1984 GPz750 - misfiring during warming up

It could be possible that the crimp where the tachometer gets its feed from the black ignition coil signal wire may have gotten loose, causing a poor connection when riding and also explaining the misfire and poor tachometer reading.

bound wrote:

It could be possible that the crimp where the tachometer gets its feed from the black ignition coil signal wire may have gotten loose, causing a poor connection when riding and also explaining the misfire and poor tachometer reading.

According to the wiring diagram for my bike (ZX750A2, non-US or Canadian model), it is the green impulse signal wire that branches off to the tachometer. 

 

If a failing crimp at that junction also temporarily disables the left coil, it doesn't explain why I sometimes have seen lack of signal to the right coil (unlit LED) during some of the stumble/misfiring bouts. 

So, following @Wookie58's guidelines,  I managed to connect a test bulb (for power supply to the coils) and (unfortunately only) one LED across the low tension pins of the right coil (the other, green LED that I intended to use unfortunately got toasted when connected to the battery, since I sourced it from a drawer with old computer parts, it probably was only rated for 5 V).  

 

During my test runs, the test bulb remained lit all the time, indicating that the +12 V supply to the coils was OK. During the usual stumbling/misfiring the LED sometimes remained lit, other times not. And then the engine died completely, also with an unlit LED. After that it took quite a while until I managed to start the engine again. In-between my starting attempts, I held up the ICU and the wires going into it, and flexed them back and forth. This made the LED light up now and then, and even staying lit for a while, indicating continuity at a time when the ignition pickup coils were stationary - which I think of as odd (?) . . . 
Once the engine started again, it kept running without the usual stumbling/misfire. When I got outside my local built-up speed limit zone, I revved it up to 4-6 kRPM. Then the engine ran as it should for a while, until it now and then began running "rough". Loss of power, reluctance to revv, but not complete lack of power, as when it stumbles/misfires at lower (2-3) kRPMs. During this "rough running" the LED remained lit, indicating that (at least) the right coil was firing, also the rev counter continued to show the RPMs the engine was running at (still firing signal to the left coil??). 

The unfortune of having only one working LED makes it difficult to draw conclusions why the engine was running "rough" in-between. My gut feeling says since the left coil did cut out now and then, but I also wonder it it could be due to failure of the advance function of the ICU. At RPMs over 4 kRPM, the ignition pulses are supposed to be at full advance - if they somehow should drop back to 10 deg BTDC, loss of power may be the result?? See - I keep suspecting an intermittent fault of the ICU. Probably in combination with some dodgy connections in the wiring loom. 

Before you condemn the igniter did you ground the black/white to eliminate the side stand switch etc as a problem 

do as wookie says & check/bypass the side stand switch, a liitle info here on igniter/pickups, if you are any good with solder iron you could replace the darlington transistors in the igniter box. cheap as chips,
RS Stock No.:790-5375P 
ignition pickups & igniter boxes - KZRider Forum - KZRider, KZ, Z1 & Z Motorcycle Enthusiast's Forum
search option on here is good & lou's KZR legacy pages are well worth a trawl IMHO

Wookie58 wrote:

Before you condemn the igniter did you ground the black/white to eliminate the side stand switch etc as a problem 

I tried, but that exercise seems slightly above my pay grade in electronics, since I couldn't find any convenient connection points to place a ground wire to the B/W one. What I did was scrape the insulation layer off the B/W wire just outside the ICU, and connect it to the battery neg post with a crocodile clip.  

 

But with that "ground connection", it turned out that the dash warning lights started flashing when I kicked the sidestand down (bike resting on centre stand). Which tells me the side stand warning circuit was still active - ?  

rangeroy wrote:

do as wookie says & check/bypass the side stand switch, a liitle info here on igniter/pickups, if you are any good with solder iron you could replace the darlington transistors in the igniter box. cheap as chips,
RS Stock No.:790-5375P 
ignition pickups & igniter boxes - KZRider Forum - KZRider, KZ, Z1 & Z Motorcycle Enthusiast's Forum
search option on here is good & lou's KZR legacy pages are well worth a trawl IMHO

Would love to replace suspect components on a circuit board. However, I've already checked, found and flipped off a 'clipsed on' back cover on my ICU, and under it was just a smooth layer of insulating  bitumen (or whatever it is, chemically) - no visible electronic components. 

That said - the parts diagrams I've found  for my bike via < www.kawasaki.com/ > show that the part number for my '84 GPz 750 should be 21119-1075, whereas the one I have seems to be 21119-1069. I also noticed that the one specified for the GPz 750 Turbo is 21119-1120. I suppose the different versions specify slightly different (advance?) timing parameters . . . ? So I cannot really be sure that the one fitted to my bike is the original one. That might affect minor running issues, but shouldn't cause significant misfiring problems. Well, my guess :-) 

OldRetroRider wrote:

rangeroy wrote:

do as wookie says & check/bypass the side stand switch, a liitle info here on igniter/pickups, if you are any good with solder iron you could replace the darlington transistors in the igniter box. cheap as chips,
RS Stock No.:790-5375P 
ignition pickups & igniter boxes - KZRider Forum - KZRider, KZ, Z1 & Z Motorcycle Enthusiast's Forum
search option on here is good & lou's KZR legacy pages are well worth a trawl IMHO

Would love to replace suspect components on a circuit board. However, I've already checked, found and flipped off a 'clipsed on' back cover on my ICU, and under it was just a smooth layer of insulating  bitumen (or whatever it is, chemically) - no visible electronic components. 

That said - the parts diagrams I've found  for my bike via < www.kawasaki.com/ > show that the part number for my '84 GPz 750 should be 21119-1075, whereas the one I have seems to be 21119-1069. I also noticed that the one specified for the GPz 750 Turbo is 21119-1120. I suppose the different versions specify slightly different (advance?) timing parameters . . . ? So I cannot really be sure that the one fitted to my bike is the original one. That might affect minor running issues, but shouldn't cause significant misfiring problems. Well, my guess :-) 

-1069 is the Ignitor.  -1075 is Ignitor + cover box

igniter (off turbo) but same inside, darlingtons 2x black transistors 3 soldered legs each & held with screw
   

I know this may seem off topic but have you done a compression test? 

rangeroy wrote:

igniter (off turbo) but same inside, darlingtons 2x black transistors 3 soldered legs each & held with screw

 

That ICU can certainly be tested down to the individual components : ) Not so easy with the one under the seat of mine, which looks like this under the back lid: 

 

F64 wrote:

I know this may seem off topic but have you done a compression test? 

Not at all; that's part of the basic checks when diagnosing poor running. Along with the valve clearances. 

I did check the compression pressure not long after I got it, ISTR it was between 125 and 135 PSI. Not great, but with a cold engine, and with the carbs in place.

For what it's worth, the engine pulls like a freight train when the ignition works.