Hey all you confused motorbike builders, I found a link that has good information on the electrical usage and requirements of LED diodes (lights),
putting away some of the mystery of WHY THE WHITE WIRE WONT RUN YOUR HEADLIGHT!!
L.E.D Basics; gaining an understanding of how to work with L.E.D.s
TOO BIG OF A BULB AND YOUR USING TOO MUCH ELECTRICAL OUTPUT AND
THERE'S NOTHING LEFT FOR YOUR SPARK PLUG TO USE! HENCE THE ENGINE DIES WHEN THE LIGHT IS TURNED ON.
hope this helps! it sure opened my eyes.
If you read instructions from various kits you will see that some recommend the kill switch be across the blue wire to ground, others say put the kill switch on the WHITE wire. Either way works because grounding out either of those wires grounds out the magneto and therefore you have no spark.....
So, if you put a higher wattage light bulb on the white wire, before it warms up it appears electrically as a short circuit, a short circuit to ground on the white wire is..... a KILL SWITCH! So that is why bikes don't like to start with higher wattage bulbs. This applies mainly to filament style bulbs, which electrically appear as a short circuit until current starts to flow and warms the filament, then the resistance goes up and it no longer appears as a short.
In my experience 6V LED lamps start off with enough resistance and so this isn't really a problem. I run 3 18LED 6V flashlights (54LEDs) with 6 AAA NiMH rechargeable batteries on my white wire, never had a problem starting and it has never effected how my motor runs. I don't run them straight off the white wire though, I built a very basic filtered ac to dc power supply circuit and put it on the white wire before the load.
When I had the top end off my motor I connected a drill to the crankshaft and put an oscilloscope on the white then blue wire while I turned over the engine with the drill to see what kind of output I was getting. I calculated there was roughly 1.5W of power available from the white wire at the speeds my drill could turn the engine. These were closer to idle and slow riding speeds, at top speed the power available from the white wire should go up a bit....
So, keep it LED and at 6V, under or around 1.5W and you should be OK. You can also put a switch on some of the bulbs that consume too much power to let the motor start, and turn them on after the motor is running, that works well with some bulbs....