After starting it runs nicely and shuts off and won't start back up only backfires on my 1975 Yamaha dt125 b 12 volt gen./starter.  I started another post similar it backfired during start up ,I was told a weak battery could be the cause. 

So Patroll330,did you check the battery then ?
Have you got a manual ?
So does the bike start when it's cooled down?
In the manual there's a simple check for the coil. I suggest you check that too if it's cutting out after that time .
Find the test, get the multi meter and when the bikes cut out ,then do the check. See if you have open circuit when the coils hot.

Watch the points & see if they go like an arc welder as it stops, in which case it's likely the condenser. But if it plain just stops & no spark or erratic & no go, it's likely the coil being dc 12 volt. Battery would work 'i think' down to 8 volts or so before walking home. Is the gene charging in that time of running.

Very good information, yes the points arc when it starts up. I recently changed the points and condenser as this bike had ser up for several years . Rebuilt my carburetor, the points was rusted and no spark . I found out these bikes hate Chinese repop carburetors. I also replaced the brushes as this is a 12 volt AC ??? OR 12 VOLT DC . When you rev the engine while blowing the horn it gets loud around 1100- 1400 rpms then the horns weak when releasing the throttle. Thanks for your help. I need to monitor it after it shuts off , Im thinking bad coil . So 12 volt AC  coil ? 

DC is what you need & hope 'that' is the problem. DC is two wire where ac is single wire. Points do arc very slightly but as i said, if they arc wildly like welder then i'd be sus of condenser or bad connection from condenser to the points. But i think you're onto it with coil--not anything else left. Well, we all say that then find something else that.s never been found before.

I would not replace parts until you get a voltmeter on the battery and see what the voltage is while running.  You'll likely find it low.

Yes , had a charged up battery I do have a manual and its probably a bad coil doesn't have enough resistance .

Also check and see if you can blow air through the breather on the gas cap.
We had the same thing here. it vapor-locked after 5-10 minutes of riding.

Very true those gas and firing issues are similar, my gas tank cap was stopped up but cleared it. 

If the bike has points and starts back running ok for a short while , then you probably have a bad condenser.