following the instruction in the tb303 manual from rolandus.com and from quote below
I have been working with irregular length patterns
these are 9 and 7 length patterns
to set irregular pattern length

How to set pattern length on tb303

Clear Pattern + Pattern number (This resets the length back to 16)
Function + Step N times sets the pattern length and resets timing info.
Enter Pitch
Enter Timing
Enter Slides etc.

enjoy the acid