1 or 2 cylinders not firing sometimes

Ok, fisrt some background. Bike is 1980 XJ650, PO said it has 35ish K km on it, so now it should be around 40. When it came to me it was running poorly, no power, no speed, idle would go to very high when the engine was warm. Discovered that the PO (genious, to say at least) had the cables set up like on the 4cyl car, instead of one coil for 1&4, one for 2&3, first fired 1&3, second 2&4. Sorted that out, did the valves, few were out of spec, they are ok now. Carbs cleaned(the right way), all seals and orings replaced, floats wet set, vacuum synched, diaphragms treated with R6R fluid, no damage on them, put bigger jets on it as it has open exhaust and pods(I know they suck, but there's no way to find an airbox and even if we could we can't mount it as the battery is resting where the filter would go). Pods are mounted on the stock air box boots tho. No leak in the intake boots(checked with propane pencil torch) New spark plugs in, oil and oil filter, fule filter new, petcock good, tank has no rust(new one custom made, all nice and shiny). Fuse box replaced with one from Len, all of the wiring done from scratch(PO did a number on it too, only luck kept the bike form bursting into flames), electrical was done by a qualified professional so no problem there. Small leak on one of the oil tubes that go to the cooler, nothing serious. Bike starts without a problem, if cold enrichment circuit does the job great. Idles just fine, altough sometimes it will hold the revs for a moment before settling down. Pulls great, friend almost fell out of the saddle first time I set the mixture right and he went on the test drive. I don't have a colortune so all the setting is done by reading the plugs. DIdn't do plug chop yet, it's on my to do list. Also, didn't check for the compression as I have no gauge, but I'll buy one if needed.

The problem is that sometimes it seems that it will cut on one or more cylinders. Didn't noticed that when the engine is cold, but rather when it's warmed up, after riding it some. For some reason it feels that, when it happens, cyl #2&3 are affected, but I still have to confirm that(was trying all day today but to o avail). Also noticed that when i remove the spark cable while the bike is running, if I do it on # 1 and 2 cyl, there's a big difference, but not so big on #3 and 4.And when the cable is put back, 1 and 2 start right away, but it seems that 3 and 4(especially 3) don't until I rev the engine a little. As for electrical system, this is what is done, test wise: tried the spark with extra plug, have it on all 4, but it is pretty weak and pale. Granted, I udes one of my Kawa VN900, and that may have some resistance in it, IDK. Resistances, plug caps, removed from the cables, 2 of them had 10K ohm, one 11K ohm, one was showing something like 30 mega ohm.ignition coil secondary both gave 12500 ohm, primary had 2,7 ohm on one, 2,8 ohm on other coil. 4pin connector on TCI had 11,9V on both grey/black and orange/black combo. Voltage on the red/white-black on TCI gave 12V while ignition was on, not cranking. One more thing I noticed, 2 of the cables gave me a little shock therapy when removed them form the running engine(and I wasn't holding the frame ATM) so I suspect bad cables? All of the caps have 10K ohm marking on them, and the one that gave bigger value still wroked when I switched the postition. Also, switching the coils didn't migrate the problem to the other set of cylinders.

I'm sorry for such a long post, I'm trying to give as much info I can because this is driving me crazy. If anyone has any idea I'm all ears.

 

Keep in mind that one coil fires 1/4 and one coil fires 2/3.

Also remember that the resistor cores screw into the plug caps and often work loose.

 

The coils are set how they should be regarding firing order. As for resistor cores, how to check if they are loose? and if they are, is there a fix or the caps need replacing?

 

There's been a lot of "bad coil" posts recently.... check the coil pack and wires for hairline cracks etc.

Also, if you have extra slack in the wires, try unscrewing the caps from the wires, cut off a cm or so, and screw the caps back on the wire.

Another thought off the top of my head, is double check your wet-set fuel level. it could be fuel starvation at higher RPM's

Do a plug chop and let us know what you see.... but I'm really leaning electrical on this....

 

Look in the plug cap where the end of the spark plug goes. On either side of the hole for the top of the plug are a pair of screwdriver slots. Use these to gently 'snug' the resistor core, it simply screws into the plug cap.

If the caps are in good shape, resistor cores can be replaced separately.

 

Please don't pull spark plug wires while your engine is running. The secondary coil circuit needs somewhere for that high tension current to go, and the engine is just another "wire."

However, since the engine and frame act as a ground for the rest of your electrical system, by pulling a coil wire, you aren't giving anywhere for the high voltage pulse to go. It will momentarily raise the voltage on the entire ground, which can cause arcing in sensitive components, like you TCI, "frying" it.

 

Thankx for the help, guys, tomorrow I'll check the stuff you mentioned and get back with the results. I'm also leaning to the electrical gremlin rather than fuel related one. Cables doesn't look that bad, but they show their years, so I was thinking about coil surgery and some quality cables. Also, new coils are also considered, but before going that route I would like to be sure that's the problem.
2 Manbot13: I didn't know that it's a no-no to pull the cable out, that was the wasy I was tought in mechanic school, but it was on the cars. Well, we learn something new every day.
One question, tho. Spark is pretty weak(compared to my VN900 or the friends FZ6), is that normal or it could be sign of a problem? I'm used to strong visible spark, but this one is barely visible only in shadow.
One other thing, not related to the problem but it's bugging me. Is it possible to put the return springs for carb butterfly too tight? When I did the carbs, it was full dismantle, when put back together I didn't noticed that oening the throttle is hard. There is no drag in the cable or the throttle grip, but when everything is put together it's hard to twist the throttle. Only thing I can think of is that maybe , when putting the springs back i put them too tight. Is that a possibility?

 

Did you switch your throttle and clutch cables? I'm not sure if the springs can even be put together too tight, it's hard enough putting them on with enough pre-load, I don't think you can get another go around.

As far as pulling the plugs, that advice is given when compression checking, to disconnect the TCI for the same reason. Maybe in a car there is much less of an affect due to a much larger frame/ground? Maybe it's an effect of the wasted spark system? But if grounding the spark plug, or disconnecting the TCI is necessary for compression testing, I think would extend to pulling the plug while running.

 

No, the cables aren't switched, I'm positive about that. I kinda doubted that I put the spings "too tight", but better to check with people who know these bikes better.

 

Little update on the problem. I haven't managed to check a compression or do a plug chop yet, bad weather and lack of time are killin me. But I took the bike to work yesterday(abou 8 miles trip), after I cut the ends of the spark cables to give the caps good contact. Cold bike, cold start, choke full, starts without a problem, all cylinders firing. If I try to release the choke and give it a little throttle, it bogs and shuts off, which is expected. Rode it to work, felt kinda lazy on overtaking(mind that I'm comparing it to my VN900, which has only 50HP, so XJ should at least preform as good). Other than that, run fine. After work, codl engine again, choke full, no problem starting. On my way home stopped at the gas station(3 mile drive max), put some gas, started the bike, not firing on all 4, by the sound I would say 2&3 are dead. If I give it choke, it wont start at all, without throttle 2 aren't firing. If the throttle is applied while the bike is running on 2 only it won't shut off, but just change the sound(don't know how to describe it). If I start the bike with the WOT, it will fire on all 4, and as long as the revs dont fall down, it will keep running. If the revs go low, sputtering starts, and then I have to shut it off and restart WOT. But when I come home, the problem is not there anymore. It's like it goes when the engine gets warmer.

 

I wondered if you fixed or found out the problem you where having on the issue of not firing occassionally on 2 and 3 cyclinders.
I have a 1982 XJ 650 with the same problem. Although 2 fires up after a few minutes but three is not firing until I run it on the highway. Any help ?
Do you guys think its a coil issue or a fuel issue

 

Sadly, no good news yet. My throttle cable snapped few days ago while testing the bike so I'm waiting replacement. Just like you, I noticed that cylinder 3 is experiencing the symptoms lot "easier" than #2. BTW, is your bike stock regarding intake&exhaust? If it is maybe that could eliminate the pods/open exhaust as the possible source of the problem.

 

I have an 81 650 with the original coils and wires. If I were interested in replacing them do any of you guys have any suggestions on which ones I should order? I've seen Dyna Coils mentioned but JC Whitney lists a few different coils that they claim would work.

 

Check your plug gaps. I had a similar issue couple years ago.

I noticed the bike was kind of slugish at low rpms. Then it became clear that it would run full power after 4-5k rpm.

Polock helped me thoroughly clean my carbs and synch them. We went for a test ride, it still suffered. I told him to ride it. He screamed back and fourth across a parking lot. He returned with a devilish grin and started pulling cables off until one had no reaction. Turns out the plug gap was to large. Tapped it gently on the head and all was good!

 

Plugs cheked and set on 0.7mm gap. Now I'm waiting for the gap in this lousy weather to go for test ride.

Sorted the herd to twist the throttle problem, tho. Got new cable on, still not as smooth like on my Kaw but soo much better than before.

 

Measured the compression today, I had between 5-5,5 bar (72-79psi) throttle closed, 6,5-7bar (94-101psi) throttle open. All 4 were in the range, no big variations. Is that normal or low compression? I am worried that rings are worn because i did the "oil in cylinder" test and that boosted the compression to almost 10bar/145psi on open throttle.

Also noticed one more symptom, when accelerating the bike will pull evenly untill certain point(certain revs, I suspect, but as there's no tach on the bike I can't be sure), then it will kinda sputter, like it's hitting the rev limiter for a second or two(sputter lasts longer in higher gears), and then it would take off like crazy. I would say that the engine is giving about 70% before sputter and full 100% after. And now I finally see why U guys love these bikes, acceleration is plain crazy!

 

the manual for my xj750 says anything below 130 is low.

but i know for sure it is different for different displacements.

i think i read something like 130 is okay for a xj550.

 

I checked the service manual and one Youtube vid, it seems that I should be getting around 150 if everything is OK. So it looks like engine tear down just got on top of my "to do" list.