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UPDATE.....
I pulled left carb off and promptly broke off the oil injector nipple. Glad I had a spare!! I need some more of those damn things. Pulled the bowl and cleaned out pilot which seemed to have a little somin somin in it, then adjusted float to what I "guessed" was a reasonable level, reassembled and she RUNS!!!!! Have not had it under a load yet due to lousy weather but hope to soon. She zinged up to 7 grand with no issues. Thanks for all the help. 

Damn that sounds messy. There goes all my nice engine paint. I'll remember to not be smoking for this little endeavor.

I saw this a while back but it did not address my elevation. Here is what I have to get me started.

34mm Mikuni's
1.25 overbore with ProX pistons
So far using oil injector
6DH3 needle on 3rd clip down
159 P-8 needle jet
270 main
45 pilot
2.5 slide cut out
1 1/8 out on air screw

I live at near '6900 but will be riding bike from around '4500 to maybe '10000 feet above sea level when hitting mountain passes.

Depends on elevation? Here is what I have in my 34's on my '77 RD. I just posted a question about my carb issues today.
6DH3 needle on 3rd clip down
159 P-8 needle jet
270 main
45 pilot
2.5 slide cut out
1 1/8 out on air screw

I live at near '6900 but will be riding bike from around '4500 to maybe '10000 feet above sea level when hitting mountain passes.

SWEET!! So do I turn carb over and try to set level so floats are even across? I will keep everyone posted on outcome and all jetting info for my elevation just for future reference.

Thanks for the reply Alain2. These carbs were on it but when I obtained the bike, the right piston had exploded, thus I sent engine to Chuck. I added the new manifolds from Chuck and cross over tube. The reed spacers were there. When I reinstalled, the carbs were pouring gas so I had to adjust the floats in both and maybe that is wrong? Carbs are spotless inside. Any idea what level should be on these? I do have manual but that is for the OEM carbs. Thanks

I know, I know, carb issues are the hardest for users to explain and hardest for forum "helpers" to diagnose which is usually why no one ever really likes to reply...or on the other hand...one will get too many replies which then can confuse the issue even more.

I have a '77 RD400 engine that was completely gone through by "Supertune" to include bottom end, bore, new matching piston set etc. Bike also has the 34mm Mikuni's with new manifold kit and crossover tube. Large Uni filters to boot. Older Neutronics (working condition) electronic iginition with Spec-II coils and pipes. (Lets keep the "get rid of the Neutronic's and Spec-II comments to a minimum please) Until I can afford or unless they are the culprits in my issue, those parts are staying put for now.

With both choke levers applied, the bike starts right up. OK guys, here we go!! Strap in!!  When I take chokes off, something magical happens to the left cylinder.  It seems to stop firing. What I mean by that is the exhaust coming out of the pipe gets colder than the right side. When I pull the cap off the plug and get a minor shock through my body and then hold it close to the head fins, I can see plenty of spark. When I pull the cap while the bike is running there is no difference to the idle meaning only the right side is firing and burning the fuel. If I push the choke lever down a bit, the cylinder will start to fire back up.

I am "ass"uming that it is a carb issue...but what?? I have had these carbs off and on more times than I want to remember. I don't want to ride it and try to "blow it out" which I would also think could "blow it up" only running on one piston. Maybe a clogged pilot jet that would come to life when choke was applied? Now that would be an easy fix and maybe I should have checked that prior to sitting here typing all this up so you guys could laugh at me. If it warms up above 20 today, I might get after it.

On a side note, both choke levers point to the left on this bike. Should the right carb be pointing out to the right? Both slide cut outs are facing the rear and are timed as close together as I can get them. Do I have two left carb's...would that even matter?