an across-the-board setting for the fuel screws, bringing it from some PO's random settings to all 2.5s, and then up to 3 because it seemed to respond a bit better. I then spent a lot of time with a mini mason jar bottle manometer that I built for the purpose to get the cylinders to agree with one another. I think that my bench sync could have been a bit better since it did take a fair bit of work to make it actually smooth out with the manometer, but in the end the result was smooth and reliable.
Valve clearances:
I still intend to check the valve clearances again before the set interval of 5k, I'm running on an interval of 2.5k instead because of how close some of the measurements were, but I still haven't hit the 2.5k mark yet. As much as I enjoy riding, and I do, I don't actually cover that much distance all that quickly. Fuel costs money and there aren't a lot of spectacular places to ride in my neck of the woods and I'm not quite at the point where I'll ride a long distance alone, though the idea of riding the Kancamagus to the north of me has been in my head for this year, assuming I can get things running again.
It was either going to be about towards the end of this riding season or the beginning of the next that I was set to check them, but I may well be able to check all of that sooner, especially since it'll give me the chance to check the timing chain, I think mine might need a bit of love, I remember being able to hear the chain when I got on the bikes accelerator and it might be in need of tightening or replacement.

I didn't realise that a person can just buy the spark plug caps. Is there anything special I need to know about the caps I'd need for the xj750mk, any kind of specific resistance they're supposed to have? Also, they unscrew from the spark plug cable? The one that fell of last year just came off with a tug, and went back on with a good push. I know that the inside of the cable I could see was lady of liberty green, and it's safe to just chop the corroded bit off?
The little fuse holder with clips seems a bit of genius, I think I can work up something like that to check the ignition fuse. I'll need to remember which of the fuses do which in the fuse box as well.
When the bike dies, it's ONLY THE IGNITION. Engine goes out, but the lights are still on, blinkers still blinking if I had them on at the moment. It just seems to me like all of a sudden the kill switch was flipped, all of a sudden there's no combustion happening in the engine which would happen with no spark. Maybe the fact that the engine's still getting fuel mix dumped into it when the spark goes could be the reason it can take a bit to get it to catch again? I know it tends to smell on the rich side after an episode.

 

The plug caps unscrew from the plug wires. You want 5k Ohm caps for the outer cylinders and 10k Ohm caps for the inner pair (though you can just run with 5k on all).
The original caps have replaceable resistors if you find that the caps themselves are not cracked.

 

And all of that makes my butt vibrate

 

Synch your carbs

 

Alright, so I'm trying to do what I can to clean up connectors and the like and at this point I'm thinking I'm just going to replace everything from the coils to the spark plug caps. My understanding is that I need the two coils, about 6 odd feet of 7mm non-resistive wire, and four caps, likely the ngk caps in 5k. I'm curious about the whole thing about the 10k inner and 5k outer resistance numbers k-moe mentioned because that's the first time I'd heard of it, but in the interest of it being harder for me to mess up I may end up going with 5ks all around.

A thing that the neighbors friend, Jonny said is that since the bike goes out like a light, that it could be the TCI box. Kind of makes sense, and it makes me wonder. It could be that one of the coils is faulty and the other one is just weak since it's 83 vintage after all, but he brought up the point that a bad TCI or CDI would likely be able to kill both coils at once, which would match the symptoms. I assume that these bikes wouldn't be able to run with a single disabled coil, that if one wasn't firing at all because of a fault, arcing, what have you that the bike isn't going to run at all on just two cylinders, but I wanted to just run that by the people who know these bikes in particular. Jonny's one of those older guys who've been wrenching since points were the thing you had to deal with but he's more of an american V-twin guy. I'm trying to figure out everything that you can do with a multimeter to test the coils themselves, but is there anything a person can do to rule out the TCI? It'd kind of suck to spent the 100+ on coils to find out it was the TCI.

 

You can mail the TCI to one of us and we can swap it into a running bike. I've done that before, no charge but postage.

 

Replace the fuse box, clean/dielectric all connections, chop the coil wires 1/4" back, add new plug caps and check the plug gaps. Your bike should start and idle without hesitation.

Gary H.

 

The CB750 Coils ,have replaceable plug wires very good feature .

 

so of all the advice you have been given which have you tried to resolve your problems?

 

I work during the weekends so I haven't been able to do much wrenching. I've got the battery on the charger since I killed it the other day. I'm now in the process of going over all of the connections with contact cleaner, now that I'm back at home I should be able to head over to the neighbors where I have the bike parked and in pieces and I can see about trimming the spark plug cables back a wee bit, cleaning up what I can see on the inside of them, see how it runs. I'll then be walking down to the auto store to do something about fuse box, see what else they have that might help me.
I'm going to be taking the multimeter to the pickups and coils, also a few bits and bobs I found from a xs650 forum I can use to maybe figure out anything useful about the TCI before I make the call to ship it off.
I still plan to check the fuel levels eventually but I'd like to make sure the electrical system is sound before that.

 

So I just got back from ohming the caps. Funny business. The two outer caps showed me about 8.4~ when I had the meter set to 20k and for the inners one was 4.8ish while the OTHER one was something like 15! Now I don't know how much variance is too much variance, but I've got the XJ parts catalog ignition page open and the bit where it says:

But those resistors in the spark plug caps........or in the spark plugs themselves, for those engines that use resistor plugs........do wear out over time. And when they do, their resistance increases, which means that they provide more resistance to the flow of electricity than is needed. So that 20,000 volts of electrical energy, instead of being discharged at the spark plug gap in a rather short (10 milliseconds) amount of time, gets "spread out" over a much longer period of time, and gets reduced in voltage, too. So when plug caps or plugs "go bad", they rarely fail to the point where no spark occurs at all, it's just that the electrical output is being "spread out" over such a long period of time that the energy being created in the spark plug gap is so low that it's not enough to fire the fuel mixture completely (or at all)........and that's what leads to hard starts and poor performance (and reduced gas mileage, too).

it has flags going off in my head since I keep hearing that these bikes can get upwards of 45mph while I don't think I ever touched 35 and I swear I'm no drag racer.

I'll be checking my numbers on the caps again, doing more reading on the caps and trying to find exactly where the leads go on the coil and what my numbers should be.
Interesting story I've personally dealt with concerning former girlfriend who left me for no reason, before she left me her car just stopped wanting to do car things one day more or less after we got stuck out in a storm (whats with humidity and electrical issues???), turns out the spark plugs were TRASHED they were TOAST it was so bad that there was no electrode on pretty much any of them and her family wasn't the kind of person to check plugs on the used car they bought. They were so damn bad that they actually fried the coils. ten dollars in plugs and a 40 dollar junkyard part and it ran just fine.
Now my spark plugs are new, they aren't all that old at all, but I'll be pulling them regardless, but I imagine it was the messed up resistance that fried the coils and the high and mismatched resistance of my bikes caps seems like it could be doing the same thing.

 

Alright, had to stop working on bike because a friend needed help with a project, he's building a drawing table and needed a hand. Before spending all the time between my last post and sundown at his place I managed to get the multimeter on the bike again for a few tests. I was right about the plug caps, even used his meter in addition to mine, had him do it too because I don't want false readings.
1-8.3k
2-16.1k
3-4.8k
4-8.4k
Now what's fun is that when I tested the secondary resistance of the coils, when I measured the left coil responsible for 1 and 4 I got 12.1k or so. within the 11k+- range.
When I measured the right coil which supplies life to the inner pair of 2 and 3, I got 4.88m. Thats an M, megaohms. Just a little bit outside of spec, and I don't think there's really a way to botch that reading with how many times I tried to take it.
I tested both by removing plug caps, jamming probes well into the cables themselves of the pairs that go to each coil, trying to get good connection.
I also tested to see if the TCI was grounding the coil (bad output driver) to check that there is a suitably high resistance. I don't know what number is suitable per se, but the numbers I were getting were definitely not in the handful of ohms or less. More like high kiloohms, low megaohms which I guess what must be appropriate.

Also took the face plate off of the TCI, went looking at the board itself with a flashlight and two pairs of sharp eyes and I couldn't see anything that looked particularly funny, paying attention to coldwelds or the like. My friend spends a lot of time restoring old electronics, has his experience with things breaking down for the kinds of reasons given in the catalog but he couldn't see anything apparent.

 

I think you have found your problem. replace the caps with new NKG aftermarket cap plugs,( I would not replace the internal resistors)
replace the bad coil or the pair.
have you tested the pickup coils yet? so often overlooked.

 

You may multiple gremlins...

I would still replace the fuse box as they often go bad with age. Cost is low, and it is good insurance. It doesn't cost much except a little time and few dollars in parts. I used inline fuse holders because I could get them locally on the Saturday I did the work, but aftermarket fuse boxes are also cheap online and are visually a bit nicer.

Working on my bike, it was a series of improvements in performance as I checked, adjusted or replaced things to bring it back to specs.

 

A series of improvements sounds good to me. When a person is catching up to 1-3 decades worth of overdue maintenance it seems pretty much impossibly onerous, the idea of doing every last thing at once. I find it better to make it a process.

I'll likely be getting rid of the old fuse box once I can find a suitable cheap replacement. It'll be something I'm looking for. There may be multiple issues indeed and I'm still planning on checking more of the electrical system before I just run out and start tossing money. I didn't get the primary resistance measured right, going to hit the pickups too because no reason not to.

 

Primary resistance for the coils was fine even though the secondary was way off for one of them. I ohmed out the pickup coils as well, and they were both returning almost exactly the 650ohms they were supposed to give me.
That part's all well and good, makes me think that the coils are definitely bust. I'd just be replacing both of them, whole system, not worth working with any of the old parts since they're obviously funny after 34 years.

The fun bit's where the neighbors friend Jonny had me turn the bike back on and try to tune the idle. Since I'd at least spent some time really cleaning pretty much every relevant contact through the whole bike he wanted to see this hanging idle issue I've got. Long story short, it does look like there's an air leak.

Now, last year when I did the carburetors I had an air leak, seems it was the cause of my issues and it was fixed with throttle shaft seals. I'd isolated them as the problem by spraying wd40, carb cleaner, what have you at the whole surface of the boots as well as the seals and I only got a change in RPM with the seals. The boots were iffy, they always have been, but they were not leaking at all. Today as I was struggling to get it to idle evenly long enough to spritz the coils with water to demonstrate the stall he went after the carbs and boots with wd40 and lo and behold the thing started acting up. Isolated it to the far left boot, seems like the seal between engine and boot itself is the location of the leak. I covered the whole damn thing with RTV black and tomorrow I'm going to spray the things again with stuff and I'm going to see if it's still leaking. If it's gone then it'll get even more RTV on it, if it's still there I guess I don't have much of a choice but to see if Chacal ever got the aftermarket boots back in stock.
I was going to do boots last year, even asked Chacal for quotes but they just weren't available at the time. From the old post I'd made -

I said it, I called it. Not happy about it. Not happy about any of this.

I'll pop back up once I've been able to make some decisions regarding battle plans. In the meanwhile I'm going to relax and maybe peruse the listed bikes on craigslist. I don't think I actually would, but the fantasy is definitely there.
I really just want to ride after all of this time stuck indoors all winter. It's all I want.

 

A hanging idle is only a problem if you ride places that require idling. So just avoid stop signs, stop lights, your driveway, etc...and you're all set.

 

If the people I share the road with didn't seem to brake check me for the sheer chuckles of it I wouldn't mind it so much. You're also from MA, so I'm sure you know how many rotaries there are around here. I live right off of a rotary even, so the traffic is always a thing unless it's about 4:10am, which is incidentally my favorite time to be on a bike anyway, but yeah, I agree. This hanging idle needs to be dealt with one way or another. The electrical fun-times need to end one way or another if I ever want to be able to get onto the highway riding out of town with any kind of sense of ease in my heart, not having to worry about some kind of breakdown where I've got no way of getting the bike home, or even myself.

I just have a fairly well founded fear of problems that become even bigger ones the moment you scratch at them. "oh, I need to remove the ancient intake boots" becomes "oh, guess I have to figure out how to remove FIVE intake bolts that broke flush" and having to consider the expense vs convenience of just buying a new head at that point. "Oh, I need to replace the coils" becomes "turns out the entirety of the wiring harness is so bad that I'd be better off rewiring every last connection on this hairball of the bike myself". I'm not a lucky person. The best that I can do is soak the daylights out of those bolts starting the second I affirm I do have an intake leak tomorrow until I finally attempt removal with the most potent stuff I can find for it, while worrying about it in the meanwhile since it's a part of my nature, and hoping that all that I do in fact need for the bike electrically is coils and their associated odds and bits.

 

Oh ye of little faith.

You are not unlucky, you are blessed with opportunities to work on your patience whilst being meticulous in refurbishing an older machine.

 

Do you use that same excuse for brakes don't work, either?