I got everything back together and turned on the petcock, only to have gas leaking out of the gasket area between the bowl and the main carb body.  Do I have the gasket on upside down?  The side that you can see is shiny, the other side is much less so.  I didn't use any gasket sealant.

From what I can see in your photo, it looks like your gasket is not fitted well into the carb body on the side just below the float pin.  Some non-OEM gaskets need to be carefully trimmed with a razor blade to fit in place.  Maybe a close-up photo would help clarify this.....

Ok, I fixed that probelm by using the old seal - it isn't leaking now.  However, I got it started and about crapped myself when the throttle was stuck wide open.  I quickly turned it off but I'm wondering what I did wrong.  I took the carb partially apart but didn't change any settings.  I don't have an air filter on it right now, I'm waiting for a new one. It seemed to run nicely at 18 million RPM but a little hard on the motor that way, especially with a new piston.  Does anyone have any thoughts? 

Sounds like perhaps you had a "runaway" circumstance, typically caused by unlimited air being available on the intake side.  Check for intake leaks at mating surfaces before the carb, or a leaking cylinder base gasket. 

Search "runaway" for more info.

Dude
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Thank you.  It starts first kick but instantly goes full throttle.  Throttle is twisting fine, doesn't seem to bound by anything, I loosened the throttle cap a bit to see if that might be it, still the same, I shut if off immediately.  Tomorrow I will check the intake gasket.  Not having an air filter on it wouldn't do that, would it?  I don't suppose it's a good idea to put gasket sealant on the intake manifold either.

Is the throttle cable binding anywhere along its entire course? Through the handlebars, under the gas tank, even where its outer cable housing should fit into the cable stop on the top of the carb cover? Is there a kink in the cable anywhere on its full length? Does the slide return with an authoritative "chunk" if you open the throttle and then let go of it? Does the same thing happen if you put the airbox back together with the old air filter in place? Have you lubed the throttle cable recently? Is the cable wrapping itself around something in the instrument panel when you turn the handlebars one way or the other. Ask me how I know that happens...usually at the front of a long line of traffic at a signal...

Carburetor slide installed incorrectly possibly, there is a vertical groove on the slide that must align with a pin on the inside of the carburetor body. I rebuilt a carburetor once and installed the slide incorrectly snd it had the same symptoms as you described including the revving to 18 million rpm, throttle tube doesn't feel right as it doesn't have the proper travel is another symptom of slide not installed correctly, easy to remedy as you only have to remove carb cap and rotate the slide 180 degrees.

A lot to check. I'm going to check the entire length of the throttle cable. I was having a little problem with it sticking before, but not instantly WOT. I took the cap off the top of the carb and the throttle cable was detached. Does anyone have a diagram of how the cable connects to the slide body? The carb blowups don't really show that connection.

Drop the needle through the center hole of the fixed floor plate inside the slide body. While compressing the return spring, pass the "lead shot" on the end of the cable through the opening in the floor inside the slide, and move the "lead shot" sideways along the track into its detent. Next the removeable plate is inserted such that the removeable plate lies over the fixed floor plate with the finger on the removeable plate hanging through the fixed floor inside the slide. Carefully release the return spring to press down firmly on the removeable plate. Then you can insert the slide into the barrel of the carb, making sure that you have the slot in the slide and the pin in the barrel wall aligned, and the needle threaded propery in the base of the barrel.

If i briefly read the story right Its because tightening the carb warps the body. loosen it off a half turn on both flange nuts & try it first by fully opening the throttle & see if it closes to the bottom by its self, then start it. Other possibilities like slide missed the guide pin so it on about half throttle. Flange gets warped from 50 years of overtightening or if an Oring too fat to compress in the mounting flange groove has been used--if it runs an Oring.