Upper thread tension that's too tight is the most common cause of tunnelling - it pulls the needle threads too taut and drags the fabric up between them.

• Ease both needle thread tensions back slightly and test on scrap fabric
• Make small adjustments one dial at a time and test after each change
• Start from the default setting if you're unsure where you are

Looper tension that's too loose can also contribute to tunnelling by not pulling the needle threads down firmly enough.

• Try tightening the looper tension slightly and test on scrap
• Make small adjustments and test after each one

Differential feed set too high can cause the fabric to gather between the needle lines and contribute to tunnelling.

• Make sure the differential feed is set to 1 as a starting point
• For lightweight fabrics, try reducing slightly towards 0.7 if gathering is occurring
• Make small adjustments and test after each change

A stitch length that's too short puts too many stitches in a small area and can contribute to tunnelling.

• Try increasing the stitch length to around 3-3.5mm and test on scrap
• A longer stitch length also tends to give a neater, more professional looking hem on most knit fabrics

If tunnelling is persistent, removing the left needle and sewing with just the centre and right needles gives a narrower stitch with less fabric being pulled up between the needle lines.

• Remove the left needle and sew with centre and right needles only
• This works particularly well on lightweight or very stretchy fabrics where the wider stitch is causing the problem

Lightweight and very stretchy knit fabrics are the most prone to tunnelling - thread choice can make a difference too.

• Use the lightest weight thread suitable for the project - heavy thread on a lightweight fabric increases tunnelling
• Consider using a finer needle for lightweight fabrics - a smaller needle creates less disturbance in the fabric as it stitches

How well the hem is prepared before it goes under the machine makes a significant difference to how it stitches.

• Press the hem firmly with a hot iron before sewing - a crisp, flat fold feeds much more evenly than an unpressed one and is one of the simplest ways to reduce tunnelling
• Make sure both needles are sewing through both layers of fabric - if one needle is only catching a single layer the risk of tunnelling increases significantly
• For lightweight or very stretchy fabrics, try applying a strip of knit stay tape or lightweight fusible interfacing to the wrong side of the hem before stitching - this gives the fabric more stability and can make a big difference

Before returning to your project, test on a piece of scrap of the same fabric with the same hem preparation. Check the hem lies flat with no ridge or bunching between the needle lines before committing to your actual piece.