Spark plug gap...Myth? Fact?

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.

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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.

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...

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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

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?

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That comparison - was that between engines from different manufacturers, or the same? If the comparison was between different mfr's engines, was there also any comparison between engines from the same mfr or brand?

I love details.

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

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Really good info!

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

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