Spark problem

Well fitted my new carb jets tonight and thats certainly done the trick! goes brill now, started first kick and pulls a lot better, seems to be more power and torque and she reached 40 mph no problems. Looking forward to a longer run out tomorrow evening. The old needle valve and main jet were quite a bit bigger than the new ones and I'm sure the main jet was fitted upside down too which would explain why the needle was at its lowest setting (circlip at the top) trying to stem the flow of fuel from the main jet whose metering orifice was at the wrong end of the jet body. Anyway all seems to be fine now (at last!). Just a bit more spit and polish and I'll post some photos soon

Hi, my bike is ST70 6 volts, I want to know how many volts from breaker contact point to ignition coil.

At least 6v.
The currency gets multiplied by xxx times in the decondary coil, so it doesn't has to be a constant or exact voltage.

I changed a new spark plug and a new contact breaker point, when I start the engine , it's has a weak spark, but when I pull out the HT cable off the spark plug(the distance about 5mm), the spark appear and the engine can start. So I think it has some problem of the ignition system.

Did you check the spark plug gap? It should be set up correctly each time you buy a new one.
Also, see if the spark plug connector is any good.

The spark plug gap is 0.7mm and the spark plug is easy to damage. I suspect the volt from contact breaker point to the ignition coil, because it has only 0.8volt when I start the engine, when I accelerate the engine it's 1or 1.2volt, I think its 'not enough volt to the ignition coil.

You're correct.
Could also be a faulty condensor.
Do you see little sparks between the breaker points? (check in the dark)

I checked the condensor and test it, I didn't see the sparks between the breaker point. The breaker point gap is 0.4mm, the ignition timing is original (mark F). Today I change a new spark plug "Champion z9y" (the old one was ND U22) and the engine runs OK, I check the volts from the breaker point to the ignition coil, the volts is same, so I don't know the volts is correct or not.

I always thought the ignition voltage should be pretty much the same as the light output.
Now I start questioning myself... As long as it runs OK, I wouldn't pay further attention to this matter.
But still, I'm convinced 1v is not enough.

The spark plug should be OK, since it's between a grade 6 and 7 (NGK).

The spark between the spark plug wire and the earthing (engine fram, spark plug cap dismounted) should be over 5mm long and blue coloured.

Today I test the bike and everything good, the spark gap between the spark plug wire and the earthing was about 5 to 6mm and colour was blue, but I still have doubt about the volts is too low.