Well, I took off the new Mikuni carb and replaced it with the original carb that I rebuilt a couple months ago. I kicked it till I was blue in the face! Nothing! Then I took out the outside spark plug and tested the spark, sparks beautifully! I then moved the spark plug wire to the outside spark plug and walla, it started right away! It ran for about 2-3 minutes very nicely so I backed it out of the garage to take a ride. I didn't give it enough throttle in first and it died. Never could get it started again! So, it's like the bike is moody! I don't understand how it can quit, and never pop again after running for a few minutes...

Bad spark plugs?
 

I test them, they spark beautifully! That's what's driving me nuts!

Not uncommon for spark to fail under cylinder pressure

just a thought:

If the VENT CAP is full of rust, what does the fuel exit valve look like and the exit inside the tank? much of what you have said, (dry as a bone, dies after starting, had to put choke on to run, etc) may be a lack of fuel supply.

This came from my DT125. I left it in Vinegar for several days, then washed it out with a pressure washer. Had tons of trash.

I did clean the tank months ago and replaced the petcock so the fuel coming out, (clear tubing) is very clean and also going through a filter...thank you for your response! I think I'm going to try a new spark plug, that's one thing I have yet to try...

Make sure that you buy from a trusted source - lots of fakes out there in internet land.

I think that some folks here recommend Autolite (?) plugs.

Well, I tried two new plugs, one a NGK and one a Autolite, bike still will not fire. It only starts when it wants to. If you do get it started, it runs fine but don't kill it! It won't start again...