spark plug gap

help need the proper spark plug gap on a 77 kz 900 stock with electronic ignition

0.028-0.031 inches...or, 0.7 to 0.8mm

0.7 - 0.8 mm per the factory spec. I keep seeing posts where people claim the electronic ignitions should be gapped wider to "take advantage" of the higher spark voltage available but I don't subscribe to that belief.

bountyhunter wrote:

0.7 - 0.8 mm per the factory spec. I keep seeing posts where people claim the electronic ignitions should be gapped wider to "take advantage" of the higher spark voltage available but I don't subscribe to that belief.

Only if you are using aftermarket hi voltage coils. Stock OEM coils then OEM gap. I run 1.5mm .060" gap with Dyna coils. I will run 2.5mm gap .100" with the MSD MC-1 box.

The thing about opening up the gap, it forces a higher secondary voltage on the coil's (which is actually a transformer) secondary winding. The higher voltage is required to make the spark jump farther. However, the design is a flyback converter where the energy is stored in the transformer primary while the switch is closed and then gets released when the spark jumps. Same spark energy is released either way regardless of gap. I'm not convinced a wider gap gains anything.

From what I read and experimented with, slightly, a larger gap may translate to faster combustion since more initial air/fuel is ignited in the larger gap, and the flamefront can spread faster.

Putting the gap back to normal will allow the spark duration to get longer, but longer duration does not translate to faster combustion on a good cylinder with a good mixture. This is becasue the flamefront spreads faster than new charge can swirl into the gap. Thus the extra duration on the spark is just sparking through exhausted gases.

On older engines with poor mixtures (which burn slower) the longer duration can really make a difference since more fresh air/fuel will get ignited as it swirls through the spark. But I don't believe a good, mostly stock KZ would fall into this category of engine.

The downside of larger spark gap is coil life and igniter life. Yes it stresses coils more, and newer coils seem to handle it just fine, but the test period is in terms of decades since the original coils typically have made it 20 to 40 years (or more).

But the often overlooked issue is stress on igniters. Since the larger gap translates to higher secondary voltage, it also translates into higher primary spike at the initiation of spark. That spike is very difficult to measure, but it is in the range of 300 to 600 volts. On the stock igniter, the clamp is probably around 400v to 450v. Raising the spike can eventually cause the output Darlington transistor to fail. There is a Zener safety clamp on the output of the transistor (to protect the transistor), but even that can fail if the spike is too overwhelming. (Learned the hard way on that.)

I think a newer igniter would probably handle the effects of larger plug gaps better than an old one. This is especially true on the HEI modules. I measured those at about 600v, so those should be able to spark through a huge gap. Like many things, though, it's probably a longevity issue rather than instantaneous failure.

Also, obviously , compression ratio is a factor. Higher compression means more insulating air in the gap, and thus higher voltage required. CDI becomes a must if the compression gets really high.

And let's not even mention 2-strokes. They need even more voltage to cut through the grease.

I've been happy with stock gap performance, but may try going bigger for kicks.

thanks for the help, i need a service manuale, is there any available and how much, if anyone knows