Does your needle penetrate the needleplate properly?
This is an often overlooked element that can create loads of problems.
If the needle is even slightly out of proper position in any direction, it will adversely affect the stitch formation and tension of the stitching.
More specifically, the needle must be positioned so that it centers in the needle plate front to back, side to side, default prositioning, and needle bar height. Setting these positions is a major part of any sewing machine repair.
Lets look at these in reverse order: The needle bar height is set by a set screw on the needle bar clamp. The point of the needle must move down through the needle plate hole, into the bobbin carrier area, and begin rising as the point of the hook passes behind the scarf of the needle. If the point of the needle passes too high or too low, poor stitches, irregular stitches, intermitten stitches, or no stitches may result.
To properly set the needle bar height, visually trace from the needle up the bar until the bar passes through the hole in the head and then up until you see a clamp with a set screw in the center. This is the adjusting screw.
Rotate the hand wheel toward you until the needle moves to its lowest position. It must not strike anything. Rotate just slightly until the point of the hook is ready to pass behind the needle. The point of the hook must pass behind the scarf or cut out in the back of the needle and above the eye. If this is not happening, loosen the set screw mentioned previously. Adjsut the bar up or down without turing it. Retighten the set screw.
Next set the machine to its default needle position. Some machines default to a center position, while others use a left needle position. In either case, make sure the needle is properly positioned. If center positioning is used, the needle must enter the needle plate in the exact center of the needle hole. If left position is used, the needle should clear the left side of the hole but match the left most position. In some machines multiple position is used. In these cases, make sure the center position matches properly.
To adjust this positioning, visually trace the needle bar up to the bracket you referenced to adjust height. Note an arm connects to the right of this bracket and goes to another connection. This connection has an eccentric screw to adjust the left to right positioning. A set screw may or may not hold this eccentric in position. Loosen the set screw and adjust the eccentric until the needle is properly positiioned. Test by rotating the hand wheel through one complete needle bar action up and down.
Next note that there is another arm running from this connection and reachinging back to the cam tracker. In the center of this arm is another set screw or eccentric to control how far left or right the needle moves. Adjust so that the needle penetrates the needle plate inside the edges of the hole on the right and left.
Finally, the needle must be positioned properly front to back. This is called the hook-needle clearance. The needle should penetrate the needle hole in the center front to back, but it must also cause the needle to be positioned as close to the point of the hook as possible without actually touching.
To adjust the hook needle clearance. trace the needle bar all the way to the top sleeve or connection with the sewing machine head. Note usually there is a set screw from the front and an eccentric or adjusting screw running through the needle bar assembly clamp. Loosen the set screw and position the needle bar assembly properly.
A major part of sewing machine repair consists of knowing what screws to turn and being able to identify these adjusting points from one model to the next. With training and experience you can be confident if properly setting the needle bar for optimum performance and repair sewing machines of any brand.