750M1 backfired, now running on only one cylinder

Day before yesterday, I went out to my bike shed and hauled out the '82 KZ750M1 to try to start it. I'd read up some on running sans battery, so I installed a 50k cap and disconnected the fuses for front and rear lights. A couple-three kicks, and we were on our way! It ran great, probably better than it has in a while, especially since it'd been sitting for a couple months while I try to figure out some title issues. Took it for a few spins up the road and back, and it was FUN FUN FUN!

So that evening, I went to put it away. Turned on fuel, ignition, and enricher, and gave a couple kicks. It stumbled on the second kick, and on the third it started, but then gave a mighty CRACK! of a backfire, and stopped DEAD. Oh, $#!t! I kicked it over again, no metallic noises... gave it another kick, and it started... on ONE cylinder. Bugger. It ran well enough on the left, but gave nothing on the right.

I limped it around the yard to line it up to the shed. It stalled once, but started back up gamely... on one cylinder. I got it halfway into the shed, and it died again. I pushed it the rest of the way and left it. No time to work on it yesterday, so it sat.

This morning I got back out there again, FINE-C and kicked a few times. It started up on one cylinder again; the right side puffed air but had no 'oomph' behind it. Going through the fuel-air-spark theories in my head, I decided to pull the right plug boot. As I did so, the right side cylinder fired just as I pulled the boot off! (I know you're not supposed to do this with CDI ignitions, but at this point I'm shooting in the dark anyway)

I put the boot back on the plug, and missed- the boot got hung up on the tip of the spark plug- and the right side ran! Huh! The contact is at least another inch up inside the boot, and here it is, firing while touching the BOOT, not the contact. I pulled it away, the cylinder quit. I put it back on, the cylinder fired. I slid the boot down to the contact... and the cylinder quit. Huh?!

It seems that when the plug boot is firmly attached to the plug, the cylinder does NOT fire. If it is loosely on the plug, or just touching the rim of the boot, it DOES fire. WTELF is this nonsense?

So, a couple-three questions:

1) Are the '82 CSR750 boots resistored, or does it use resistor plugs? Perhaps this is why it fires on the boot but not the socket, as any carbon-tracking in the boot may approximate an appropriate resistor for the ignition.
2) Since the right side does fire under certain conditions, can I rule out the CDI as a culprit?
3) Any other suggestions of what to look at before I do something stupid?

TIA,
Nick

Running it without a battery may have damaged the ignition.

so I installed a 50k cap

Is that a 50,000 uF capacitor? The battery filters the voltage ripple coming from the alternator. That cap may have caused voltage spikes on the system.

It may be something else, but it definitely sounds like the ignition is fouled up.

May also have a bad coil.

Thing about twins is the spark plugs are wired in series, so it's hard to get one to fire without the other also sparking. If you truly are running on one cylinder, might have a fuel/carb problem on one side or low compression.

I would check compression, fuel, and spark on each side first.

Turned on fuel, ignition, and enricher, and gave a couple kicks. It stumbled on the second kick, and on the third it started, but then gave a mighty CRACK! of a backfire, and stopped DEAD..

When my 750 twin did that it was from one of two reasons:

1) bad valve seal let oil run down and foul the plug. As I tried to start it, gas would "wash" the oil off and it would eventually fire.

2) One of the carbs was flooding and so it couldn't fire on one side until I cranked it enough to clear some gas out.

Remember the two plugs are wired in series so if something fouls one of the plugs enough that it can't spark, it keeps the other one from sparking too. Gas accumulates as you crank and when the plugs finally spark you hear the loud BANG that souns like a .357 went off.

The funny thing is, it ran better and smoother than it ever had, on two trips that day! Irritating.

Just got back in from being outside. Man, kicking for compression test SUCKS! What also sucks is finding that BOTH your compression testers are fouled up- one has a bad gauge that won't read anything, the other has a bad hose that won't keep pressure on! Bleh. The best I could get was 60psi from either side. What it is for real is anyone's guess.

Yes, it's a 50kuF, 15V coil, which was suggested in several threads on the subject. I could have gone as high as 50V, but this is the one that came up. Had no problem running on it prior to this issue, so I still don't think the ignition is at fault.

I do know that there's a bad valve seal on the right- when I first got the bike running well, it smoked like a midnight poker game out the right side, until it cleared up. On Friday's run, I don't recall it smoking at all, even though it'd been sitting for almost two months, so it was probably stuck rings, although with 20k on the clock, it could probably use new seals anyway.

The plugs are NGK B7ES- whether or not that's appropriate for this engine I don't yet know, I just know that's what it came with. (just checked the NGK site, that's what's called for) The one from the right (the previously non-running cylinder) was black and wet with gas, so it doesn't seem like fuel flow or air flow is an issue, just spark. The one from the left was a kind of middle-grey, running just a bit lean.

I got tired of kicking the bike to get a compression reading, so I put a plug back in to the left cylinder and hooked it up. Just for S & G, I put the gas-wet one in. I left the right boot unplugged, and threaded the compression tester in to the right cylinder. I kicked and I kicked and kicked and kickkickkickkick<THWACK>"G-d d-m-it!"Kickkickkick<pantpantpant>kick..kick.. kickickick<THWACK>... bleh. I even tried putting the plug in and laying it on the head in case, as you noted, it needed a full circuit to fire. Still nothing. My leg is sore, front and back. It seems Kaw took their kicker design direct from the '65 XLCH.

So, I unthreaded the compression tester from the right and threaded the grey plug back in, and plugged it in, just to see if I could get it to run AT ALL. It stumbled on the first kick, so I pushed the enricher in one notch, and BARROOM on the next kick. It was running again! But wait... it's still shaking like mad, not wanting to idle... but it's running ON THE RIGHT. What?!

I pulled the plug boot off the LEFT plug (the black and wet one formerly on the right, recall), and had the same result as Friday: it would run sometimes with the boot unplugged and touching the rubber, but as soon as I plugged it in, it quit. I plugged it in, then unplugged the right, and the bike died instantly.

So, the problem followed the plug, which leads me to believe the plug itself is at fault. I haven't yet tried swapping the wires, but I think the problem will remain with the plug. The wife is going to stop at the APS tomorrow to get a new set, and maybe tomorrow night we'll get some resolution.

Just got back in again. Swapped the plug wires left-to-right, leaving the suspect plug in the left cylinder. When I FINALLY got it to start, it ran ONLY on the right, except when I pulled the left plug boot and touched it to the plug. So, the problem still remains with the plug. New ones tomorrow night, we'll try it again then.

Try this -

The coil fires both plugs at the same time becasue it uses a "wasted spark" ignition. So, swap the spark plug wires & caps at the spark plugs (leave the connections at the coil alone) so the cap that currently is on cylinder one moves to cylinder two and vice versa. If the problem follows the wires there is a problem with the coil, wires, or caps. If the problem remains on the same cylinder as before the swap the problem is not ignition related. Ed

Ed, that part I already did: "Swapped the plug wires left-to-right, leaving the suspect plug in the left cylinder." The problem stayed on the left, regardless of which plug wire went to that side.

I'm just glad it's not the CDI! I've rebuilt the Yamaha SR and XJ CDIs in the past, but I don't know anything about the KZ CDIs. If it ever goes belly-up, I hope the parts don't fry beyond recognition- they're so simple to work on, it's a wonder more people don't just rebuild theirs rather than trying to find a replacement.

None of the KZ's have CDI, far as I know the last air cooled Kawasaki motorcycle to use CDI was the two stroke triples. The KZ's use transistorized ignition with an igniter. Loudhvx has a website with instructions on building your own igniter using an HEI module.

Now, as to your problem, it doesn't make any sense to me, so I'm probably reading something wrong.

You removed the plug wire from the left side spark plug while engine is running, and it kept on running? If so, then your ignition coil has an internal short to ground, and should be replaced.

I get hung up on that one and can't go any further.

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The boot may have a trace of oil or other contaminant holding moisture and allowing the spark to travel down the boot. When you seat the boot, it allows the spark to short to the base of the plug. Lifting the boot forces the spark to travel to the plug's electrode. If it's the plug, obviously, get a new one.

It may seem like the plug's spark gap would be the easier path for the spark to travel, but under compression, the plug gap is 10 times bigger, so it is often easier for the spark to find another path nearby. When in doubt get new caps and plugs (and wires if they look bad). When it gets running, run it at night in total darkness and look for sparks jumping around on the engine. If there are any, replace the wires.

And as far as pulling the boot while running... don't do that. You'll be damaging the coils and/or igniter. It may get damaged partially, where it will start and maybe even idle, but will fail under high RPM and heavy throttle. That type of failure often gets diagnosed as carbs, but is actually a marginally damaged coil or igniter.
I realize you had to check for spark, but it's better to check with a known gap. When checking, don't go more than 1/2 inch in open air.

You need a different capacitor. 50,000 uF is probably a bit of overkill on capacitance, but the voltage rating of 15v is way too low. Spikes can easily go above that and higher voltage caps are cheap so there is no reason to take the risk. A battery naturally tries to contain voltage to about 14 or 15v, but a capacitor will let the voltage rise to whatever level it wants to. You should get a cap in the 5000 to 10,000 uF range and 50v minimum.

Also, if you have not re-wired the brown wire of the regulator, you need to do that before riding it. There is potential risk of destroying the cap if you don't hard wire the brown wire of the regulator to the red (or white) output wire of the regulator. This will ensure the regulator is working all the time. If not, and you shut off the ignition while coasting in gear, the rectifier will pump full alternator output to the capacitor while no other power is used. This can push the voltage to around 100v on the cap... not good.

Here's the $20 home made igniter Steell was mentioning.
home.comcast.net/~loudgpz/GPZweb/Ignition/GPZgmHEImod.html
Scroll all the way down for the twin's version.

Loud: THANK YOU! Excellent info, especially the HEI mod. I converted my old '82 CJ-8 to HEI, and it was an excellent mod for that. I'll also start looking on ebay for a larger cap- I rather like not having to worry about a dead battery, and since the start switch is kaput, I have to kick anyway, so there's no big loss. Thanks for the heads-up about the plug/boot issue- is there any way I can test the boots or wires to ascertain their fitness, or is it just one of those 'it's old, might as well replace it' things?

Steell, I didn't realize it was transistorized- I could swear the box under the side cover said CDI, but just goes to show what I do and don't pay attention to. I'll test the coil to see if it shows an internal short. I have a few other twin-output coils, can I just swap another one in provided the primary resistance is the same?

The wife brought home new plugs tonight, but other plans precluded installation. I hope to get out some other night this week and get those in there.

Capacitor Discharge Ignition at the Wiki

It's easy to spot a CDI ignition, one wire from the coil goes to ground while the other wire goes to the CDI igniter. The CDI sends a high voltage to the coil which is then multiplied before being sent to the plug. If a coil has a 12V connection then it's not CDI.

Just an FYI.

The 79 KZ750 twin in my avatar pic has a 750M1 motor with the electronic ignition. And I have another KZ750M1 motor under the bench for my next project.