huge 2X spark plug gap needed for CDI? WHY?

I have a Gerex electronic ignition, kz1000 1978, and the install manual says:

"Re-gap plugs to .063" - .065"

which in metric is: 1.6mm - 1.65mm.


Meanwhile, the kz1000 factory shop manual calls for a spark plug gap of:

0.7mm to 0.8mm.


Wow man. Big difference!

Funny thing too, because the Gerex install manual says this in the same section:

"Don't attempt to draw a long spark from spark plug! Your ignition coil may be destroyed."

It's like "Excuse me, you just told me to double the normal plug gap then you say 'don't draw a long spark' ??

Any ideas on why this or any Electronic Ignition (CDI) would require a double-large plug gap?

And why then say 'don't draw a long spark' ??

Is this huge a plug gap needed for CDI electronic ignitions? Or should I just stick with the stock plug gap of .7mm - .8mm ??

newOld_kz1000 wrote:

I have a Gerex electronic ignition, kz1000 1978, and the install manual says:

"Re-gap plugs to .063" - .065"

which in metric is: 1.6mm - 1.65mm.


Meanwhile, the kz1000 factory shop manual calls for a spark plug gap of:

0.7mm to 0.8mm.


Wow man. Big difference!

Sounds like baloney to me. I have run an aftermarket CDI on my 1979 KZ-750 for 28 years. Use the stock plug gap. You probably can open the gap up a LITTLE and still run OK because the CDI can put out a little more spark voltage. I see no advantage to doing that.

newOld_kz1000 wrote:

I have a Gerex electronic ignition, kz1000 1978, and the install manual says:

"Re-gap plugs to .063" - .065"

which in metric is: 1.6mm - 1.65mm.


Meanwhile, the kz1000 factory shop manual calls for a spark plug gap of:

0.7mm to 0.8mm.


Wow man. Big difference!

Funny thing too, because the Gerex install manual says this in the same section:

"Don't attempt to draw a long spark from spark plug! Your ignition coil may be destroyed."

It's like "Excuse me, you just told me to double the normal plug gap then you say 'don't draw a long spark' ??

There is a grain of truth there in the warning. A coil's windings are designed to handle a certain amount of voltage. A CDI ignition has the ability to produce more voltage across the coil which can potentially damage it. Worst case is when you draw the longest spark possible since it is firing at the highest potential the ignition can force across the coil, which is probably 2X stock voltage. Stock coils really aren't designed to handle CDI, but most do for a long time. I ran my stock coil about 20 years before it failed, and it may just have died of old age related disease.:laugh:

You are correct that doubling the plug gap will double the coil secondary voltage when the plug fires which doubles the voltage stress on the secondary winding, which will shorten it's life. I would not do it because it doesn't increase performance.

Take a look at small block chevy's. For years there was a fairly consistent plug gap. Then the HEI came along, with the integral coil, and also a much higher spark plug gap. What warranted that? Could it be a similar technology?

I know with older two strokes, the CDI bikes used a different coil vs. the points bikes. Not sure why, but the above posts make sense.

HEI works on the same principle as standard Kettering style points ignitions. It just uses a transistor instead of points, and some Zener diodes for protection while doing away with the condenser.

CDI uses a very short duration pulse but is capable of producing higher secondary peak voltage before the spark arcs (as Bountyhunter said). What they mean by "don't draw a long spark" is probably don't draw a long DURATION spark. Meaning time, not distance. Or possibly they mean to not draw a long (distance) spark longer than what they prescribe.

They are probably telling you to widen the gap in an effort to compensate for the short duration. More spark area ignites a larger flame kernel, same effect as a longer duration spark. If a small kernel spreads quickly, a larger kernel is not needed. While this may be necessary on some motors, a KZ in good condition probably does not need it and won't benefit from it. Keep the gap stock until you know you need to widen it. If it idles smoothly and revs all the way to redline smoothly, I wouldn't worry about it.

As mentioned, a wider gap will shorten the coil's life expectancy.

Something to think about: when the points open, the voltage builds up on the primary winding and couples across to the secondary winding (spark plug side). It increases until the voltage is high enough to jump the plug's air gap and then the coil's energy gets dumped. This means when the plug fires it acts as a clamp to limit the coil's secondary voltage.

Most data shows that a standard points ignition can muster about 25kV or maybe 30kV on a good day, and a good CDI can kick 50kV - 60kV. This is OPEN CIRCUIT voltage, since the spark will clamp it at a much lower voltage in actual operation (I think the plug fires at maybe 5kV ballpark?).

What people don't remember is if you foul a plug so it won't fire, then the ignition does build up to the open circuit voltage which means a CDI is putting the full 60kV onto the coil's secondary winding until that plug clears and starts firing. That is actually more stessful to the coil than running "wide gap plugs" IMHO.

I don't think running wider plugs will increase performance in a KZ. There has been a ton of internet information and BS recently about spark energy and duration because of the new "super duper" plugs with the eaxtra hot spark but I have never seen any credible evidence they do anything. The published dyno tests show "increases" of maybe 0.3% which is actually within the measurement error of the tests.

I always say the same thing: bike makers are dying to get a couple of extra horses out of their engines. If a new plug or ignition technology would do it, why aren't they using that?

The aftermarket ignitions like Dyna increase performance because they can tailor the spark advance curve to the engine and also because their output does not drop off at high RPM. They do increase performance a little but you wuld need a dyno to see it.

I use an aftermarket CDI with original points (which was state of the art 28 years ago when I got it) and it has the advantages of:

1) Points last forever because they only have low voltage on them.

2) Hotter spark when cold cranking.

3) More spark reserve to fire a reluctant plug.

It runs very well and I never have to buy points or condensers.

bountyhunter wrote:

Something to think about: when the points open, the voltage builds up on the primary winding and couples across to the secondary winding (spark plug side). It increases until the voltage is high enough to jump the plug's air gap and then the coil's energy gets dumped. This means when the plug fires it acts as a clamp to limit the coil's secondary voltage.

Most data shows that a standard points ignition can muster about 25kV or maybe 30kV on a good day, and a good CDI can kick 50kV - 60kV. This is OPEN CIRCUIT voltage, since the spark will clamp it at a much lower voltage in actual operation (I think the plug fires at maybe 5kV ballpark?).

What people don't remember is if you foul a plug so it won't fire, then the ignition does build up to the open circuit voltage which means a CDI is putting the full 60kV onto the coil's secondary winding until that plug clears and starts firing. That is actually more stessful to the coil than running "wide gap plugs" IMHO.
........

Excellent -- great info. I have a bkgnd in electronics and while the conceptual underpinnings of a bike's electricals and ignition system are pretty basic -- I've never had the ignition part of it regarding the primary and secondary coil windings and especially the 'clamping' effect of the plug -- explained so clearly -- thanks. Thanks also to Loudhvx for the clarity on the 'drawing a long spark' issue.

Based on everyone's feedback, I'm leaving the stock plug gap for now and will adjust if I need to -- the old plugs had the stock gap and the prior owner said the bike ran great.