Spark plug gap...Myth? Fact?

Bikeguy Joe · Feb 21, 2010

Here's something I have been wondering for a long time.

Spark plug gap. Consensus seems to say Around .025"

Now, is this from "back in the day" when points ignitions were gapped at smaller settings (.012"-.018'')?
I know with magneto ignitions, the gap is small(er) too.
But, we keep calling our ignitions "CDI" ignitions, which generally have plugs gapped as high as .065".

So, what is the truth? Has anyone tried to change the gap, say run one at .045" or .050"? Why not?

It's winter here, no riding, many questions.....so, what say you on this matter?

The forum is open.

stv1jzgte · Feb 21, 2010

I have a Mechanic mate who services whippy motors etc for the council that i run stuff by and he looked at me funny when i say the plug gap should be .025 he said thats crazy small and recomended at least .050 i settled on .040
Like i said earlier i go through plugs faster than most and this may be why? At .025 it deffinatly starts easier but is slower by 10kmh in the top end.

thatsdax · Feb 21, 2010

Hi.. CDI simply Means.. Capacitor Discharge Ignition. It is electronic . A capacitor is Charged, this voltage is then passed through a step up coil during discharge phase which causes a spark. A Trigger voltage is sent to a Darlington Pair of Transistors which causes this discharge Phase of the Capacitor through the coil causing spark. Two Transistors hooked emitter to base are called Darlington Pairs. Since all these Values are fixed with these 2 stroke motors, the only adjustment you have is with spark Plug gap. You can advance your timing a some by simply lowering the gap. Since these Ignition systems are not real high voltage, I am guessing only 10K volts, I am guessing here.. It may be lower... But.. Since they are not hot spark systems, a lower gap is better. I usually run no larger than 15 thousands on my gap. 12 is good too. This will advance timing a little, and provide a more consistent spark for these little 2 strokes. And..Since these new oils are so good, there is little or no fouling. Mix 32:1 and use a quality 2 stroke oil such as Valvoline. Enjoy the ride...

Retmachinist · Feb 21, 2010

I wonder why it is that everyone seems to use a different spark plug gap and they all have a favorite. I spent one afternoon last summer just experimenting with different gaps and test riding two different bikes (I can only do this because I am retired!) I finally settled on .035. Any other gap that I used had an adverse affect. But yet I have seen alot of people swear by .020 or .025. I guess since no two of these Chinese motors are exactly the same it is just what ever works best on your particular set up. Oh, I always use better plugs than the kit plug. N.G.K. or Bosch!

John

2door · Feb 21, 2010

I totally agree with Duane here but I'm going to muddy his waters a bit. He explains the CDI system accurately and he is correct when he says that the ignition timing can be altered slightly by increasing/decreasing the gap. What he didn't add was that it has been our experience that these engines are not consistant in magneto rotor (magnet) position on the crankshaft. Comparing different engines we have found them with as much as four degrees difference as to where the keyway is indexed on the crankshaft in relation to piston TDC and magneto coil position. This advanced/retarded condition significantly impacts how an engine performs. In other words what works for some may have the opposite effect with another engine. I typically experiment with my engines and out of four, I have found four different plug gaps that work for me. Each engine is different in some respects. .024 to .035 is the range I work with and one engine, my doggiest, likes the smaller gap to run its best. The rotor position on this engine is 3 degrees retarded when compared with my best running engine which performs well with a wide gap, .035. Did I confuse things?

2door · Feb 21, 2010

Three of the four were the same manufacturer. I used a cam shaft degree wheel and a hub that I made on a lathe to attach the wheel to the crank. I did the check several times using a dial indicator to assure I had TDC (top dead center) on the piston. The keyways on the cranks are apparently cut, in-the-ballpark at the factory, probably due to imperfect machining practice or loose tolerances in their milling/broaching equipment. Generally speaking one or two degrees wouldn't be much to be concerned with but four degrees is pushing it some. Ignition timing is a critical aspect for achieving top performance. Altering rotor (magnet) position has been tried by several members with mixed results.

Tom

Russell · Feb 21, 2010

Really good info!

Tad Bit Tipsy · Feb 21, 2010

I just think of what Ferdinand would do, and like my Volkswagen I gap mine at .28, tschüs!!!

corgi1 · Feb 21, 2010

1 match book cover thick for the points and 2 thick for the plug,VW's forever....

bairdco · Feb 21, 2010

i read this this morning, and i've been wondering the same thing. i normally run mine at .025, but i've had it at .018 and .030 with no difference.

so i tried it at .040 today, and still no difference. at least, nothing i could notice.

Creative Engineering · Feb 21, 2010

The plug gap is directly proportional to the output voltage of the ignition system.

I have no idea what the actual voltage is...but I would guess about 20KV.

A good rule of thumb is .012" per 10KV

Jim

Bikeguy Joe · Feb 22, 2010

Food for thought? Yes, this is a buffet!
I will be trying some of my own "experiments" later this year when it's warm enough to ride.

I put the question out there because we always see/read/advise somewhere around .025", which to me is too small.

Duane says .012"! bairdco said .040"!! I say as long as the spark is jumping, and the timing is about right, it doesn't matter.

bairdco · Feb 22, 2010

in my old honda autocross car, i always gapped my plugs pretty small, figuring it would be a faster spark at high rpms. seemed like it was a little harder to start, but as long as it started, i didn't care.

these engines i just don't know. maybe one week i'll change it everyday. gap it so it'll barely start, then widen it till it won't run anymore.

but that seems like a lot of work for a little performance, so i'll probably just keep that in the back of my "things to do to make my bike better that i'm probably not gonna do but maybe i should..." list.

GearNut · Feb 22, 2010

Be careful concerning the "widen it 'till it won't run anymore". If the spark cannot jump the spark plug gap, and the engine is repeatedly cranked over, the spark energy will eventually find a way to discharge itself. That will ruin the CDI unit. They are sensitive little buggers.

Bikeguy Joe · Feb 22, 2010

Good point.

bairdco · Feb 22, 2010

if it didn't start after 2 or 3 times, i'd quit. it'd be pretty obvious why. and then i'd have a smoke instead of pedaling up and down the block.

but i figure if i started at .010 or something, and finished off about .050, and never felt a difference, i'd probably gap it in the middle, at .030. y'know, where it was in the first place...

Creative Engineering · Feb 22, 2010

3. Gapping (taken from the NGK website)

"Since the gap size has a direct affect on the spark plug's tip temperature
and on the voltage necessary to ionize (light) the air/fuel mixture, careful attention is required. While it is a popular misconception that plugs are pre-gapped from the factory, the fact remains that the gap must be adjusted for the vehicle that the spark plug is intended for. Those with modified engines must remember that a modified engine with higher compression or forced induction will typically require a smaller gap settings (to ensure ignitability
in these denser air/fuel mixtures). As a rule, the more power you are making, the smaller the gap you will need.

A spark plug's voltage requirement is directly proportionate to the gap size. The larger the gap, the more voltage is needed to bridge the gap. Most experienced tuners know that opening gaps up to present a larger spark to the air/fuel mixture maximizes burn efficiency. It is for this reason that most racers add high power ignition systems. The added power allows them to open the gap yet still provide a strong spark.

With this mind, many think the larger the gap the better. In fact, some aftermarket ignition systems boast that their systems can tolerate gaps that are extreme. Be wary of such claims. In most cases, the largest gap you can run may still be smaller than you think."

Jim

Bikeguy Joe · Feb 23, 2010

So the question remains...what IS the ideal/correct gap for our engines?

BarelyAWake · Feb 23, 2010

*shrug* I run the mundane average .030, tightened it up a bit to .020 in order to run Norm's epic lights ofn the white wire alone - and noticed not a bit of difference even via GPS statistics... or even with the lights on/off for that matter (save a slight stuttering at idle - thus the gap reduction).

I suspect our lil HTs haven't the tolerances for profound effects stemming from just minor plug gap variances... but w/o a dyno I can't be sure

professor · Feb 23, 2010

If it wasn't for the emissions stuff, car mfg. would have never gone to high output ign. They went that way because lean mixtures are harder to ignite and the big gaps exposed the mix to more spark.
From the sounds of the posts on the HTs, they are rich to begin with, so you likely see no change fooling with the gap.

Spark plug gap...Myth? Fact?

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