Breaking Needles On My Model 18

[CustomDoug]

Hi my Consew 18 has been sewing along pretty well for a couple of days (despite the loose feeling bobbin,race,shuttle). But now it's started breaking needles. It seems to be doing this since I started using a zipper foot and sewing through 3 layers of fairly thick upholstery leather right up to the edge (which is thicker than before the zipper foot was added). When it started doing this I was using #18 (135x16) needles and #92 thread... I switched to #20 (135x16) needles and they broke too. It'll sew for a while (around 20 stitches or so) then the needle will start tapping the outside of the hook as the needle is deflected to the left, and quickly break if i don't stop with the warning clicks. I've been very conscious not to pull the leather as I am sewing so I don't think that's the problem. I plan to swap the tensioner apparatus out for a new one, 'cause admittedly this one binds up a little... but it seems to still do this even when I make sure that the tensioner is acting right or when I manually pull a little slack in the thread. As I stated already I'm using the 135x16 system of needles and actually this machine was designed for 16x63 needles. I made no changes to the machine when I started using these needles ( I just read it was possible to use them, so I did) - so maybe something needs to be adjusted or altered for this needle system to be usable? Any comments or suggestions? Thanks ya'll.