Hello All!
Im new to the forum and looking to soon purchase a 1972 dt2 250.
Doing my research I happened upon this website/forum, which has been really helpful for information. I wanted to check if anyone can help troubleshoot idling problems.

The engine runs with starter fluid sprayed into the air filter, but won't idle. When it does start on starter fluid, it revs on what sounds likw full open throttle - which I assume is from the starter fluid being rich.

The seller suspects its a problem with the fuel line or carburetor. He did mention that he had cleaned the carb, just with carb cleaner and used a rebuild kit on the original Mikuni carb. Are there any other reasons that would cause idling issues?

I've attached a picture of the bike and a close-up of the carburetor  


 

 from near Salisbury, UK.

Junk the brassware from the rebuild kit and buy original Mikuni - jets should still be available from either Yamaha or a Mikuni dealer.

Check that the tiny jet/hole in the bottom of the float chamber is clear - I use a 0.4mm piece of copper wire.

Check the condition of the 'O' ring on the setter jet - these are still available from Yamaha if yours is worn and/or brittle.

Agree with everything you say - if the carb has been rebuilt using but not using genuine Mikuni parts you need to take the carb apart and rebuild with genuine Mikuni.
Mikuni alternatives are known to give issues.
If the bike has not been used for a while, it will be a good idea to strip / clean the carb anyway, the aparrrent running on full throttle could be caused by the lighter fluid burning off and effectivley becomming a weak "mixture" which would make the engine race.
If it will not start / run / idle then you definatley need to strip / check carb.

Sounds like an air leak to me. 
cliff

1. Make sure throttle slide is closing... should "clunk" closed. (If slide is not installed with guide pin lined up it will be stuck at full throttle.)

2. Pilot jet in carb could be plugged

3. Air leak.

After reading the symptoms the first thing that came to me is to check fuel flow by starting at the fuel tank then petcock, when I work on a unknown condition bike and it exhibits the symptoms the OP describes I would hook up my known good I.V. fuel tank and keep troubleshooting down the line. If there is a what we use to call a “sequence of operations” I am pretty confident it would start with check fuel supply & delivery first. No need to jump to conclusions and rebuild the engine yet keep it simple. Or as an old wise boss use to tell me K.I.S.S. 

Thank you all for your feedback and suggestions!
That's a good point, to start with the fuel delivery system to see if that fixes the problem. Could save me some time and effort.

And if needed, I'll definitely look to get OEM Mikuni parts to rebuild the carb.

I have purchased the jets and such individually when I had to. I use the OEM part # from the parts manual and search. Usually can find on evilBay.

And about the air leak,This is one possible way an air leak happens.
I see it is a flange mount carburetor.
Flange mount carbs sometimes bend when the bolts holding it on are super tight.
I would take the carb off and flat plain the flange with some wet/dry 1500 grit sandpaper on a piece of glass or something real flat. ( I use a piece of granite kitchen counter leftover.) Tape a whole sheet of the sandpaper down flat, add a little water to the sandpaper, in a figure 8 pattern on the flange. Do that for a couple of minutes and check to see if you have high and low spots. If you do, keep going til it's shiny all the way around and clean it up real good with carb cleaner. Please don't crank down those bolts.Snug is good.
Should be able to hear that throttle snap like Mark T said.
Spray some soapy water around the flange and look for bubbles when you get it running