All good advice...run 32 to 1 yamalube. Pull the hose off the carb (plug nipple on card) and let the bike run until you see constant oil out the hose. let me tell you...it isn't much oil but it is consistant. Most of the old DT's were jetted way fat...like 4+ steps fat. That is to your benefit right now.
If you haven't cleaned the carb, you should asap. My 2 78's (DT175 and DT250) start with 1 or 2 kicks and run clean (little to no smoke). A handful of jets, a clean carb and a little time works wonders. The 250 was a pig to start until the rejetting.
As for the running...there is a screen in the bottom of the petcock, the cap screws off with a 10mm wrench. See if it is plugged with debris (rust). This will happen with one that has sat a long time them made run again. Over an hour or so enough fuel passes through the screen to fill the carb, it starts, runs the carb dry, then won't start again until it slowly refills the carb. This is easy and quick. Also, pull the fuel line, turn on the petcock and see how much fuel flows...bet you it is nil.