I bought a '75 DT250 several weeks ago and it ran fine on the test ride but got it home and tried to crank it several weeks later and no fire.  Actually with the plug out and grounded (wire wrapped around threads of new plug and connected temporarily to coil mount bolts) it has a weak spark which gets weaker the more I kick it and finally pretty much goes away.  I ran a wire from the black wire coming out of the stator directly to the bullet connector on the coil to eliminate anything in the harness or switches and that made no difference.  I also filed the points just to see if there was corrosion there and that didn't help either.  The fact that the spark goes from weak to VERY weak and then disappears after 10 or 12 kicks is something I've never seen before and after it sits a while it'll usually repeat that cycle.  I'm curious if it's more likely the condenser or the coil.  I have a new set of points and condenser that I bought for my RT1 that has NO fire but don't want to throw parts at the DT250 if it's not necessary PLUS I HATE setting the timing on a Yamaha points type ignition.
 

Disconnect that black wire from the stator to secondary coil and put a multimeter set to 1ms update and 400vdc scale. On kicking over, you should see 200-300 volts peak. If so, the problem is in the secondary ignition coil. If not, the problem is in the stator. A partially shorting condenser could cause the problem, but a source coil is more likely. You could try disconnecting the condenser as a test, but points will spark and wear quickly without it. Source coil breakdown is not easily measured unless you have a megger or hi-pot to test at high voltage.
Alnarv

Intermitant spark is more annoying than no spark at all. Timing won't affect it but dirty points might, or zero gap to the eye, or for that matter very very wide gap off the scale. Damned annoying i know. Crook resistor cap.

I had this issue recently and determined my frustration was made more intense by not being certain I was reading the multi-tester ohms resistance correctly. In the end the parts were inexpensive enough to just replace Coil, points, condenser, plug and non-resistor cap. Everything new and got that fat beautiful blue spark and everything got better from there for a bit. The dry side crank seal was leaking and I thought it threw some oil on the points once it started but turns out I didn't have the points wire tucked out of the way enough so it pulled the points wire off the condenser and I lost spark again. Fixed that and I have a good spark every time now. This brings to mind my 74 DT250 had a poor routing of the points wire also and I wore off the side of the points wire covering, grounding it out once. Might be a good thing to check at your end.

Take the coil off. After your done cooking in the oven throw it in there but leave the oven off. Try the coil after it cools down, if your spark looks good and runs then you know your coil is bad. Works great on the lawn mowers but im yet to try it on a DT250. If you have one of those cheap chinese coils on there those are no good in my experience.