Had sort of similar issues with a 250.
Would either rev far too high or not at all.
Incorrect sized pilot jet. one size bigger cured.
Also if the carb is loose in any way between itself and manifold it will never run right. This needs rectified first. If unable to tighten it could be cheap S**t that is undersized and is fit only for the bin. No O ring required.
See if you can borrow from somewhere an original BVF carb to see if it makes a difference. Doesn’t matter if its worn or not will simply prove where the issue is.