Tension is the first thing almost everyone adjusts when a stitch looks wrong. It's the most visible dial on the machine, so it gets blamed first. But a large share of what looks like a tension fault is actually a threading mistake, lint packed into the bobbin case, or a bent needle catching the thread wrong. Adjusting tension before ruling those out usually makes the real problem harder to find, not easier.
The fix is a simple diagnostic we use on the shop floor called the pull test.
Testing upper thread tension: Thread the machine normally with the presser foot down. Hold the thread just above the tension discs and pull it gently but firmly. You should feel steady, even resistance — not a free slide, not a hard stop. Now raise the presser foot and pull again. The thread should come almost completely free, since lifting the foot releases the tension discs.
What the results actually mean:
Even resistance with the foot down, and a free release with the foot up, means the tension mechanism itself is fine — the problem is somewhere else.
If resistance doesn't change when you lift the foot, the tension discs likely aren't releasing properly, often from lint buildup or a bent release mechanism.
If the thread slides freely even with the foot down, lint or thread debris is probably lodged between the discs, stopping them from gripping. A strip of clean cotton fabric "flossed" between the discs with the foot down usually clears it.
If resistance feels rough or inconsistent along the thread's length, that's often a burr on the disc itself or a damaged thread guide further upstream.
Bobbin tension is worth checking with equal suspicion, especially on machines running heavy daily use. Hold the bobbin case by the thread tail and let it drop. A properly tensioned case drops an inch or two and then holds — not a free-fall, not a unwillingness to drop at all. A bobbin case that's lost its spring tension will produce inconsistent stitches no matter how carefully you set the upper tension.
If the pull test comes back clean on both ends, the real cause is almost always one of these: an incorrect threading path (a skipped guide, or thread not seated in the take-up lever), a bent or dull needle changing how thread catches at the hook, lint packed into the hook or bobbin area, or the wrong needle system or size for the machine, which throws off hook timing.
We see the same pattern constantly at the shop — someone's convinced the tension assembly has failed, and it turns out to be a bent needle or a lint-packed bobbin case. Running the pull test first saves a lot of unnecessary part-swapping and guesswork.
We wrote up the full method with more detail on genuine bobbin case and tension guide parts here: Sewing Machine Tension Problems: The Pull Test Method
— Aashu, Stitch Partner
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