So now that the MX is finally started, it kicks over and idles incredibly high with no regard to idle screw placement. 

Carb? - It has been cleaned extensively in my ultrasonic cleaner and rebuilt. Air screw is set to 1.5 turns out. 

Air Leaks? This is what i'm thinking too. The idle is so high that spraying carb cleaner around the rubber has no affect, even if it were leaking. Here is a picture of my intake boot from the box>carb:

 

As you can see, with the carb all the way forward, there is an ominous amount of gap that air may be seeping into on the intake boot. 

Is this the correct boot for my 73 mx250? It has "364" stamped on it similar to this one on ebay:

 

But this boot stamped "365" also comes up on ebay and seems like the bend in it may fit my application better:

 



Any insight into the high idle or boot??

I don’t know about mx bikes or the correct part numbers but in general make sure the throttle slide works easily and you hear a thunk when releasing the throttle as it returns home. Sounds a little like it may be sticking or the bore is out of round? Been there with other bikes. Good luck.

That boot will let dirt into the engine.  Would not be a reason for high idle.

Making sure slide closes solidly is first.  Then boot on engine side, intake manifold, base gasket, and crank seals are common places for air leaks.

Oh, 364 is the model code for the MX250.  365 is the model code for the MX360.  So for a 250 you want the 364. 

It's possibly shrunk or something...  Clauss Studios seems to have a lot of good reproductions for various models so you could check with them for a replacement. 

great info, so it is the correct boot, but it's clearly shorter for some reason. 

The carb slide is positively closing all the way and the throttle opens/closes freelly. 

there doesn't appear to be any cracks in the boot on the cylinder side of the carb.

I just ordered a replacement "364" boot off ebay as well as a replacement LH crank seal. 

Quick question about the crank seal though, how would you remove the stator from the crank on the left side as seen below? I'm sure it's just pressed on, but with the magneto plate off I still can't get a 3-jaw puller behind it to pull it out and off:

 

 

Get the correct PVL rotor puller or be prepared to buy a new ignition.  Do not use a jaw puller.

I can't tell from your pictures but thought PVL does what Suzuki did and used a bolt puller...  should have threaded holes in rotor. 

Some auto parts stores rent "steering wheel" or similar bolt pullers for free. 

Here's a link to PVL version for reference.  pvlignitions.com/shop-us/product/rotor-puller/

I don't know if the high idle thing is still an issue [my page opened on that for some reason] but i did cause 'that' problem years ago altering slide cutaway & taking too much away, probably ended up 5 if there is such a thing. So spun some off the base to sort of bring down the cutaway a bit but ended up with the slide bottoming on the needle jet sticking up bit & stopped the slide at more than a fast idle. so that was another "Fail" lol.

I had the same problem. It almost sent me into a busy street. I simply didn’t have the throttle cable router correctly over my tank dampers. I’m sure you’ve already done this and not made the same stupid mistake I did but just something to double and triple check. Make sure the cable isn’t under any tight tension. And make sure you can turn the bars with the idle staying the same in all positions.

It's always good to double check the simple stuff, thanks for the reminder YAMJAB. 

Good to know about the special puller. I'll look at it and either make one quick or buy it. 

Does the rotor press back on with the tightening of the nut then?