I have bought new needles for my Riccar overlocker now the stitching is loopy and messy

Verify you are using the correct needles for your machine. Some needles vary by a fraction of an inch but will make a big difference in serging success.

Remove all the threads from the machine.
(Always cut the threads below the spools, raise the presser foot, and pull the threads DOWN through the needles--Never pull up!)
ALWAYS RAISE the presser foot when threading.
Be sure to always thread each thread in the proper order--threading out of order will usually fail.
Rethread from the beginning and give each thread an extra little tug when threading the tension disk.

If a thread breaks, it is usually less frustrating in the end to remove all the threads and rethread from the beginning. Trying to rethread a broken thread will most likely cause problems.