How to adjust tension on bottom stitch?

You need to tighten the bottom and top stitches both. First, tighten the bobbin slightly. Now, tighten the top stitches the same amount of pressure as the bobbin. The attempt is to have both the bottom and top stitches exactly the same so the final stitch ends up in the center of the fabric. Observe the stitch and readjust if necessary.

According to the manual, you should not need to adjust the bobbin tension (and I located other sites which stated the bobbin tension cannot be adjusted on this machine). Apparently, all the tension adjustment is accomplished through the upper thread::
"EN Thread Tension Upper thread tension Basic thread tension setting: "4". (1) To increase the tension, turn the dial to the next number up. To reduce the tension, turn the dial to the next number down. A. Normal thread tension for straight stitch sewing. B. Thread tension too loose for straight stitch sewing. Turn dial to higher number. C. Thread tension too tight for straight stitch sewing. Turn dial to lower number. D. Normal thread tension for zig zag and decorative sewing. Correct thread tension is when a small amount of the upper thread appears on the bottom side of fabric. Lower thread tension The bobbin tension has been set correctly at the factory, so you do not need to adjust it. Please note: - Proper tension setting is important for strong seams. - There is no single tension setting appropriate for all stitch functions, thread or fabric. - A balanced tension (identical stitches both top and bottom) is usually only desirable for straight stitch construction sewing. - 90% of all sewing will be between "3" and "5". - For zig zag and decorative sewing stitch functions, thread tension should generally be less than for straight stitch sewing. - For all decorative sewing you will always obtain a nicer stitch and less fabric puckering when the upper thread appears on the bottom side of your fabric. 1 A B C 22 D"

Doing further research, I found this statement on a machine review site:
"After reading reviews online from where I've bought my drop-in bobbin machines, I think many of the negative reviews are due to the bobbin thread coming up without laying across the bobbin. It can cause the stitches to look very sloppy and no amount of tension adjusting can fix the stitches."

There are several machine review sites wherein 4423 owners stated their disappointment in the 4423's performance. Some said the machine failed almost immediately and others said the machines developed problems when sewing heavy fabric.

You bobbin tension CAN be adjusted. First, thread your machine with different colors in the top and bobbin, set your top tension at its default, and run a line of straight, and then a line of zigzag stitches, and compare the results at the top and bottom. You will be adjusting your bobbin to your top tension. If the top tension looks like a straight line instead of stitching and the zigzag looks like "chicken tracks" instead of a zigzag, but the bottom looks like a zigzag, your bobbin tension is too loose.

Remove the bobbin case, and locate the straight-slot screw. There are generally two; one is a Phillips and one is straight--you want the straight one. Leave the other alone.

The rule is "lefty-loosy, righty-tighty." If your stitching displays the above symptoms, it needs to be tightened, so turn 1/4 turn to the right--no more--then test again. You will need patience, but don't turn more than that. It is easy to do, but if you don't do it systematically, you can make things worse, and very slight adjustments can alter the stitching significantly.

Thread tension is a BALANCE between the top tension and the bottom tension. It appears that your bobbin tension is sightly tighter, causing the top thread to be pulled down. Run a zigzag stitch. If your top looks fine, but your bottom looks something like chicken footprint tracks, see if you can adjust your top tension a little tighter to compensate. If, in doing that the stitches are balanced but just too tight, you will need to adjust your bobbin tension SLIGHTLY looser. You will need a tiny screwdriver to do this. Take the case out and locate two screws on the side. Once is a Phillips--leave it alone. The other next to it is usually on the left of the Phillips, and, is a straight-blade screw. Lefty-loosey, righty-tighty. In your case, you want to adjust it a LITTLE looser to match the top tension--no more than 1/4 turn. Put it back and test it again; you should see some improvement. If it still needs a little adjusting, reinsert and repeat, but ONLY 1/4 turn at a time.

DON'T fool with it at all if you can compensate by tightening your top tension, and before you do anything, clean and oil your bobbin area with ONE drop and ONE only of sewing machine oil.ONLY. You might find that just cleaning and oiling will solve your problem right there.

top tension must be adjusted to fix loose stitches of bottom stitches
adust the top tension one letter or number higher and sew on a scrap
piece of fabric till stitches look correct,, here is a free manual
http://www.singerco.com/uploads/download/36952a1312a6d8436174ef874f3662471a946b64.pdf

The bottom stitch is affected by the TOP thread tension :) First go back and make sure your bobbin tension is correct since you adjusted it. It should drop slightly when held suspended by danging it by the thread. If it reels out to the floor it is too loose. If it doesn't drop at all it is too tight.

Now that you have that adjusted look at your top tension. If it is loopy on the bottom side of the stitch the TOP tension is too loose. Increase the top tension.

Now take some scrap material and set machine to a wide zig zag, medium length stitch and sew a bit. Compare the top and the bottom sides of the stitch. They should look identical. Remember the TOP side effects the bottom and the BOTTOM tension effects the top side. Adjust accordingly until they look identical with no loose threads or loops and no puckering (puckering means it's too tight).

Hi! The bobbin tension rarely needs to be adjusted. First, make sure you're using the appropriate size needle and thread for your fabric (I'd probably use either a 'denim' needle or a universal size 14) and also that the upper thread and bobbin are threaded absolutely correctly. These can throw off the tension! Also, take the time to clean out the bobbin area. That can throw your stitch off also.

If the stitch on the bottom is loose, so that you see the top threads, then the top tension is too loose. If the top stitch is too loose and you see the bobbin threads on top, then the top tension is too tight.

It helps to thread the machine with different color thread on top and in the bobbin, and then stitch an inch or so on the same type of fabric as you're using in your quilt. Check the stitch. If you need to adjust the top tension, make sure the pressure foot is down! Adjust a little, stitch a few inches, and check again. Repeat until you don't see any (or very little) bobbin thread on the top or top thread on the bottom.

If you go through all these steps, and the bobbin thread is still loose (and you've tightened up the top tension), then there is a tiny little screw on the side of the bobbin case that you can use to adjust the bobbin tension. Turn the screw just a little bit (less than a quarter turn), stitch, check, etc. Once the bobbin tension is set, you shouldn't ever have to change it.

Let me know if this helps, ok?

Happy sewing!

increase top thread tension slowly till u get similar stitches on both sides

pucker is caused by too loose or, to tight adjustment of: the thread tensioners both the bobbin tensioner and, the needle tensioner need to be adjusted: to adjust the needle tensioner turn the side knob clockwise is tighter counterclockwise loosens; the bobbin tensioner is a tiny screw on the bobbin holder adjust both tensioners until your stich loops' both top and, bottom are tight but, the machine does not break the thread

You don't want to adjust the bobbin tension, you'll want to try adjusting your stitch length, as well as the regulator tension.