He are a couple pictures of the bike and the $30 carb I have been trying to use. 

Says Mikuni right on it, but I've never seen one like it. Looks way newer than what you need.

OMG, I have no idea what that is.  I will try to take a picture of what the setup should be.  I have a 68, 70 and 71 and that carb ain't close.  the main jet cannot possibly  be enought to run a 250 especially with a 1/8 fuel line. 

 

Ok. it's a 4 stroke carb, a common Honda carb hence the small hose fitting. Bought a new Chinese 'very cheap one for my Honda 185 & ran perfect out of the box. Anyway, if you look at the flange you'll see the mount holes are offset from the center So ya lucky it worked without letting air past either underneath or above--can't remember which way its offset now. All Hondas inlets are like that. Choke is just a butterfly strangling type. I know it says Mikuni & normally are Keihin carbs but still think i'm right lol.
Just to add, it'd probably work at a pinch if the manifold sealed & also if you jetted it up heaps but slide needle jet might be too lean even on richest setting, & pilot jet--ok forget i said it. 4 strokes run way leaner jetting.

This type of carb but not quite. Did you open out the mount hole to make then into slots or bought it like that. Ofset up & down is there but not that obvious as not a straight on pic. 

Yeah. I cut the holes open to fit it to manifold and studs. I had no idea it was for a 4 stroke. That explains a lot. That carb is never going back on the bike. I have wasted enough time.  I like to think of myself as a better than average DIY home mechanic. However, right now I feel pretty stupid. Oh well. At least it enabled me to actually get it running well enough to ride it around my neighborhood and verify that everything else works. So, I'll chock this up as a good learning experience. 

I'm going to take the advice of RTD2 and contact Sudco on Monday morning. As it turns out they are only about 20 minutes from my house. A buddy of mine called them on Friday and apparently they can assemble and jet a carb for my bike in an hour. Hopefully I will have it running Monday afternoon. I'll let you all know what happens. Thanks again. This forum is a great resource. I should have used it a long time ago. 

Progress report:  I bought a carb from Sudco. I talked with a guy named Chad. He was very helpful. I installed the carburetor and got the bike running. I immediately knew this new carburetor was better but I knew I still had other issues. The bike backfired a lot. I finally pulled the head and properly set the points and no more backfiring. Now I am trying to dial it in. I have played with the air screw and moved the needle pin up one and down one clip position still runs like it has a fouled plug. This has the head that holds two plugs. I put new B8ES plugs in it when I started to get it running. I have sand blasted them a couple times. So I am going to get new plugs and see where it takes me. At least at this point I can see the end in sight. 

I have a question for RTD2. You mentioned that you purchased a carb from Sudco for (I think) your 68 dt250 and it ran great out of the box... What was the pilot jet? What was the main jet? 
The carb I got came with a 30 pilot jet and a 160 main jet and the needle pin is on the center clip. The original carburetor had a 35 pilot and a150 main. Just curious. 

Thanks. 

One more thing I would like your thoughts on. I am not using oil injection. May I ask what you guys mix your fuel/oil ratio at? I am currently mixing at 40-44/1. 
Thanks. 

I've possibly mentioned the exhaust baffle & deeper down in the end section. This is a performance killer if restricted.