Well, believe me, if the carb is leaking gas from the overflow constantly its possible you got some dirt in the inlet valve and the carb is flooding. A badly flooded carb will shut you down like a switch. Wont start for **** either. Ive had it happen on my '72 Kaw 350.
Also, remeber this, if its been sitting a while the floats may be full of gas, and they lose their ability to shut off the fuel flow. Ive had that happen too, and it causes constant flooding. WIth your DT I think it had composite floats (plastic) so that may not be a problem.
As for spark, unscrew the plug and hook it to the boot, and lay the bottom of the plug on the cylinder head and kick it over.
Just remeber the basics, as they say- Fuel, Compression, and spark at the right time.
FUEL:
Heres a crude carb function test:
Try a new plug, and of your getting a nice fat spark, try dribbling a little gas down the spark plug hole directly into the cylinder, maybe 1/4 cup, then replace the plug and see if it will fire up. If it fires breifly and dies, then you got a carb problem because what its doing at this point is burning off what you dumped in, and if it dies very shortly, that means the carb is not suppling gas to the engine.
Carbs are not that bad, Ive done somewhere around 30 of them.
FIRE:
DO the plug test mentioned above
COMpression, well it would run, just crappy if that was all, unless the rings just cracked right apart...
Let me know what you find!
The DTs are my second favorite old bike besides the Bighorn!
A buddy of mine had a '76 DT years ago, and I spent alot of time riding it. Awesome tractor like torque!