Normally a thirty thousandths gap will serve just fine. Make sure you have the correct plug, and then obey the commandments for making two-stroke engines reliable: Clean air Clean fuel Good quality (or better) air-cooled two stroke oil. No "outboard two-stroke oil" - that's for water cooled engines.
If you've been running without an air cleaner, there will be plenty of dirt to live with for a while, and that will end up "bridging" the spark plug gap and the bike will refuse to run properly, or maybe at all.
The correct spark plug for this model is the NGK B-7ES.
Check for loose wires, then most likely a bad coil or rusty mag maybe bad cdi or stator
1974 yamaha dt 125
One at a time
i am pretty sure it is 1974
If it's points or a Dyna S ignition you set the plugs at 28, other wise you set the plugs between 38 - 40
2 cycle.... since it a 2 stroke
what is the calibration of the spark plugs in a forklift clark 4 cyl. 1974
The dwell for the points should be 30 degrees. The spark plug gap should be .040".