Joe,
Is there a difference in running between the Slimline and the Bisson? How do the two compare volume wise?
Switching oil while running in the engine is a bad idea. MVVS supplies Mobil 1 2t Racing just to make sure you use the right oil for the process. When changing oils, it takes a long time before the previous oil is washed away completely. As the two may not be compatible very well, during that period the engine is jeopardized.

Your setting up problem: I take it the carb is clean, so make sure of that first. Also make sure the carb is secured well.

Regardless of factory settings, get the engine to run well at WOT, then little by little reduce throttle, whilst keeping the engine running well using the idle needle only. In between open the throttle again, and adjust to best speed with the H needle only. Then proceed as before.
Continue, until you can very slowly reduce the engine speed to near idle without problems.
Now check throlle response, and adjust with idle needle only, all the time keeping best WOT power using the H needle.
Make a note of the needle settings. They belong to your particular carb-engine combo.

If using this method, you cannot get the engine to run well:
Keep using the same fuel, and check plug gap ( 0.6mm = 0.024" ), replace plug. Also check out the mufflers for difference. These pitts type mufflers choke the engine quite a lot.
If all fails, check for damaged carbon reeds, and replace as required.

Edited:
It helps to realize that the H needle is active only from 30% throttle upwards. The idle needle however is active over the complete range from idle to WOT. That is why I suspect a lean idle needle setting.