This issue has been resolved.He brought it back to us & since he had a Ferdco 440 for some unknown reason he thought the top tension should be as tight as the Ferdco .So we adjusted the top tension (loosened it a lot) for him & he sewed on a lot of leather & was so happy he bought a CB3200 for his holsters.
Looping underneath is always upper thread either too loose or incorrectly threaded,with a home machine you could be using a thread to large for it can also cause looping.So double check the threading & then keep tighten the top needle tension as far as you need to get it to stop looping.
Either it's not threaded correctly or the top tension is too loose. So double check the threading first, when you know for sure it's right then tighten the top thread 1/2 a turn at a time until it stops.
IF it's threaded correctly,the tension is tight enough ,make sure you pull the handle all the way forward & back & you always hold the thread for the first 3 stitches it shouldn't be jamming up.Miss anyone of the above & any Industrial might jam.
I recently sold a machine to a customer who wanted a fine looking stitch in English bridles & used a #14 with size #69,it looked great! A size 20 will make too big of a hole & won't look nice.If it isn't real thick leather use size16