This question crops up all the time especially with BSA unit singles, I agree with what Gordon and others have written above and my thoughts are as follows:-
- make sure you have the right springs and cups fitted, its common to find mismatched parts so check and measure everything.
- screwing in the adjusters to stop slip is a playoff between easy or hard handlebar action, personally, I go for screwing them in all the way and then out enough to allow full disengagement with the handlebar lever pulled in.
- check that the right oil is being used, it should be a motorcycle wet clutch compatible (JASO MA spec), a classic car oil no higher than API SF/SG or auto trans oil type F, you might need to clean the plates with solvent if the wrong oil has been used.
- the adjusters should be lock-wired as they tend to loosen easily.
- consider doing the 6 plate conversion as specified on the BSA B50 website Here
- use surflex clutch plates which are grippier than the originals.
The B44 clutch can be made slip-free with a bit of time and effort if you keep at it.