Excess oil can be a problem when setting up a two stroke, especially on first start up. The high pipes don't help as the oil tends to run back toward the engine. There may be a lot of oil in the crankcases from the rebuild and it tends to build up if you've had the engine ticking over while the engine speed and mixture is adjusted.
Ideally she needs a run to clear the pipes, you then have a short window to make adjustments before she loads up again. From what you say the left cylinder is probably firing nicely, it's the right cylinder that may need adjustment.
Ah the joy of setting up two stroke twins
. Too much oil is better than too little at this stage, check the oil pump adjustment and stroke setting if all else fails.
Brian, ( also trying to set up a GTO with similar issues but can't take it for a run until she is UK registered. Now have lots of crop circles on big lawn !!)